May 15, 1993 / Vol. 18, No. 10 / OPTICS LETTERS 817

Continuously tunable diode-pumped UV-blue laser source

Larry R. Marshall, A. Kaz, and Orhan Aytur

Fibertek, Inc., 510 Herndon Parkway, Herndon, Virginia 22070

Received January 25, 1993

Continuous tunability and high efficiency are obtained from an intracavity-doubled optical parametric oscillatorpumped by a frequency-doubled diode-pumped Nd:YAG laser. The optical parametric oscillator is tunable from

760 to 1040 nm with 30% efficiency, giving 380-520-nm tunability after intracavity doubling with 40% efficiency.

The utility of diode-pumped Nd:YAG lasers iscurrently limited by the lasers' inherent lack of tun-ability. Although tunable lasers such as forsteritelcan be diode pumped, laser diodes do not performwell at the pump wavelengths required, and thisnecessarily limits the diode power available.2 Wereport continuously tunable output between 380and 520 nm, using a diode-pumped Nd:YAG laserto pump an intracavity-doubled optical parametricoscillator (OPO). The OPO generates tunable mid-IR radiation between 880 and 950 nm, which is thendoubled into the UV-blue, giving a continuouslytunable UV-blue source with an electrical efficiencyof 0.4%.

The diode-pumped Nd:YAG laser consists of a6-mm Nd:YAG rod, side pumped by sixteen diodearrays, arranged in two groups of eight, each groupwith fourfold symmetry, with one group rotated by450 from the other, to give an overall eightfoldsymmetry. The optical cavity employed consists of a3-m concave high reflector located near the laser headand a Gaussian-reflectivity output coupler3 on a 4-mconvex substrate separated from the high reflector by50 cm. The output beam forms a waist of 1.5 mm(l/e radius measured with a CCD camera array),approximately 240 cm from the output coupler. Thebeam divergence was measured to be 1.1 ± 0.05times diffraction limited. When operating at roomtemperature, this device produces 54 mJ of TEMOO,Q-switched, 1.064-Am output in a 10-ns pulse, fora diode pump energy of 550 mJ and an electricalefficiency of 3.5%.

The OPO employs a 10 mm X 10 mm X 15 mmcube of KTP, antireflection coated at 911 nm,cut for normal incidence at 0 = O°, 0 = 690,corresponding to a signal wavelength of 911 nmfor type-II phase matching. The OPO tuning curveis shown in Fig. 1, together with the calculatedwalk-off, Deff, acceptance angle, and threshold pumpintensity for the present experimental arrangement(calculated including walk-off effects). We estimatedthe maximum possible tuning range by comparing theangular range allowed by the geometrical aperture ofthe crystal (70°), the total internal reflection (670),and the Fresnel loss (500). Fresnel loss presentsthe main limitation for tuning this OPO, giving anestimated tuning from 740 to 1040 nm. Operationof an OPO this far from noncritical phase matching

(i.e., at 0 = 90°) incurs the penalty of significantwalk-off and a limited acceptance angle. However,the use of a 532-nm pump significantly lowers thethreshold intensity compared with that in our earlierresearch with 1064-nm pumped OpO's,6 which allowslarger beam diameters to be employed, which in turnreduces the significance of walk-off. The thresholdpump intensity calculated for the present OPO is-7 MW/cm2 (where walk-off is included).7

Intracavity doubling of an OPO was first reportedby Bey and Tang.8 The layout of our intracavity-doubled OPO is shown schematically in Fig. 2. The1064-nm output pulse of the diode-pumped Nd:YAGlaser passes through a 10-mm cube of KTP cut forfrequency doubling. Some 45 mJ of 532-nm outputis generated by the doubler, and this serves topump the OPO. The waist of the pump beam ispositioned at the center of the OPO cavity, andsince the cavity is only 5 cm long the pump can beconsidered a plane wave. The 532-nm pulse entersthe OPO cavity through a flat dichroic turning mirror(MI; highly reflecting at 800-950 nm and highlytransmitting at 440-540 nm). The 532-nm pulse

6

aCM

"I

90

Theta Angle (0)

Fig. 1. Tuning curve of the KTP OPO pumped at 532 nm,showing walk-off angle, Deff, and signal and idler wave-lengths, calculated by using the dispersion relation ofRef. 4 and the Deff values of Ref. 5.

