- Email: [email protected]
Non-linear optical wavelength conversion in Ti:LiNbO3 waveguides
Thin Solid Films, 136 (1986) 29-36 ELECTRONICS AND OPTICS
29
N O N - L I N E A R O P T I C A L W A V E L E N G T H C O N V E R S I O N IN T i : L i N b O 3 WAVEGUIDES* GUNNAR ARVIDSSON AND FREDRIK LAURELL
Institute of Optical Research, S-10044 Stockholm (Sweden) (Received March 12, 1985; revised July 10, 1985; accepted September 2, 1985)
Phase matching temperatures for second harmonic generation were measured for fundamental wavelengths from 1.06 to 1.24 gm in a planar waveguide and compared with bulk crystal values. Furthermore, both second harmonic generation and sum frequency generation were achieved in a channel waveguide, the latter by mixing the N d - Y A G laser wavelength at 1.064 gm with Raman-shifted light from the same light source. A conversion efficiency for second harmonic generation in a channel waveguide of 4.2~ at a fundamental power of 120 mW was obtained. Some material-related aspects on waveguides for this type of application are also discussed.
1. INTRODUCTION Utilizing the optical non-linear properties of suitable materials it is possible to achieve wavelength conversion based on phenomena such as frequency doubling (second harmonic generation (SHG)), sum frequency generation, difference frequency generation or optical parametric oscillation. To be efficient these interactions require phase velocity matching between the pump and the generated optical fields. When this condition is fulfilled the conversion efficiency increases with the optical intensity and with the interaction length. If a laser beam is focused in a bulk crystal to achieve high intensity, the beam diverges after the focus point and the effective interaction length decreases. If, however, a waveguide is used to confine the light, a high optical intensity may be maintained over a longer interaction length. Waveguides are therefore promising for enhancing the efficiency of this type of interactions. It has been predicted 1 that even such comparatively weak pump sources as semiconductor lasers may be used in this context in the near future, thus making it possible to realize, for example, new miniaturized coherent (tunable) light sources.
* Paper presented a t the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13-17, 1984. 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
30
O. ARVIDSSON, F. LAURELL
2. EXPERIMENTS
Wavelength conversion, primarily SHG, was experimentally investigated in bulk L i N b O 3 and in titanium-diffused lithium niobate (Ti:LiNbO3) waveguides (planar waveguides as well as channel waveguides). Phase matching was achieved in the conventional manner in this case, i.e. by using the birefringence to compensate for the wavelength dispersion. The direction of propagation was at 90 ° to the optical axis of the crystal and the phase matching was fine tuned by varying the temperature of the crystal. This method utilizes the temperature dependence of the birefringence, i.e. the fact that the ordinary and the extraordinary indices of refraction have different temperature dependences. An N d - Y A G laser at a wavelength of 1.064 ~tm, Q switched and in most cases also mode locked, was used as a light source. In many measurements this light was passed through a single-mode fibre (60 m long), and the Raman effect was utilized to transform the 1.0641~m wavelength to other wavelengths 2. Apart from the 1.064 p.m line, the output spectrum from this fibre contained radiation maxima at approximately 1.12 and 1.18 tam (Fig. 1). Also, weaker parts of the spectrum around these maxima were used for some of the measurements.
Iz
_z
1.020
1.064
I.'120 1:180 I.'243 WAVELENGTH (pm)
Fig. 1. Measured output spectrum from the Raman fibre.
2.1. Second harmonic generation in a bulk crystal Figure 2 illustrates the results from S H G measurements in a bulk crystal, a y-cut x-propagating plate of LiNbO3. A beam of light with a diameter of slightly less than 1 m m was passed through the plate as indicated in the figure and at different height coordinates. The fundamental wavelength was 1.064 lam generated from an N d - Y A G laser. The curves represent the measured second harmonic intensity as a function of the crystal temperature at four different positions in the crystal. It is seen that the phase matching temperature varies slightly, as well as the shape of the curve. It should be noted that each curve was quite reproducibly obtained when the
NON-LINEAR OPTICAL WAVELENGTH CONVERSION IN
Ti: LiNbO~
31
~ -12.8 °C
il
E x Z
O p-
~
-12.0 °C
c/) O
4
_f " ~2'0.....
