Thermal lensing measurement and compensation in acontinuous-wave mode-locked Nd:YLF laser
Herman Vanherzeele
Central Research and Development Department, E.l. DuPont de Nemours & Company,Experimental Station, Wilmington, Delaware 19898
Received October 29, 1987; accepted February 25, 1988
Thermal lensing effects in a cw-pumped Nd:YLF rod have been characterized, permitting the optimization of thelaser resonator. In the mode-locking regime, a well-designed system generates an average output power similar tothat of a typical Nd:YAG laser with, however, an increased stability and pulse durations that are significantlyshorter.
Investigations of the spectroscopic and physical prop-erties of Nd:YLF have suggested that this materialmay be a better candidate than Nd:YAG for generat-ing short pulses with high peak power.i-7 A recentreport on a cw-pumped Nd:YLF mode-locked oscilla-tor and regenerative amplifier has further confirmedthe -predicted advantages of this material.8 In thisLetter I describe performance degradation due tothermal lensing in a cw pumped Nd:YLF laser andshow how it can be overcome in an optimized system. 9
It has been shown10 that Nd:YLF has a relativelylarge bandwidth (1.35 nm) compared with Nd:YAG(0.45 nm),. Consequently, Nd:YLF can deliver shorterpulses than those available from Nd:YAG. This is adistinct advantage for many applications in nonlinearoptics as well as for synchronous-pum-ping dye lasers.Moreover, Nd:YLF exhibits a strong natural birefrin-gence, which overwhelms the thermally induced bire-fringence and thus eliminates thermal depolarizationproblems commonly encountered with isotropic mediasuch as Nd:YAG.5 The output of Nd:YLF is natural-ly polarized: either a(E I c) at 1053 nm or 7r(E 11 c) at1047 nm. For the latter transition a slightly largercross section (a) than the 1064-nm transition ofNd:YAG was reported recently, while for the 1053-nmline the cross section is smaller than in YAG.7 Thefluorescent lifetime (r) in YLF is about twice as long asin YAG, and consequently the product -ri- in YLF isgreater than in YAG. Cw threshold is inversely pro-portional to this product, so Nd:YLF has a lowerthreshold if other factors are equal. As an overallresult, one expects about the same output power froma cw-pumped Nd:YLF rod as from a similar Nd:YAGrod. However, during initial experiments in our lab-oratory with a 4 mm X 79' mm Nd:YLF rod inside aresonator similar to the one described in Ref. 7, wegenerated less than 5 W of average mode-locked powerat 1053 nm, whereas a similar Nd:YAG rod typicallydelivers as much as 10 W under the same pumpingconditions. Attempts to improve the efficiency byincreasing the mode volume in the rod resulted instrongly elliptical beams with only a slight increase in
output power. These experimental results clearlysuggest adverse thermal lensing effects in Nd:YLF. -
Thermal lensing measurements on-Nd:YLF havebeen reported for pulsed operation (at a fixed inputenergy).5 However, data are given only for 7r polarizartion; for a polarization the thermal lens was reportedto be too weak to be measurable, and therefore only anassumed lower limit was given. For this work -moredetailed and accurate data are needed. A Quantronix116 laser head with one krypton lamp and a 4 mm X 79mm Nd:YLF rod (1% doping) are used. The rod axisis along the crystallographic a axis. For investigationof the effect of the pump power dissipated in the rodon the transmitted wave front, a polarized 633-nmHe-Ne laser illuminates the full aperture of the rod.The He-Ne beam is highly collimated with a 30Xtelescope. The thermal lens f of the rod is inferredfrom the measurement of the 633-nm power I (or Io forzero lamp power) transmitted through a narrow slit(rather than a circular pinhole as commonly used forthese measurements in isotropic materials):
(1)
where d is the distance between the slit and'the imageprincipal plane of the rod. The slit is positioned ei-ther parallel or perpendicular to c. The accuracy ofthe experimental technique was checked by replacingthe rod by a lens with a known focal length. As de-scribed above, the thermal lensing effects in Nd:YLFare measured as a function of lamp power P in the (a,c) and (a, b) planes for both a- and 7r-polarized light.The experimental results are shown in Fig. 1. Thedata are accurate to within ±25%. These error barsinclude observed differences in the thermal lensingeffects in three otherwise identical rods. The focallength f of the thermal lens clearly is different for bothpolarizations, and, moreover, for a given polarization adifferent lens effect is measured in the (a, b) and (a, c)planes. For a-polarized beams, for which the lens isweaker [see Fig. 1, curve (1)], f < 0 in the (a, c) plane,while f > 0 in the (a, b) plane; the magnitude of the lens
Fig. 1. Measured thermal focal lengths f(m) at 633 nm forboth a and 7r polarizations in two orthogonal planes (II c andIc) for a 4 mm X 79 mm Nd:YLF rod as a function ofelectrical input power P(kW) to the lamp: (1) a-polarizedlight (f < 0 in the c plane, f > 0 in the plane perpendicular toc); (2) 7r-polarized light in the c plane (f < 0); (3) 7r-polarizedlight perpendicular to the c plane (f < 0). The squaresrepresent the experimental data.
