Low noise

Low-noise62W CW intracavity-doubled TEM00 Nd:YVO4green laser pumped at888nm

Louis McDonagh and Richard Wallenstein

Fachbereich Physik,Technische Universit?t Kaiserslautern,67663Kaiserslautern,Germany Received November7,2006;revised January7,2007;accepted January8,2007;

posted January10,2007(Doc.ID76865);published March5,2007

We present a CW intracavity frequency-doubled TEM00Nd:YVO4laser oscillator pumped at888nm,pro-ducing62W of green light at532nm with M2=1.05and RMS noise of0.05%,thanks to the simultaneous oscillation of a large number of longitudinal modes.A diode-to-green optical ef?ciency of29%is achieved by the use of a noncritically phase-matched lithium triborate crystal inserted in a periodic resonator containing two Nd:YVO4crystals pumped with a total of211W at888nm.?2007Optical Society of America

OCIS codes:140.3480,140.3530,140.7300.

High-power,high-beam-quality green lasers emitting in CW operation are needed in many applications, such as pumping of Ti:sapphire or dye lasers,pro-cessing of materials exhibiting low absorption in the infrared,annealing and crystallization of glass,and even large-scale laser https://www.360docs.net/doc/4513692948.html,rge-frame argon-ion lasers were the?rst to offer powers in excess of30W in a near diffraction-limited beam,but at the expense of very low ef?ciency,requiring several tens of kilo-watts of electrical power and high-?ow-rate water cooling.With the advent of high-power laser diodes and high-quality neodymium-doped vanadate and nonlinear crystals,intracavity frequency-doubled la-sers emitting at532nm became commercially avail-able10years ago,offering at?rst5–10W of stable noise-free output power in a diffraction-limited beam.1,2Although the power of such systems has in-creased up to23W in a laboratory setup and18W commercially,3they still have not equaled large-frame ion lasers in terms of power,with the exception of a frequency-doubled Yb:YAG thin-disk ring laser providing up to50W at a wavelength of515nm.4 The major issue in the development of high-power intracavity-doubled solid-state lasers is the rise of power instabilities in the microsecond to millisecond range,known as the“green problem.”5This strong noise is due to sum-frequency mixing gain saturation between the different longitudinal modes oscillating in the cavity.Several solutions have been proposed that rely on simple resonator designs without added intracavity elements.Passive stabilization can be achieved by shortening the cavity until the relaxation-oscillation pulses are short enough to un-dergo signi?cantly stronger conversion to the green than stable CW,the laser operating then in a CW mode so as to minimize its power loss.6Another method of achieving stable cw green output is to re-duce the cavity length to maximize mode spacing, thus limiting the number of oscillating longitudinal modes,and place the doubling crystal gain medium in speci?c locations inside the laser cavity so that the spatial overlap of different oscillating modes in the gain medium and the doubling crystal is minimized.7,8The ef?ciency of sum-frequency mixing and mode coupling in the gain medium are reduced, leading to a stabilized output.However these solu-tions intrinsically need short resonators and/or very speci?c locations for laser and doubling crystals, which prevents their use in high-power systems gen-erally requiring relatively long resonators in which the position of elements is already determined by geometrical and mode-matching considerations. These constraints lead to the prevalence of two solu-tions for the realization of high-power systems.One solution is to operate the laser in a single-longitudinal-mode regime by designing a unidirec-tional ring oscillator with a Faraday rotator and an intracavity Fabry–Perot etalon for single-mode selection.2–4Another approach for solving the green problem is to build a long linear resonator with a high-gain laser medium to allow many longitudinal modes to oscillate simultaneously,thus averaging out the effect of sum-frequency mixing between modes.1,9–11The latter solution was chosen here as it requires fewer intracavity components than a single-mode unidirectional ring resonator and can be ap-plied by simply extending an existing linear resona-tor.

This Letter presents a62W intracavity-doubled CW TEM00Nd:YVO4laser oscillator,producing what we believe is the highest CW power at532nm in a diffraction-limited beam.Nd:YVO4was chosen as the laser medium for its high gain and natural bire-fringence and was pumped at888nm to optimize ab-sorption uniformity,minimize crystal stress,and maximize optical ef?ciency.888nm pumping of vana-date allows high pump powers to be optimally ab-sorbed in long crystals,thanks to a low and isotropic absorption at this wavelength.12Furthermore,the low quantum defect leads to a reduced thermal load and higher optical ef?ciency,all contributing to achieving a high output power.Noncritically phase-matched lithium triborate(LBO)(?=90°,?=0°, T=150°C)was selected for type I second-harmonic generation for its high damage threshold,absence of walk-off,and large temperature and angular acceptances.13Figure1depicts the oscillator setup that can be operated with one laser crystal or as a pe-