/._ ;1'0. . . .
TEMPERATURE
o
(°C)
Fig. 2. Measured second harmonic intensity as function of crystal temperature at four different positions in a bulk crystal (fundamental wavelength, 1.064 pm).
temperature was both increased and decreased as well as when the same position in the crystal was returned to. The variation may, to a certain degree, be due to a nonhomogenous temperature distribution in the crystal. Another reason may be optical inhomogenities due to compositional variations within the crystal (varying stoichiometry). This might occur for the crystal quality class (acoustic grade) used in this case.
2.2. Second harmonic generation in planar waveguides Wavelength conversion was studied in titanium-diffused waveguides of both planar and channel types. In both cases the light was propagated into the waveguide by end-fire coupling. With the waveguide samples mounted on a metal block which was cooled or heated, the second harmonic intensity was recorded as a function of the temperature. A monochromator was used to select the appropriate wavelength from the Raman spectrum. Figure 3 illustrates results for a planar waveguide which was fabricated in a y-cut crystal. The titanium thickness was 700 ~ and the diffusion was carried out in static air for 6 h at 1050 °C. The diagram gives the phase matching temperature for different possible mode combinations as a function of the wavelength. The full curves represent calculations based on Sellmeier's dispersion formulae and measured effective indices for the waveguide. The Sellmeier formulae for the ordinary and
32
G. ARVIDSSON, F. LAURELL
200
~
l
i
i
l
,
l
i
i
i
,
i
,
r
i
i
i
,
?
,6oI
j.j
./
/.#, y ~Z
~l
/ ,~
1.06 0.53
/
~
F'J /
y
2~ TM~'TEo
/ 1.12 0.56 WAVELENGTH
1.18 0.59 (iJm)
1.24 0.62
P,fund ~sec harm
Fig. 3. Phase matching temperature for frequency doubling as function of the wavelength for different possible mode combinations in a planar waveguide. The curves correspond to calculations based on measured effective indices and Sellmeier's dispersion formulae including the temperature dependence. The crosses correspond to measurements carried out using Raman-shifted light from an N d - Y A G laser at a wavelength of 1.064 p.m. The broken curve and the squares represent the bulk case, which is included for comparison.
extraordinary indices of refraction are both based on a combination of refs. 3 and 4. A small further adjustment was made to obtain agreement for the crystals that we used. The extraordinary index depends on the exact composition (or stoichiometry) of the crystal. (The slight variation in the literature values of phase matching temperatures for congruent undoped LiNbO3 probably depends mainly on this sensitivity to the composition, combined with slight deviations from the nominal congruent compositions.) The crosses in the diagram represent measured values. The broken curve and the squares correspond to the bulk case. It should be noted that phase matching at a certain wavelength may, in a waveguide, thus be achieved both below and above the phase matching temperature for the bulk case. As is seen in the figure, good agreement was obtained between theoretical and experimental results. The conversion efficiencies for the different possible mode combinations are very different. The reason for this is the varying overlap between the lateral distributions for the generating field and the guided second harmonic field in the different cases. The conversion efficiencies for the different interactions were measured relative to the strongest interaction in some cases. Table I gives the result for a planar waveguide fabricated in the same way as that previously discussed but using a titanium layer 400 • thick. The table also contains theoretical values TABLE I CONVERSION EEFICIENCIES FOR DIFFERENT INTERACTIONS IN A P L A N A R W A V E G U I D E N O R M A L I Z E D TO THE STRONGEST INTERACTION
Interacting modes Experimental results Theoretical results
TMo -~ TEo 0.47 0.35
TMo ~ TEa 1.0 1.0
TMo ~ TE2 0.04 0.08
NON-LINEAR OPTICAL WAVELENGTH CONVERSION IN
Ti: LiNbO3
33
obtained by a numerical evaluation of the so-called overlap integrals predicting the relative conversion efficiencies. The agreement between experimental and theoretical values is fairly good. The deviation is greatest for conversion to the highest order transverse electric mode TE z. The main reason for the deviations is probably that the index profiles used for the calculations are not accurate enough. The interaction with the highest order mode would be the most sensitive to such an error. It might further be expected that the highest conversion efficiency is obtained between the two fundamental modes. As demonstrated by this example (see also ref. 5), this is not always the case. This is basically a consequence of the different wavelengths and the different index profiles for the two interacting modes. S H G in z-cut y-propagating waveguides has also been investigated. Using temperature tuning we found it more difficult to achieve reproducible results for this crystal cut. The main reason is probably that, in accordance with the observations reported in ref. 6, the pyroelectric effect has in this case a more pronounced influence on the phase matching condition. This may make temperature tuning more difficult to use for waveguides in z-cut material. 2.3. Second harmonic and sum frequency generation in channel waveguides Wavelength conversion in channel waveguides was also investigated. Figure 4 illustrates some experimental results for S H G in a channel waveguide. The measured second harmonic intensity is shown as a function of the temperature. Two different mode combinations are observed. The left-hand peak corresponds to the fundamental mode at both wavelengths. For this measurement, a short single-mode fibre was mounted with epoxy resin against the end face of the channel waveguide to facilitate an input coupling which was stable during temperature tuning. The channel waveguide used was fabricated in a y-cut substrate from a titanium stripe 10 tam wide and 300/~ thick. The length of the waveguide was 27 mm.