in both planes is the same within the experimentalerrors. On the other hand, for 7r-polarized beams [Fig.1, curves (2) and (3)], f < 0 in both planes, with aweaker action in the (a, c) plane. A least-squares fit ofthe experimental data gives the following results:
fL(m) = - f/c(m) = 98 [P(kW)]- 6, (2a)
f'j,(m) = - 53 [P(kW)]' 1 6, (2b)
fj_(m) = - 12 [P(kW)]-11 l. (2c)
Equations (2a), (2b), and (2c) correspond to curves(1), (2), and (3), respectively, in Fig. 1. From thesedata it is obvious that the thermal lensing effect insidea Nd:YLF resonator will be anything but negligible,despite the fact that the lens is much weaker than forNd:YAG, as was pointed out in Ref. 5. The anisotro-py of the lens effect for a given polarization particular-ly merits attention. This effect, if not compensatedfor, causes astigmatism. It gives rise to ellipticallydistorted beam profiles both inside and outside theresonator, thereby reducing the maximum extractableTEMoo power from the rod.
Several resonator candidates were theoretically an-alyzed to ensure a combination of all the followingdesirable properties: (i) cavity length set for our 100-MHz mode locker (see below), (ii) optimum rod fillingfor TEMoo operation (ideally, the rod should be thelimiting aperture), (iii) good astigmatism compensa-tion of the thermal lensing over a wide range of lamppower, (iv) acceptable pointing stability and mirror-misalignment sensitivity (in the sense defined in Ref.11), and (v) suitable beam parameters outside the res-onator (low beam divergence). A resonator designthat adequately combines all the above properties,while requiring only one set of optics for both 1047-and 1053-nm operation, is schematically representedin Fig. 2. A spherical lens (f = 38 cm) ensures anoptimum filling of the rod (the beam radius, HW 1/e2M, inside the rod is 1 mm). A cylindrical lens (f =
470 cm) corrects the astigmatism (resulting from thethermal lensing) to better than a few percent over theentire range of useful electrical input power (3.5-5.5kW). The mode volume in the rod does not changeover this range of lamp power. The far-field beamdivergence (FW 1/e2 M) is 1.6 mrad at both wave-lengths. Notice that, without the cylindrical lens, theresonator is unstable. The pinhole helps to make thesystem quieter in the mode-locking regime but doesnot significantly reduce the mode volume in the rod.The resonator offers a good dynamic stability and hasa low mirror-misalignment sensitivity (because of thespherical lens). As a result, ab initio alignment of theresonator can be done easily, despite the presence ofthe intracavity lenses.
