Study of mirror coatings for eye-safe optical parametric oscillator

Transcription

1 Study of mirror coatings for eye-safe optical parametric oscillator P. K. Bandyopadhyay*, A. Ghosh and N.S. Vasan Instruments Research & Development Establishment, Dehradun, India. ABSTRACT Eye-safety is of paramount importance in the use of laser based instruments. Nonlinear crystal based optical parametric oscillators have been used in state-of-art military range finders for providing eye-safe output in the nm band. In this paper, design and fabrication of input and output mirror coatings for a KTP based eye-safe optical parametric oscillator have been developed on BK7 glass substrate. The input mirror was designed for high transmission at the pump wavelength 164 nm and for high reflection at the eye-safe laser wavelength 1571 nm. The output mirror coating was designed for high reflection at 164 nm and for partial reflection at 1571 nm. Both mirror coatings have a high transmission at the idler wavelength of 3297 nm. Comprehensive search method of design was used without a starting design. The optimized design was achieved with thirty one and thirty two layers of alternate stack of hafnium oxide and silicon di-oxide coating material combination respectively for the input and output mirror. This mirror coating has been fabricated by using electron beam gun evaporation system in Balzers BAK- vacuum coating unit. The result achieved for input mirror was 94% transmission at 164 nm and 99.9% reflection at 1571nm. For output mirror it was 99.9% and 5% reflection at 164 nm and 1571nm respectively. The coated samples passed the durability tests and transmission reproducibility characteristics in different coating cycles. Keywords: Eye-safe laser, Input mirror, Output mirror, Optimised design, Durability tests, Reproducibility characteristics. 1. INTRODUCTION Parametric down conversion in non-linear crystals provides a convenient means to generate tunable output from a Q-switched solid-state laser. In this technique the incident pump photon is subdivided into two photons of smaller energies termed as signal and idler in such manner that the sum total of their energy and momentum are conserved. By altering the phase-matching condition either by changing the pump incident angle or the crystal temperature, as the case may be, the wavelength of the generated photons can be tuned over a broad range. In a singly resonant optical parametric oscillator (OPO), a low pump threshold is obtained by placing the crystal between a pair of mutually aligned mirrors, which are coated to transmit the pump beam and to reflect the generated signal or idler beams between them. A portion of energy at the desired wavelength is extracted through the output mirror. The pump beam is folded back through the crystal for stable operation and to separate it from the OPO output. Eye-safe signal output at 157 nm or 15 nm can be readily obtained from a noncritically phase matched X- or Y-cut KTP OPO pumped by a high PRF 164 nm Nd:YAG laser[1,2]. The OPO can be placed either externally to the pump laser cavity or be made an integral part of it as with the Raman cell[3]. Application of eye-safe lasers has vastly increased over the years in the areas of scientific research, engineering, medicine, commerce, optical fiber communication and military technology[4]. In this paper the design and fabrication of OPO input and output mirror coatings on BK7 glass substrate were reported. Various specifications of input and output mirrors were found in literature[5,6] as well as technical catalogue. For input mirror the specified coating was high transmission of Nd-YAG laser wavelength used for pumping purpose (164nm) and non-resonated idler wavelength 3297 nm while high reflection at signal wavelength (1571 nm) used for lasing action. For output mirror the specified coating was high reflection at pumping wavelength and partial reflection (5%) at signal wavelength.