rr
<_.
[ m, o
z_ - 50
-40 TEMPERATURE
-30
-20
-10
(°C)
Fig. 4. Measured second harmonic intensity for a channel waveguide as a function of temperature (1.064 p_m ~ 0.532 lam). The left-hand peak corresponds to the interaction between the two fundamental (transverse magnetic and electric) modes (TMoo to TEo0 ). The other strong peak corresponds to TMoo to TElo. This identification is based on comparison with theory.
34
a . ARVIDSSON, F. LAURELL
Sum frequency generation was also obtained in the same channel waveguide. This is illustrated in Fig. 5. The generating wavelengths are 1.064 gm and the Raman peak at approximately 1.12 tam. The intensity at the sum frequency, corresponding to 0.546 gm, was measured and is shown in the figure as a function of the temperature. These peaks are much broader than those in the previous figure. This is due to the large spectral width of the Raman peak. The same two mode combinations, however, are observed as in the SHG case. Furthermore, the phase matching temperatures for these two peaks are in good agreement with calculations based on the SHG case. i
i
i
m Z
i
/
1()
2C)
/
3'0
40
TEMPERATURE
5'0
60
7b
(°C)
Fig. 5. Measured intensity for sum frequencygeneration in the same channel waveguideas in Fig. 3. The Nd YAG laser line at 1.064p.m and a broader Raman-shifted line at 1.12~tmgenerate light at 0.546gm. The conversion efficiency for SHG in a channel waveguide was also measured. Another channel waveguide fabricated from a titanium stripe 15 lam wide, on the same sample, was used. The other parameters for the waveguide were the same. The 1.064 ~tm wavelength from the N d - Y A G laser was used and the laser was Q switched (400 ns pulses; repetition rate, 1250 Hz) but not mode locked. For a fundamental peak power of 120 mW a conversion efficiency of 4.2~o was measured. If this value is normalized to a standard situation as proposed by Ostrowsky v it may be estimated that this result agrees well with the best values reported from other laboratories (ref. 7 and references cited therein). 3. DISCUSSION To utilize fully the advantages with the guided wave approach for phasematched non-linear interactions, the phase matching condition between the interacting modes has to be fulfilled over the whole interaction length. This condition imposes a strict requirement of a constant index profile along the waveguide. Therefore, extremely constant lateral dimensions and homogeneous material are required for the waveguide. It is also important that the overlap of the transverse intensity distributions for the interacting modes be as large as possible.