802OPTICS LETTERS/Vol.32,No.7/April1,2007

0146-9592/07/070802-3/$15.00?2007Optical Society of America

riodic resonator comprising two crystals.The single-crystal resonator contains a 4?4?30mm a -cut,0.5%at.doped Nd:YVO 4crystal C 1,end pumped by a 888nm,400?m diameter,0.22NA ?ber-coupled di-ode laser system.The pump light is focused on a 1350?m diameter pump spot in the center of the crystal,providing an almost collimated pump volume along the crystal length.The unabsorbed pump light is retrore?ected by a lens–mirror combination to achieve high and smooth absorption of the pump light,up to a total absorption ef?ciency of 95%,thus spreading the thermal load on the whole crystal length and limiting the critical thermal load on the input facet.The residual nonabsorbed pump light does not affect the diode’s operation,and overheating of the ?ber ferrule is avoided by the use of a free-standing end.The folded resonator is formed between ?at end mirror M 5,convex pump mirrors M 3and M 4with r =+1000mm,highly re?ective (HR)at 1064nm and highly transmissive (HT)at 888nm,concave mirror M 2with r =?75mm,HR at 1064nm and HT at 532nm,and concave end mirror M 1with r =?100mm,HR at 1064and 532nm.M 2focuses the laser mode down to a waist radius of 35?m in the 20mm long LBO crystal while the laser mode is 510?m in the laser crystal,as illustrated in Fig.2.The radii of curvature of M 1and M 2and their rela-tive distances to the LBO were optimized to achieve the desired focus size and position,while the distance from M 1to the LBO was adjusted to compensate for the dispersion of air and minimize phase mismatch between the green beams generated in the two direc-tions.Thus,the green radiation generated from M 2to M 1is backre?ected on M 1and combined in phase with the green light generated in the opposite direc-tion,both exiting the cavity in one single beam through dichroic mirror M 2.The resonator is further extended as a periodic resonator by duplicating the

IR cavity between M 2and M 5to an identical arrange-ment between M 5and M 8and removing mirror M 5to let the cavity oscillate between M 1and M 8.Figure 2illustrates the periodicity of the mode pro?le and the unchanged focusing in the LBO when the cavity is extended.

The single-crystal oscillator was ?rst operated be-fore heating the LBO crystal,thus without generat-ing any green light,and was optimized for TEM 00op-eration at a pump power of 108W,providing 55.5W at 1064nm when the end mirror M 5was a 35%trans-mission optimum output coupler.The LBO crystal was then heated up to achieve noncritical phase matching,and end mirror M 5was replaced by a 1.5%transmission output coupler to limit intracavity power.The oscillator provided an output power of 34W at 532nm,corresponding to an IR-to-green con-version ef?ciency of 61%,with respect to the avail-able fundamental power without frequency doubling and with optimum output coupling,and a diode-to-green optical ef?ciency of 31%.The one-way circulat-ing intracavity power was calculated from the IR power transmitted through M 5to be 180W.The os-cillator was then extended to the periodic resonator con?guration,producing an IR power of 115W for a pump power of 211W with a cold LBO and an output coupling of 65%.Once the LBO was brought to tem-perature,and M 8replaced by a 5%transmission mir-ror,the oscillator provided an output power of 62W at 532nm,corresponding to an IR-to-green conver-sion ef?ciency of 54%,and a diode-to-green optical ef-?ciency of 29%,while the one-way intracavity power was calculated at 250W.In both con?gurations,fur-ther reducing the transmission of end mirror M 5or M 8did not permit higher IR intracavity power,or higher green output power,as the resonator then suf-fered high nonlinear losses through off-axis ampli?-cation of the laser mode wings in the high-gain edges of the aberrated thermal lens.This effect caused by thermal lensing and aberrations in the LBO crystal led to off-axis laser light being ampli?ed and heating the pump mirror mounts,thus disturbing the sys-tem’s stability.A beam quality factor of M 2=1.05was measured in both cases with a Coherent ModeMas-ter,and the beam pro?le was monitored with a CMOS camera,as illustrated in Fig.3.

Figure 4illustrates the output power monitored with a 20ns risetime silicon PIN photodiode and a 1GHz bandwidth oscilloscope.The residual noise was mainly a result of relaxation oscillations at 25kHz,also visible as a single peak on a RF spec-trum analyzer.The RMS noise induced by the 25kHz residual oscillations measured on the oscilloscope amounted to 0.20%at 34W (left)and 0.05%at 62W (right).The output power remained stable over sev-eral hours of operation once the air and mechanical components had reached thermal equilibrium,al-though a sealed temperature-regulated cavity would reduce warm-up time and limit the perturbations in-duced by air ?ows and convection.

To explain the lower noise for the two-crystal con-?guration,we measured the optical spectrum of

the

Fig.2.Mode pro?le inside the 1.5m long

resonator.

Fig.1.Oscillator setup.

April 1,2007/Vol.32,No.7/OPTICS LETTERS 803

fundamental laser mode with a 150GHz free-spectral-range (FSR)scanning Fabry–Perot interfer-ometer,as illustrated in Fig. 5.The single-crystal and periodic resonators exhibited average spectral widths of,respectively,7and 12GHz,for longitudinal-mode spacings of 175and 100MHz cor-responding to 40and 120oscillating modes,respec-tively.The low-noise operation can therefore be at-tributed to the large number of simultaneously oscillating modes,therefore providing a constant overall output,although the IR spectrum’s pro?le is constantly changing.However,to maintain the ran-dom excitation of the different modes,care should be taken to avoid any spectrally selective effect,such as an etalon.10Therefore the LBO crystal was slightly tilted off its axis to avoid any parasitic re?ections along the resonator axis.

In summary,we have demonstrated a 62W intracavity-doubled Nd:YVO 4oscillator,which we believe the highest output power for a cw laser at

532nm with diffraction-limited beam quality,more than three times the highest commercially available power.3Furthermore,it provides higher output power than large-frame argon-ion lasers,with the ad-vantage of low power consumption,compactness,re-liability,and ease of use.

The authors thank Achim Nebel with Lumera La-ser GmbH for discussions and ?nancial support.L.McDonagh’s e-mail address is mcdonagh@physik.uni-kl.de.References

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Fig. 3.(Color online)Output beam and corresponding horizontal and vertical

pro?les.

Fig.4.62W green output monitored with a PIN photodi-ode.The insets are close-up views with AC coupling for the one-and two-crystal

resonators.

Fig. 5.IR mode optical spectra for the one-and two-crystal resonators.

804OPTICS LETTERS /Vol.32,No.7/April 1,2007