2 2. DESIGN CONSIDERATIONS Comprehensive search method of design was used without a starting design[7-8]. In this method the desired characteristics(transmission/reflection) were designed by optimising the thicknesses of high and low refractive index films. Selection of coating materials is based on their refractive index values, physical stability characteristics, their transparency in the desired wavelength regions and interface compatibility[9]. The commonly used high index coating material are Titanium di-oxide, tantalum oxide, hafnium oxide and zirconium oxide. The low index coating material silicon di-oxide does not show columnar growth and used to increases durability of multilayer coatings[1]. Hafnium oxide is chosen as high index and silicon di-oxide is chosen as low index coating material since this combination yields high laser damage threshold[11]. The number of layers increases the cumulative stress and sometimes limits the efficiency of the device. Therefore, it is better to use minimum number of layers in this design and the best possible optimized design was achieved with thirty one and thirty two alternate layers of hafnium oxide and silicon di-oxide combination for the input and output mirrors respectively. The theoretically design transmittance curve is given in figure-1 for input mirror and the corresponding index profile is given in figure-2 (design indices with their physical thickness). As shown in figure-1, the theoretical transmission achieved was 97% at 164 nm (pumping wavelength) and 95% at 3297 nm (idler wavelength) while reflection achieved was 99.9% at 1571 nm (signal wavelength). Transmittance (%) Wavelength (nm) Fig. 1: OPO Input mirror transmittance vs wavelength 2

3 BK7 GLASS MASSIVE Fig. 2: OPO Input mirror Refractive index vs Optical distance from medium(physical thickness) wavelength Similarly, for output mirror the theoretical design transmittance is given in figure-3 and the corresponding index profile is given in figure-4. In figure-3 the theoretical reflection achieved was 99.9% at 164nm for pumping wavelength and 5% at 1571 nm for signal wavelength while transmission achieved is 95% at 3297 nm for idler wavelength. Transmittance (%) Wavelength (nm) Fig. 3: OPO Output mirror transmittance vs wavelength 3

4 Refractive Index BK7 GLASS MASSIVE Optical Distance from Medium Fig.4: OPO Output mirror Refractive index vs Optical distance from medium(physical thickness) wavelength High local electric fields can also be a cause of defects and low laser damage threshold in the coating. Electric field enhancement of the order of a factor of 4 is possible with cracks and grooves in layers[12] as well as with three-dimensional nodular defects in multilayer mirror coatings[13]. The layer interface is more responsible for local thermal stress compared to the inner part of the layer material. So the electric field across the layers was also taken into account during the design process. It is found from literature and laboratory experiences that higher electric field inside the low refractive index layer can be sustained more easily than in the layer interface and high index layer[14-15]. 3. EXPERIMENTAL HIGHLIGHTS The multilayer stack designed for the mirror coating has been fabricated using electron beam evaporation system in Balzers BAK- vacuum coating unit. Prior to deposition the substrates were cleaned using ultrasonic cleaner followed by vapour degreasing process with alcohol solvant. Inside the vacuum chamber the optics were subjected to high-tension glow discharge cleaning for ten minutes before coating. Local high absorption generally occurs due to loss of stoichiometry inside the film which is minimised by depositing the film in presence of oxygen atmosphere. The evaporation took place at the working vacuum range 4x1-4 mbar to 2x1-4 mbar. The rate of evaporation of the coating material is a critical parameter responsible for thin film microstructure. The rate of evaporation in case of silicon di-oxide was 1nm/sec and for hafnium oxide.8nm/sec[16]. The job was rotated with respect to the central point of coating chamber for coating uniformity. The substrate was heated upto 3 C+1 C for three hours inside the vacuum chamber. The substrate temperature during deposition was maintained at 25 C within tolerance +5 C. After deposition process the optics was post heated to 25 C +1 C for two hours. 4. RESULTS & DISCUSSION The experimental results are shown in figures 5 and 6(measured in Perkin-Elmer spectrophotometer of model no. Lambda 9). The achieved result for input mirror coating was 94% transmission at 164nm and 99.9% reflection at 4