N O N - L I N E A R O P T I C A L W A V E L E N G T H C O N V E R S I O N IN
Ti:LiNbO3
35
Therefore, on the basis of detailed knowledge and control of the waveguide fabrication process and waveguide properties, the waveguide has to be thoroughly designed. Further requirements on the waveguides are low loss and high resistance against optical damage. Comparison of experiments of this type carried out in different laboratories shows that by far the best results up to now have been achieved in Ti:LiNbOa waveguides 1'8'9. This is due to the relatively high non-linear coefficients of the material combined with the possibility of fabricating high quality waveguides in the crystal. The diffusion technique has inherent advantages in comparison with many other waveguide fabrication techniques for fulfilling the strict requirement of a constant index profile along the waveguide. A major difficulty with LiNbOa is the tendency to "optical damage" as a result of the photorefractive effect, which limits the power densities that can be used. Research on improved material properties, however, is being conducted at various laboratories, to a large degree motivated by the extensive use of LiNbOa for various integrated optics devices. Progress has also been reported1 o,~ ~ in the use of LiNbO3 in the bulk form for wavelength conversion in high power laser systems. Further improvements in waveguide design are also possible, e.g. ifa practical configuration could be found which permits use of the d 3 3 coefficient ~2. Such improvements might in the future lead to commercial use of non-linear phase-matched interactions in LiNbO3 waveguides. A crucial question is whether other material with more marked non-linear properties will be developed faster. One interesting class of such materials is organic crystals ~3 such as 3-methyl-4-nitropyridine-1-oxide, 2-methyl4-nitroaniline, m-nitroaniline and p-chlorophenylurea. So far it seems that much further research is needed before high quality waveguides can be realized in these materials. 4.
SUMMARIZING REMARKS
We obtained good agreement between theoretical and experimental results on the phase matching temperatures and other data for SHG and sum frequency generation in T i : L i N b O 3 waveguides using 1.064 p.m radiation from an N d - Y A G laser and Raman-shifted light from the same source. This knowledge is essential for further improvements in this type of interaction. So far we have obtained a conversion efficiency for SHG in a channel waveguide of 4.2~o at a fundamental power of 120 mW. ACKNOWLEDGMENTS
The authors express their thanks to K. Bergvall and A. Sj6berg for fabricating the waveguides, to B. Stoltz and U. Osterberg for many discussions as well as assistance concerning the N d - Y A G laser and to D. Schadt for performing some of the calculations. This work has been supported by the Swedish Board for Technical Development, Telefonaktiebolaget L. M. Ericsson and the Swedish Telecommunications Administration.
36
G. ARVIDSSON, F. LAURELL
REFERENCES
1 W. Sohler and H. Suche, Proc. Soc. Photo-Opt. Instrum. Eng., 408 (1983) 163. 2 R.H. Stolen, Non-linear properties of optical fibers, in S. E. Miller and A. G. Ctiynoweth (eds.), Optical Fiber Telecommunications, Academic Press, New York, 1979, p. 125, and references cited therein. 3 M.V. Hobden and J. Warner, Phys. Lett., 22 (1966) 243. 4 D.F. Nelson and R. M. Mikulyak, J. Appl. Phys., 45 (1974) 3688. 5 W. Sohler and H. Suche, Appl. Phys. Lett., 33 (1978) 518. 6 N. Uesugi, K. Daikoku and T. Kimura, Tech. Dig. Int. Conf. on Integrated Optics and Optical Fibre Communication, Tokyo, July l 8 22, 1977,1nstitute of Electronics and Communication Engineers of Japan, 1977, p. 209. 7 D.B. Ostrowsky, Parametric processes in LiNbO3, in H. P. Nolting and R. Ulrich (eds.), Integrated Optics, Proc. Eur. Conf. on Integrated Optics, 1985, Springer, Berlin, 1985. 8 N. Uesugi, Radio Sci., 17(1982) 197. 9 E.M. Zolotov, V. M. Pelekhatyi, A. M. Prokhorov and V. A. Chernykh, Soy. Phys. JETP, 49 (1979) 603. 10 D.A. Bryan, R. Gerson and H. E. Tomaschke, Tech. Dig. 13th Int. Quantum Electronics Conf., Anaheim, CA, June 1984, Optical Society of America, 1984, p. 28. 11 Huifa Wu, Huide Xu, Bing Xiao, Liangying Xu and Haoran Tan, Tech. Dig. Conf. on Lasers and Electro-optics, Anaheim, CA, June 1984, p. 142. 12 S. Neveu, J. P. Barety, M. De Micheli, P. Sibillot, D. B. Ostrowsky and M. Papuchon, Optical Society of America 7th Topical Meet. on Integrated and Guided Wave Optics, Kissimmee, FL, 1984, Optical Society of America, 1984, paper PD6. 13 M. De Micheli, J. Zyss and A. Azema, Proc. Soc. Photo-Opt. Instrum. Eng., 401 (1983) 216.
