Growth and second harmonic generation characterization of Er 3 doped bulk periodically poled LiNbO 3 V. Bermu ´ dez, J. Capmany, J. Garcı ´ a Sole ´ , and E. Die ´ guez Departamento Fı ´sica de Materiales C-IV, Universidad Auto ´noma de Madrid, 28049 Madrid, Spain Received 9 April 1998; accepted for publication 2 June 1998 Samples of Er 3+ doped bulk periodically poled lithium niobate have been grown by the Czochralski method. The efficiency, tuning and thermal tolerances of infrared to green second harmonic generation have been measured, showing good accordance with a 4%–5% dispersion in the domain period length. © 1998 American Institute of Physics. S0003-69519804931-6 Nonlinear optics has become a useful way of extending availability of laser radiation to determined bands and wave- lengths demanded by technological and scientific problems. Efficient second order nonlinear processes become more ef- ficient at high nonlinear coefficients and when phase match- ing conditions among the interacting waves are fulfilled. Be- sides the very restricted use of anomalous dispersion, phase matching can be accomplished in birrefringent crystals in two different ways 1 types I and II. Another way of achiev- ing high efficiency in nonlinear processes is by means of periodic modulation of the nonlinear coefficient in what is known as quasi phase matching QPM. 2,3 Technologically, periodic modulation of the sign of the nonlinear coefficient can be obtained in ferroelectric crystals by periodically alter- nating the sign of the electric field of their ferroelectric domains, 2 a technique known as periodic poling, and where the length of the domain periods plays a fundamental role in wavelength tuning of high efficiency in the nonlinear pro- cesses. In this way LiNbO 3 LN is an excellent nonlinear and very well known crystal which is worldwide grown in large quantities. The processes involved in creating waveguides for integrated optics have been extensively investigated. In- tegrated waveguide lasers and amplifiers in LN have been demonstrated already. 4,5 Moreover, there is an actual interest in periodically poled lithium niobate PPLN for it has been demonstrated to serve as the basis of efficient, widely tun- able parametric oscillators, 6 which can be miniaturized and even diode pumped, as well as demonstration of very high efficiency in harmonic generation. 7 This last property is of great use for covering the recent demand in technology and science of compact and efficient laser systems emitting in the blue-green region by nonlinear frequency doubling of infra- red lasers. PPLN displays some advantages over conven- tional birefringent phase matched bulk LN for an extended use in efficient nonlinear processes. The effective nonlinear coefficient for second order nonlinear processes is greatly increased because beams propagating normal to the ferro- electric axis can benefit from the high value of the d 33 non- linear coefficient, which cannot be accessed by birefringent phase matching. This situation also eliminates Poynting vec- tor walk-off. In addition, it is known that LN has a low resistance to photorefractive damage for applications in the visible and near infrared. This resistance to photorefractive damage in LN is considerably increased by periodical poling, 8 thus avoiding the need of doping with oxides 9,10 leading to improved optical quality of crystals. Another ad- vantage is the possibility of QPM at short wavelengths, be- low 1 m, where birefringent phase matching is not possible. Several techniques reviewed in Ref. 11 have been ap- plied to obtain the PPLN structures, either directly during the growth process or after growth. The most common of after growth processes is the use of patterned electric field poling. This technique requires application of electric fields of order 20 kV/mm 11 to the samples, and leads at best to devices with a useful thickness about 0.3 mm. Such devices are too thin to allow for wide angle tuning of QPM parametric oscillators. However, the advantages of PPLN structures obtained during crystal growth are the possibility of obtaining thicker struc- tures, leading to greater useful surfaces, and to avoid the subsequent poling process. On the other hand, incorporation of optical ions in PPLN structures opens up the possibility of realizing optical de- vices which take additional advantage of the optical pro- cesses occurring in the ion. In this way Er 3+ is an optical ion in which a considerable number of useful optical processes take place. 12–14 Besides its most useful optical property of producing optical gain and laser oscillation for wavelengths around 1.5 m of a great interest in guided optical commu- nications, other optical processes have been demonstrated, like up-conversion, optical gain and up-conversion lasers at several bands in other crystals. This leads to an extense num- ber of potential applications in different fields, particularly in optical communications and eye-safe laser rangefinding. In addition, Er 3+ has absorption bands suitable for laser diode pumping. 15 Efficient diode-pumped all-solid-state lasers and amplifiers at 1.52 m have been demonstrated already 4,5 in Er 3+ doped bulk LN. Moreover, because the segregation co- efficient of Er in LN is greater than unity, it improves the formation of growth striations 16 which can be created with a desired period length by suitable choice of the growth con- ditions. The periodic structures have been obtained by the Czo- chralski crystal growth technique with the rotational axis dis- placed from the symmetry axis of the temperature field. This situation creates a periodic temperature fluctuation able to generate growth striations. 17 The crystals were grown along the a axis, which was displaced 5 mm away from the rota- tional symmetry axis of the temperature field. The starting material was a congruent lithium niobate melt doped with Er 0.5 mol % in the form of oxide, contained in a Pt crucible of diameter 5 cm. This dopant concentration is below the satu- APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 5 3 AUGUST 1998 593 0003-6951/98/73(5)/593/3/$15.00 © 1998 American Institute of Physics