0146-9592/93/100817-03$5.00/0 © 1993 Optical Society of America

818 OPTICS LETTERS / Vol. 18, No. 10 / May 15, 1993

M3

-IzI: LBBO

532-nm Pump

760-1040-nm signal

- # ~ KTP OPO m~~~~~ 1 ----

\ w .xssw~x~v~---..- 1MX> I, I

Ml

380-520-nm second harmonic

Fig. 2. Layout of intracavity OPO, showing the differ-ent pump wavelengths involved: the 532-nm pump, the760-1040-nm signal, and the 380-520-nm intracavity-doubled signal.

passes through the OPO crystal and is reflected offthe flat rear cavity mirror (M2; highly reflecting at532 and 880-950 nm), to leave the cavity throughmirror Ml after making a second pass through theOPO crystal. All mirrors have minimal reflectivityat the idler wavelength, making the OPO singlyresonant. The intracavity 911-nm flux is reflectedby the turning mirror (Ml) to pass through a 3-mm-long pl-barium borate (BBO) type-I doubling crystal(in an index-matched housing, antireflection around1000 nm). The final cavity mirror (M3) is highlyreflective at both 880-950 and 410-480 nm, whichallows the doubled output to be extracted throughthe turning mirror (Ml) after making anotherpass through the doubling crystal. The use oftwo-pass nonlinear generation in both the doublerand the OPO significantly improves the conversionefficiency of these processes. The advantage of thisconfiguration is that the KTP crystal is not exposedto the blue-WN flux, which could cause damageproblems.

Tunability in the near-IR is obtained by removingthe doubler and replacing M3 by a 10% transmittingoutput coupler at 911 nm. In this configuration, theOPO can be tuned from 760 to 1040 nm. The tuningrange appears to be limited only by the reflectivity ofthe mirrors. The BBO doubler has sufficient angularsensitivity to permit doubling of wavelengths from680 to 1100 nm with the use of a single crystal andis therefore ideal for the present broad tuning rangeapplication.

Oscilloscope traces of the input 532-nm pump, thegenerated 911-nm signal (without intracavity dou-bler), and the intracavity-doubled output at 455 nmare shown in Fig. 3. In the absence of the dou-bler, the buildup time of the OPO compresses the911-nm output pulse length to -6 ns, compared withthe 10-ns pump pulse. However, when the doubleris inserted the frequency-doubled pulse is broadenedcompared with the 911-nm pulse obtained in thedoubler's absence. This broadening is typical of thatobserved for intracavity-doubled lasers9 and occursbecause the doubler acts as a variable-output cou-pling whose reflectivity scales with intracavity flux;thus the peak of the 911-nm pulse is flattened some-what owing to higher output coupling, while thewings are enhanced owing to lower output coupling.

The output energies obtained at the variouswavelengths involved are shown in Fig. 4, plotted asa function of electrical input energy. The absoluteelectrical efficiency obtained for each wavelengthis as follows: 3.5% at 1064 nm, 2.9% at 532 nm,0.9% at 911 nm, and 0.4% at 455 nm. The OPOconversion efficiency of 532-nm pump to 911-nmsignal is 30%, while the effective doubling efficiencybased on this optimum 911-nm conversion is 40%.The threshold pump energy for 911-nm operation(conventional OPO) is 7 mJ. This corresponds to athreshold pump intensity of 10 MW/cm2, which isin good agreement with the calculated threshold of7 MW/cm2. The repetition rate of the pump laserwas varied from 10 to 50 Hz, with negligible changein OPO efficiency.

We have plotted in Fig. 5 the conversion efficiency(to 455 nm) of the intracavity OPO and the effi-ciency of the optimized 911-nm OPO without dou-bling. Also shown is the theoretical dependence ofefficiency on the factor by which the pump power ex-ceeds the threshold for parametric oscillation. 6 Al-though the intracavity doubler acts as an intensity-dependent output coupler for the OPO, the OPO effi-ciency is relatively independent of output coupling,6'8so we expect that the same theoretical curve willbe followed by the intracavity-doubled OPO as for aconventional OPO. The 911-nm efficiency increaseswith factor above threshold at a lower rate thanpredicted from theory, giving a maximum efficiencyof 30%, in contrast to the calculated maximum ef-ficiency of 42%. We have found this theory to beaccurate in the case of noncritically phase-matched

-AI

Time (5 ns/Div)

Fig. 3. Oscilloscope traces of the 532-nm pump pulse(solid curve), the 911-nm signal in the absence of theintracavity doubler (dashed curve), and the intracavity-doubled output at 455 nm (dotted curve).

c;

0E

c

soo 1000 1500Electrical Input Energy (mJ)

2000

Fig. 4. Summary of energies obtained at various wave-lengths generated in these experiments, plotted as afunction of electrical input energy into laser-diode arrays.