29
N O N - L I N E A R O P T I C A L W A V E L E N G T H C O N V E R S I O N IN T i : L i N b O 3 WAVEGUIDES* GUNNAR ARVIDSSON AND FREDRIK LAURELL
Institute of Optical Research, S-10044 Stockholm (Sweden) (Received March 12, 1985; revised July 10, 1985; accepted September 2, 1985)
Phase matching temperatures for second harmonic generation were measured for fundamental wavelengths from 1.06 to 1.24 gm in a planar waveguide and compared with bulk crystal values. Furthermore, both second harmonic generation and sum frequency generation were achieved in a channel waveguide, the latter by mixing the N d - Y A G laser wavelength at 1.064 gm with Raman-shifted light from the same light source. A conversion efficiency for second harmonic generation in a channel waveguide of 4.2~ at a fundamental power of 120 mW was obtained. Some material-related aspects on waveguides for this type of application are also discussed.
1. INTRODUCTION Utilizing the optical non-linear properties of suitable materials it is possible to achieve wavelength conversion based on phenomena such as frequency doubling (second harmonic generation (SHG)), sum frequency generation, difference frequency generation or optical parametric oscillation. To be efficient these interactions require phase velocity matching between the pump and the generated optical fields. When this condition is fulfilled the conversion efficiency increases with the optical intensity and with the interaction length. If a laser beam is focused in a bulk crystal to achieve high intensity, the beam diverges after the focus point and the effective interaction length decreases. If, however, a waveguide is used to confine the light, a high optical intensity may be maintained over a longer interaction length. Waveguides are therefore promising for enhancing the efficiency of this type of interactions. It has been predicted 1 that even such comparatively weak pump sources as semiconductor lasers may be used in this context in the near future, thus making it possible to realize, for example, new miniaturized coherent (tunable) light sources.
* Paper presented a t the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13-17, 1984. 0040-6090/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
30
O. ARVIDSSON, F. LAURELL
2. EXPERIMENTS
Wavelength conversion, primarily SHG, was experimentally investigated in bulk L i N b O 3 and in titanium-diffused lithium niobate (Ti:LiNbO3) waveguides (planar waveguides as well as channel waveguides). Phase matching was achieved in the conventional manner in this case, i.e. by using the birefringence to compensate for the wavelength dispersion. The direction of propagation was at 90 ° to the optical axis of the crystal and the phase matching was fine tuned by varying the temperature of the crystal. This method utilizes the temperature dependence of the birefringence, i.e. the fact that the ordinary and the extraordinary indices of refraction have different temperature dependences. An N d - Y A G laser at a wavelength of 1.064 ~tm, Q switched and in most cases also mode locked, was used as a light source. In many measurements this light was passed through a single-mode fibre (60 m long), and the Raman effect was utilized to transform the 1.0641~m wavelength to other wavelengths 2. Apart from the 1.064 p.m line, the output spectrum from this fibre contained radiation maxima at approximately 1.12 and 1.18 tam (Fig. 1). Also, weaker parts of the spectrum around these maxima were used for some of the measurements.
Iz
_z
1.020
1.064
I.'120 1:180 I.'243 WAVELENGTH (pm)
Fig. 1. Measured output spectrum from the Raman fibre.
2.1. Second harmonic generation in a bulk crystal Figure 2 illustrates the results from S H G measurements in a bulk crystal, a y-cut x-propagating plate of LiNbO3. A beam of light with a diameter of slightly less than 1 m m was passed through the plate as indicated in the figure and at different height coordinates. The fundamental wavelength was 1.064 lam generated from an N d - Y A G laser. The curves represent the measured second harmonic intensity as a function of the crystal temperature at four different positions in the crystal. It is seen that the phase matching temperature varies slightly, as well as the shape of the curve. It should be noted that each curve was quite reproducibly obtained when the
NON-LINEAR OPTICAL WAVELENGTH CONVERSION IN
Ti: LiNbO~
31
~ -12.8 °C
il
E x Z
O p-
~
-12.0 °C
c/) O
4
_f " ~2'0.....
/._ ;1'0. . . .