5 1571nm; for output mirror coating 99.9% and 5% reflection respectively both at 164nm and 1571nm. The theoretical and experimental reflectance curves are in close agreement with each other. There is some discrepancy in the experimentally achieved transmission values compared to theoretical values. They are because of the absorption and scattering losses due to columnar microstructure, voids and dispersion of the deposited coating materials. The coated sample can withstand 5MW/cm 2 of laser radiation having 3ns pulse width. The samples were tested for characteristic repeatability and durability as shown in table1 for regular commercial production. The coated samples passed the adhesion and abrasion tests as shown in table1. Six samples coated in different runs were found to have nearly identical transmission characteristic as tabulated in table1. 9 Transmission(%) Wavelength in nm Fig. 5: OPO Input mirror transmittance vs wavelength Transmission( Wavelength in nm Fig. 6: OPO Output mirror transmittance vs wavelength 5

6 Table 1 Sample test record for dichroic coating Run No Reflection(%) For Input Mirror For Output Mirror 164 nm nm nm nm Durability Test Specification Adhesion 1 pull by scotch tape ok ok ok Abrasion 5 rubs by cheese cloth ok ok ok ACKNOWLEDGEMENTS The authors are grateful to Mr. J.A.R. Krishna Moorty, Director, IRDE, Dehradun, India for his encouragement in publishing this work. The authors are indebted to Mr. G. K. Sharma, Joint Director, IRDE, Dehradun, India for his valuable suggestions in carrying out the above work. REFERENCES 1. L.R Marshall, Jett Kasinsky and R.L. Burnlam; Diode-pumped eye-safe laser source exceeding 1% efficiency, opt. Lett. 16 (16). 2. L.R. Marshall and A.Kas; Eye-safe output from noncritically phase matched parametric oscillators, J.Opt. Soc.Am. B 1, 173 (1993). 3. Yuri Yashuir and H. M. Van Driel; Passivels Q-switched 1.57 µm intracavity optical parametric oscillator; Applied optics, 38, 12, (1999). 4. M. J. Weber; Science and Technology of laser glass; J. Non-Cryst. Solids; 123, 8-222, (199). 5. N.Srinivasan, H. Kiriyama, M.Ohmi, M. Yamanaka, Y. Izawa and S. Nakal; Efficient low energy near-infrared KTiOPO 4 optical parametric converter; Optics Letters,11, (1995). 6. A.L.Shah, S.N. Dutta, A.R.Singh, K.N.Chopra and Suranjan Pal; Nd-YAG Laser pumped KTP crystal based intracavity optical parametric oscillator; ,proceedings of National Laser symposium(3). 7. J. A. Dobrowolski; Completely automatic synthesis of optical thin film systems; Applied Optics; 4, (1965). 8. A. L. Bloom; Refining and optimization in multilayers; Applied Optics;, (1981). 9. M. R. Kozlowski, I. M. Thomas, J. H. Canpbell and f. Rainer; High power optical coatings for a mega joule class ICF laser; SPIE Proc.; 1782, (1993). 1. M. R. Kozlowski; Damage resistance Laser coatings in Thin films for optical systems Edited by F. R. Flory; Marcel Dekker, Inc., USA(1995). 6

7 11. M. R. Kozlowski, R Clow and I. M. Thomas; Optical coatings for high power laser applications in handbook of Laser Science and Technology; Suppl. 2, Optical Materials, M. J. Weber (Ed.), CRC, Boca Raton, FL, (1995). 12. N. Bloembergen; Role of cracks, pores and absorbing inclusions on laser induced damage threshold at surface of transparent dielectrics; Applied Optics; 12, (1973). 13. J. Deford and M. R. Kozlowski; Modifying of Electric field enhancement at nodular defect in dielectric mirror coatings; SPIE proceedings;1848, (1993). 14. J. H. Apfel; Optical coating design with reduced electrical field intensity; Applied Optics; 16, (1977). 15. F. Demichelis, E. Mezzetti-Minetti, L. Tallone & E. Tresso; Optimisation of optical parameters and electric field distribution in multilayers; Applied Optics; 23, (1984). 16. Karl Heint Miiller; Dependence of thin film microstructure on deposition rate by means of a computer simulation; J. Appl. Physics 58(7), (1985). *PKB@irde.res.in; phone ; fax