May 15, 1993 / Vol. 18, No. 10 / OPTICS LETTERS 819

30-

cJ 20 la-.-0 CS

10- calculation0/ 0 91 Inm

0 455nm

0O0 2 4 6 8 10

Factor Above Threshold

Fig. 5. Energy conversion efficiency (pump to signalonly) of the conventional 911-nm OPO and the intra-cavity-doubled OPO, together with theoretical efficiencyplotted as a function of factor above threshold.

OpO'S,6 where walk-off is negligible. In the presentcase we are operating far from 900 phase matching,and the walk-off becomes large (2.50). Since walk-offreduces the overlap between pump and signal beams,it prevents parts of the pump beam from participat-ing in the parametric process and therefore reducesthe apparent conversion efficiency. An even largerreduction in slope is seen for the frequency-doubledoutput. We attribute this larger divergence fromtheory to the additional walk-off (2.750) introducedby the BBO doubler.

A rough estimate of this effect can be made asfollows: as the signal mode walks off from the pumpmode, the peak of the signal mode sees a pump inten-sity lower than the peak of the pump. If the signal isto remain TEMOO, then the maximum conversion canoccur only at this region of lower pump intensity; thusthe maximum possible efficiency has been reduced by7 the ratio of the pump intensity at this point tothat at the peak of the pump mode. This factor canbe approximated by

[1(4l tan p 77w = exP 1j- an

where 4, is the crystal length, p is the walk-off, andw0 is the beam waist.

This rough approximation necessarily overesti-mates the reduction in efficiency since the actualbeam separation is zero at the start of the crystaland reaches 4, tan p only at the end, while we haveassumed a separation of l tan p over the entirecrystal length. For the 911-nm OPO, the reductionfactor is 77w = 0.82, which reduces the maximumtheoretical conversion efficiency from 42% to 34%.The calculation does not take into account the pulsecompression caused by the OPO, and this further

reduces energy conversion efficiency, in practice.Given this, the observed reduction is in reasonableagreement with the discrepancy observed in Fig. 5.If we now consider the doubler, and note that thesignal mode size is reduced by 0.7 from the pumpspot size, then we calculate a further reductionfactor of y77 - 0.94 that is due to the 5-mm-longdoubler. While these are rough approximations,it does seem justified to attribute the observedreduction in efficiency to walk-off.

The acceptance angle of BBO is quite small,which limits the extent to which pump beamsmay be focused to increase doubling efficiency.Intracavity doubling in lasers leads to significantlyhigher doubling efficiencies because of the greaterflux available within the laser cavity, and thesame is true with an OPO. Furthermore, in theintracavity configuration the optical beams arefairly parallel (flat OPO mirrors), and the limitedacceptance angle of BBO is less of a problem.We showed previously9 that the inclusion of thedoubling crystal within a laser cavity can lead tosignificant instabilities in laser output, especially formultilongitudinal-mode operation. However, in theintracavity-doubled OPO this does not appear to be asignificant problem, presumably because of the short-lived nature of the optical fields involved. This isthe typical behavior observed for intracavity-doubledQ-switched lasers. The observed pulse broadeningis also typical of such lasers.

These high efficiencies, together with broadbandtunability in an all-solid-state design, permit accessto a number of new applications for diode-pumpedNd:YAG lasers, including IN-blue spectroscopy,lithography, and underwater communications.

References

1. V. Petricevi6, A. Seas, and R. R. Alfano, Opt. Lett. 16,811 (1991).

2. J. G. Endriz, M. Vakili, G. S. Browder, M. A. DeVito,J. M. Haden, G. L. Harnagel, W. E. Plano, M. Sakamoto,D. F. Welch, S. Willing, D. P. Worland, and H. C. Yao,IEEE J. Quantum Electron. 28, 952 (1992).

3. K. J. Snell, N. McCarthy, and M. Pich6, Opt. Commun.65, 377 (1988).

4. K. Kato, IEEE J. Quantum Electron. 27, 1137 (1991).5. R. C. Eckhardt, H. Masuda, Y. X. Fan, and R. L. Byer,

IEEE J. Quantum Electron. 26, 922 (1990).6. L. R. Marshall, J. Kasinski, and R. L. Burnham, Opt.

Lett. 16, 1680 (1991).7. S. J. Brosnan and R. L. Byer, IEEE J. Quantum Elec-

tron. QE-8, 361 (1972).8. P. B. Bey and C. L. Tang, IEEE J. Quantum Electron.

QE-8, 361 (1972).9. L. R. Marshall, A. D. Hays, A. Kaz, and R. L. Burnham,

IEEE J. Quantum Electron. 28, 1158 (1992).