TEMPERATURE
o
(°C)
Fig. 2. Measured second harmonic intensity as function of crystal temperature at four different positions in a bulk crystal (fundamental wavelength, 1.064 pm).
temperature was both increased and decreased as well as when the same position in the crystal was returned to. The variation may, to a certain degree, be due to a nonhomogenous temperature distribution in the crystal. Another reason may be optical inhomogenities due to compositional variations within the crystal (varying stoichiometry). This might occur for the crystal quality class (acoustic grade) used in this case.
2.2. Second harmonic generation in planar waveguides Wavelength conversion was studied in titanium-diffused waveguides of both planar and channel types. In both cases the light was propagated into the waveguide by end-fire coupling. With the waveguide samples mounted on a metal block which was cooled or heated, the second harmonic intensity was recorded as a function of the temperature. A monochromator was used to select the appropriate wavelength from the Raman spectrum. Figure 3 illustrates results for a planar waveguide which was fabricated in a y-cut crystal. The titanium thickness was 700 ~ and the diffusion was carried out in static air for 6 h at 1050 °C. The diagram gives the phase matching temperature for different possible mode combinations as a function of the wavelength. The full curves represent calculations based on Sellmeier's dispersion formulae and measured effective indices for the waveguide. The Sellmeier formulae for the ordinary and
32
G. ARVIDSSON, F. LAURELL
200
~
l
i
i
l
,
l
i
i
i
,
i
,
r
i
i
i
,
?
,6oI
j.j
./
/.#, y ~Z
~l
/ ,~
1.06 0.53
/
~
F'J /
y
2~ TM~'TEo
/ 1.12 0.56 WAVELENGTH
1.18 0.59 (iJm)
1.24 0.62
P,fund ~sec harm
Fig. 3. Phase matching temperature for frequency doubling as function of the wavelength for different possible mode combinations in a planar waveguide. The curves correspond to calculations based on measured effective indices and Sellmeier's dispersion formulae including the temperature dependence. The crosses correspond to measurements carried out using Raman-shifted light from an N d - Y A G laser at a wavelength of 1.064 p.m. The broken curve and the squares represent the bulk case, which is included for comparison.
extraordinary indices of refraction are both based on a combination of refs. 3 and 4. A small further adjustment was made to obtain agreement for the crystals that we used. The extraordinary index depends on the exact composition (or stoichiometry) of the crystal. (The slight variation in the literature values of phase matching temperatures for congruent undoped LiNbO3 probably depends mainly on this sensitivity to the composition, combined with slight deviations from the nominal congruent compositions.) The crosses in the diagram represent measured values. The broken curve and the squares correspond to the bulk case. It should be noted that phase matching at a certain wavelength may, in a waveguide, thus be achieved both below and above the phase matching temperature for the bulk case. As is seen in the figure, good agreement was obtained between theoretical and experimental results. The conversion efficiencies for the different possible mode combinations are very different. The reason for this is the varying overlap between the lateral distributions for the generating field and the guided second harmonic field in the different cases. The conversion efficiencies for the different interactions were measured relative to the strongest interaction in some cases. Table I gives the result for a planar waveguide fabricated in the same way as that previously discussed but using a titanium layer 400 • thick. The table also contains theoretical values TABLE I CONVERSION EEFICIENCIES FOR DIFFERENT INTERACTIONS IN A P L A N A R W A V E G U I D E N O R M A L I Z E D TO THE STRONGEST INTERACTION
Interacting modes Experimental results Theoretical results
TMo -~ TEo 0.47 0.35
TMo ~ TEa 1.0 1.0
TMo ~ TE2 0.04 0.08
NON-LINEAR OPTICAL WAVELENGTH CONVERSION IN
Ti: LiNbO3
33
obtained by a numerical evaluation of the so-called overlap integrals predicting the relative conversion efficiencies. The agreement between experimental and theoretical values is fairly good. The deviation is greatest for conversion to the highest order transverse electric mode TE z. The main reason for the deviations is probably that the index profiles used for the calculations are not accurate enough. The interaction with the highest order mode would be the most sensitive to such an error. It might further be expected that the highest conversion efficiency is obtained between the two fundamental modes. As demonstrated by this example (see also ref. 5), this is not always the case. This is basically a consequence of the different wavelengths and the different index profiles for the two interacting modes. S H G in z-cut y-propagating waveguides has also been investigated. Using temperature tuning we found it more difficult to achieve reproducible results for this crystal cut. The main reason is probably that, in accordance with the observations reported in ref. 6, the pyroelectric effect has in this case a more pronounced influence on the phase matching condition. This may make temperature tuning more difficult to use for waveguides in z-cut material. 2.3. Second harmonic and sum frequency generation in channel waveguides Wavelength conversion in channel waveguides was also investigated. Figure 4 illustrates some experimental results for S H G in a channel waveguide. The measured second harmonic intensity is shown as a function of the temperature. Two different mode combinations are observed. The left-hand peak corresponds to the fundamental mode at both wavelengths. For this measurement, a short single-mode fibre was mounted with epoxy resin against the end face of the channel waveguide to facilitate an input coupling which was stable during temperature tuning. The channel waveguide used was fabricated in a y-cut substrate from a titanium stripe 10 tam wide and 300/~ thick. The length of the waveguide was 27 mm.