Henceforth we will restrict ourselves to the 1053-nmlaser. The resonator was built starting from a com-mercial unit (Quantronix 416). It is harmonicallymode locked with 12 W of rf power at 100 MHz. Theaverage mode-locked output power as a function oflamp power is shown in Fig. 3 (lasing was restricted to1053 nm by proper alignment of the rod with respect toa Brewster-plate polarizer inside the cavity). Thetransmission of the output coupler is 12% (a standardvalue for Nd:YAG lasers). No attempts were made tooptimize this transmission. From a glance at Fig. 3 itbecomes immediately evident that the average outputpower of an optimized Nd:YLF resonator can indeedbe as large as that of a typical Nd:YAG laser. Theoutput power is stable to within 1.2% (peak-to-peakvalue) without the use of any stabilizing feedbackmechanisms. This is better than what one usuallyobserves in Nd:YAG. (An enhanced stability has alsobeen reported in Ref. 2 for a passively mode-lockedsystem.) The beam profile, measured at several loca-tions, proved to have an excellent Gaussian shape,without measurable ellipticity. Experimentally ob-tained data for the spot sizes agreed to within a fewpercent with the ones calculated from the ABCD ma-trix for the resonator, thus confirming the thermallens data. The pulse duration, measured with a real-time autocorrelator, is 36 psec (FWHM) assuming aGaussian pulse shape. Spectral measurements ob-tained with a scanning Fabry-Perot interferometerfurthermore showed that the pulses are nearly band-width limited. The excellent short- and long-term'temporal stability was further evidenced by monitor-
Dl D2II I
II n n r , , nII
U
HR SL CL rod
J . . U
PH POL ML OC
Fig. 2. Schematic representation of the Nd:YLF resonatordiscussed in the text: HR, high reflector (R = 100 cm); OC,12% output coupler (R = -120 cm); CL, cylindrical lens (f =470 cm); SL, spherical lens (f = 38 cm); ML, mode locker;PH, pinhole; POL, polarizer. For 1047-nm operation, DI =55 cm, and for 1053-nm operation DI = 48 cm; D2 = 9 cm inboth resonators.
(1)
(2)
(3)
.4
1
I
I
May 1988 / Vol. 13, No. 5 / OPTICS LETTERS 371
an optimized resonator has been designed that com-bines good TEMoo mode quality with high averageoutput power and excellent short- and long-term sta-bility in the mode-locking regime. Since the pulsewidth is significantly shorter than in similar Nd:YAGlasers, we believe that an optimized Nd:YLF system asdescribed in this Letter may become the choice formany applications. Further system improvements,including, e.g., an electronic cavity-length control toeliminate thermal-drift problems in the mode-lockingregime; as well as a more detailed description of our
2 3 4 5 6 improved KTP doubler will be presented in a forth-input power (kW) coming publication. 14
0
Fig. 3. Average mode-locked output power at 1053 nm as afunction of electrical input power to the lamp. The squaresrepresent the experimental data.
ing the timing fluctuations (only a few picoseconds) ofthe optical pulses in the resonator. For these mea-surements, a method similar to the one described inRef. 12 was used. Detuning the cavity length by ap-proximately 0.5 gm from its ideal length (either side)resulted in a 200-MHz mode-locked operation of thelaser with no change in average output power. How-ever, some pulse broadening was observed in this re-gime, and the timing fluctuations were greatly en-hanced.
Frequency doubling the Nd:YLF output in a 5-mm-long KTiOPO4 (KTP) crystal,13 cut for Type II sec-ond-harmonic generation at 1053 nm (phase-matchingangle 34 deg with respect to the X axis in the X, Yplane), generates 25-psec (FWHM) pulses with 2-Waverage power. The corresponding second-harmonicpeak power is substantially higher than for a similarfrequency-doubled Nd:YAG system (because of boththe higher average power and the shorter pulse widthat 526 nm in YLF). Optical damage in KTP has beenobserved at these high average power levels. Howev-er, we now are able successfully to avoid catastrophicdamage in KTP (up to the power levels here reported)by maintaining the crystal at a temperature, above100 0C.14
In summary, the thermal lensing of a cw-pumpedNd:YLF rod has been characterized. With these data,
The valuable assistance of J. Kelly during the courseof this research is very much appreciated.
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