rr
<_.
[ m, o
z_ - 50
-40 TEMPERATURE
-30
-20
-10
(°C)
Fig. 4. Measured second harmonic intensity for a channel waveguide as a function of temperature (1.064 p_m ~ 0.532 lam). The left-hand peak corresponds to the interaction between the two fundamental (transverse magnetic and electric) modes (TMoo to TEo0 ). The other strong peak corresponds to TMoo to TElo. This identification is based on comparison with theory.
34
a . ARVIDSSON, F. LAURELL
Sum frequency generation was also obtained in the same channel waveguide. This is illustrated in Fig. 5. The generating wavelengths are 1.064 gm and the Raman peak at approximately 1.12 tam. The intensity at the sum frequency, corresponding to 0.546 gm, was measured and is shown in the figure as a function of the temperature. These peaks are much broader than those in the previous figure. This is due to the large spectral width of the Raman peak. The same two mode combinations, however, are observed as in the SHG case. Furthermore, the phase matching temperatures for these two peaks are in good agreement with calculations based on the SHG case. i
i
i
m Z
i
/
1()
2C)
/
3'0
40
TEMPERATURE
5'0
60
7b
(°C)
Fig. 5. Measured intensity for sum frequencygeneration in the same channel waveguideas in Fig. 3. The Nd YAG laser line at 1.064p.m and a broader Raman-shifted line at 1.12~tmgenerate light at 0.546gm. The conversion efficiency for SHG in a channel waveguide was also measured. Another channel waveguide fabricated from a titanium stripe 15 lam wide, on the same sample, was used. The other parameters for the waveguide were the same. The 1.064 ~tm wavelength from the N d - Y A G laser was used and the laser was Q switched (400 ns pulses; repetition rate, 1250 Hz) but not mode locked. For a fundamental peak power of 120 mW a conversion efficiency of 4.2~o was measured. If this value is normalized to a standard situation as proposed by Ostrowsky v it may be estimated that this result agrees well with the best values reported from other laboratories (ref. 7 and references cited therein). 3. DISCUSSION To utilize fully the advantages with the guided wave approach for phasematched non-linear interactions, the phase matching condition between the interacting modes has to be fulfilled over the whole interaction length. This condition imposes a strict requirement of a constant index profile along the waveguide. Therefore, extremely constant lateral dimensions and homogeneous material are required for the waveguide. It is also important that the overlap of the transverse intensity distributions for the interacting modes be as large as possible.
N O N - L I N E A R O P T I C A L W A V E L E N G T H C O N V E R S I O N IN
Ti:LiNbO3
35
Therefore, on the basis of detailed knowledge and control of the waveguide fabrication process and waveguide properties, the waveguide has to be thoroughly designed. Further requirements on the waveguides are low loss and high resistance against optical damage. Comparison of experiments of this type carried out in different laboratories shows that by far the best results up to now have been achieved in Ti:LiNbOa waveguides 1'8'9. This is due to the relatively high non-linear coefficients of the material combined with the possibility of fabricating high quality waveguides in the crystal. The diffusion technique has inherent advantages in comparison with many other waveguide fabrication techniques for fulfilling the strict requirement of a constant index profile along the waveguide. A major difficulty with LiNbOa is the tendency to "optical damage" as a result of the photorefractive effect, which limits the power densities that can be used. Research on improved material properties, however, is being conducted at various laboratories, to a large degree motivated by the extensive use of LiNbOa for various integrated optics devices. Progress has also been reported1 o,~ ~ in the use of LiNbO3 in the bulk form for wavelength conversion in high power laser systems. Further improvements in waveguide design are also possible, e.g. ifa practical configuration could be found which permits use of the d 3 3 coefficient ~2. Such improvements might in the future lead to commercial use of non-linear phase-matched interactions in LiNbO3 waveguides. A crucial question is whether other material with more marked non-linear properties will be developed faster. One interesting class of such materials is organic crystals ~3 such as 3-methyl-4-nitropyridine-1-oxide, 2-methyl4-nitroaniline, m-nitroaniline and p-chlorophenylurea. So far it seems that much further research is needed before high quality waveguides can be realized in these materials. 4.
SUMMARIZING REMARKS
We obtained good agreement between theoretical and experimental results on the phase matching temperatures and other data for SHG and sum frequency generation in T i : L i N b O 3 waveguides using 1.064 p.m radiation from an N d - Y A G laser and Raman-shifted light from the same source. This knowledge is essential for further improvements in this type of interaction. So far we have obtained a conversion efficiency for SHG in a channel waveguide of 4.2~o at a fundamental power of 120 mW. ACKNOWLEDGMENTS
The authors express their thanks to K. Bergvall and A. Sj6berg for fabricating the waveguides, to B. Stoltz and U. Osterberg for many discussions as well as assistance concerning the N d - Y A G laser and to D. Schadt for performing some of the calculations. This work has been supported by the Swedish Board for Technical Development, Telefonaktiebolaget L. M. Ericsson and the Swedish Telecommunications Administration.
36
G. ARVIDSSON, F. LAURELL
REFERENCES
1 W. Sohler and H. Suche, Proc. Soc. Photo-Opt. Instrum. Eng., 408 (1983) 163. 2 R.H. Stolen, Non-linear properties of optical fibers, in S. E. Miller and A. G. Ctiynoweth (eds.), Optical Fiber Telecommunications, Academic Press, New York, 1979, p. 125, and references cited therein. 3 M.V. Hobden and J. Warner, Phys. Lett., 22 (1966) 243. 4 D.F. Nelson and R. M. Mikulyak, J. Appl. Phys., 45 (1974) 3688. 5 W. Sohler and H. Suche, Appl. Phys. Lett., 33 (1978) 518. 6 N. Uesugi, K. Daikoku and T. Kimura, Tech. Dig. Int. Conf. on Integrated Optics and Optical Fibre Communication, Tokyo, July l 8 22, 1977,1nstitute of Electronics and Communication Engineers of Japan, 1977, p. 209. 7 D.B. Ostrowsky, Parametric processes in LiNbO3, in H. P. Nolting and R. Ulrich (eds.), Integrated Optics, Proc. Eur. Conf. on Integrated Optics, 1985, Springer, Berlin, 1985. 8 N. Uesugi, Radio Sci., 17(1982) 197. 9 E.M. Zolotov, V. M. Pelekhatyi, A. M. Prokhorov and V. A. Chernykh, Soy. Phys. JETP, 49 (1979) 603. 10 D.A. Bryan, R. Gerson and H. E. Tomaschke, Tech. Dig. 13th Int. Quantum Electronics Conf., Anaheim, CA, June 1984, Optical Society of America, 1984, p. 28. 11 Huifa Wu, Huide Xu, Bing Xiao, Liangying Xu and Haoran Tan, Tech. Dig. Conf. on Lasers and Electro-optics, Anaheim, CA, June 1984, p. 142. 12 S. Neveu, J. P. Barety, M. De Micheli, P. Sibillot, D. B. Ostrowsky and M. Papuchon, Optical Society of America 7th Topical Meet. on Integrated and Guided Wave Optics, Kissimmee, FL, 1984, Optical Society of America, 1984, paper PD6. 13 M. De Micheli, J. Zyss and A. Azema, Proc. Soc. Photo-Opt. Instrum. Eng., 401 (1983) 216.