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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 17 — Aug. 20, 2007
  • pp: 10842–10854
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Rapid thermal annealing in high repetition rate ultrafast laser waveguide writing in lithium niobate

Amir H. Nejadmalayeri and Peter R. Herman  »View Author Affiliations


Optics Express, Vol. 15, Issue 17, pp. 10842-10854 (2007)
http://dx.doi.org/10.1364/OE.15.010842


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Abstract

For the first time to our knowledge, bulk modification of lithium niobate using high repetition rate ultrashort laser pulses has been studied. A fiber based ultrafast laser has been applied in a range of 0.1 to 1.5 MHz repetition rate to directly inscribe optical waveguides in z-cut lithium niobate. Circularly polarized light with stretched 600 fs pulses produced waveguides with nearly circular mode profiles that guided in the telecom band of 1300 nm. Higher laser repetition rate of 700 kHz was found to offer smooth waveguides with low propagation loss of 0.6 dB/cm, matching the best reported value so far, with the advantage of 50 fold faster writing speed. At repetition rates of 250 kHz and higher, the tracks exhibited a cladding-like modification zone that extended outside the main laser interaction volume, yielding smoother structures, despite higher net fluence delivery, providing concrete evidence of heat accumulation and thermal annealing effects. We also present the first observation of periodic micro-structures in the bulk laser interaction volume of a non-glass material.

© 2007 Optical Society of America

1. Introduction

Since the 1996 demonstration of ultrafast laser refractive index modification inside bulk glasses,1 considerable effort has been made to enhance the technique, not only in terms of the quality of the optical structures and the fabrication speed, but also advancing its applicability to a broader range of optical materials. Along this path, Nolte and coworkers2 were the first to demonstrate laser formation of optical waveguides in a crystal (quartz), while Gui et al.3 made further extension to lithium niobate (LN), one of the most widely used nonlinear optic media today. Gui et al. demonstrated single-mode guiding of visible light, exhibiting waveguide propagation losses of ≈1 dB/cm.

Later, several groups,4–7 including ours8 extended the LN work by optimizing various exposure conditions amongst a seven dimensional parameter space of pulse energy, scan speed, wavelength, focusing geometry, repetition rate, pulse width and polarization. Thomson et al.,4 Burghoff et al.5 and Lee et al.7 used linearly polarized laser pulses at 1–5 kHz repetition rate having tens to hundreds of femtosecond duration, energy of hundreds of nJ to tens of µJ, and scan speeds of 10–100 µm/s to generate waveguides with propagation losses of 9, 2.4 and 2.3 dB/cm, at respective wavelengths of 650, 1064 and 1550 nm.

Gui et al.3 reported that waveguides in z-cut samples were formed directly in the laser interaction zone. Thomson et al.4 found two types of waveguides in similar samples. The first type were formed in volumes immediately adjacent to the laser damage tracks, guiding both polarizations—perpendicular to the sample surface, hereafter referred to as transverse magnetic (TM), and parallel to the sample surface, referred to as transverse electric (TE). Their second type of waveguides were reported to guide in the laser interaction zone. Burghoff et al.5 also demonstrated TM modes in z-cut samples, located in the fabrication laser focal spot, while two TM modes were observed in x-cut samples on both sides of the laser interaction zone. Waveguides of Lee et al.7 were made in z-cut periodically poled lithium niobate (PPLN), supporting the TM polarization at regions around the laser interaction zone. The laser damage tracks are believed to cause compressive stresses in the surrounding crystalline media that lead to these polarization dependent refractive index changes.6

One promising approach that we investigate in this paper to address these two issues— fabrication speed and waveguide mode symmetry—is to harness repetition rate as a new optimization parameter for crystals in the aforementioned seven dimensional space.

2. Experimental arrangement

The material used in this work was z-cut congruent LN (Crystal Technology 99-60011-01). The laser beam was focused ≈110µm below the surface of LN samples, with an aspheric lens (New Focus 5722-C) having a numerical aperture of 0.55. The samples were placed on two-axis air-bearing motion stages (Aerotech ABL1000) having bidirectional repeatability of 50 nm, and straightness/flatness of better than 400 nm over a 10 cm travel range. Modification tracks were then made by scanning the motion stages orthogonal to the laser propagation (perpendicular to LN z-axis), with the expectation of forming birefringent waveguides. Multiple tracks were made at each exposure condition with laser repetition rates of 0.1, 0.25, 0.7, 1, and 1.5 MHz, and respective on target energy in ranges of 170–700, 170–700, 150–561, 150–390, and 120–264 nJ/pulse. The uppermost energy for the three latter repetition rates was limited by the maximum output power available from the laser. Scan speeds were varied between 1 and 80 mm/s.

After processing, the sample facets were ground and polished to optical quality to access the laser-formed waveguides. The waveguides were then probed in free space by a lens firing method, exploiting two external cavity tunable laser sources (Photonetics, Tunics) emitting wavelengths in the telecommunication bands of 1300 and 1550 nm. An IR detector (Newport, 818-IG) and a vidicon camera (Electrophysics, 7290A) were used respectively for measuring power and recording waveguide mode profiles. As previously described,8 a Fabry-Perot technique28 was used for assessing waveguide propagation losses.

Fig. 1. Cross section (first row) and top section (second row) optical microscope images of laser-formed tracks in z-cut lithium niobate at 46 mm/s scan speed and pulse energy of ≈270 nJ for different values of repetition rates. Laser exposure direction is from the bottom in the top row images.

3. Results and discussion

3.1. Morphology

Optical microscope top and cross-section views of several modification tracks formed at the aforementioned repetition rates are shown Fig. 1. The scan speed was fixed at 46 mm/s and the pulse energy was held at ≈270 nJ. Circular polarization and ≈600 fs pulse duration was applied. Dramatic changes are seen in the morphology and the structural modifications as repetition rate increases from 100 to 1,500 kHz. However, these changes are counterintuitive to the expectation of higher modification contrast and damage as the net fluence exposure rose 15-fold over this increase in repetition rate. The weakest contrast and smoothest modification structures are noted for the 1.5 MHz sample, which we attribute later to a heat accumulation and thermal annealing mechanism. The most significant damage is seen in the lowest exposure sample of 100 kHz, where nonuniform structural changes, together with microcracks, are apparent in both top and cross-section views. An angled crack is also prominent in the side view. The modification features get slightly smaller at 250 kHz, while at 700 kHz, the structure size, microcracks, and overall contrast are dramatically reduced. At 1 MHz the laser modified zone becomes almost cylindrically symmetric and consists of a high-contrast zone surrounded by a weak modification envelope (not visible in the figure) of ≈9.6 µm diameter. These features exceed the theoretical ≈1.2 µm (1/e2) focal diameter, and ≈4.8 µm depth of focus of the writing laser. This contrasts with an absence of any weak modification envelope at lower repetition rates, where only high contrast ≈4.3 µm structure—alongated in the laser propagation direction—are noted.

Higher magnification images of structures formed at 250 and 700 kHz are shown in Fig. 2. The tracks reveal a very strong laser modification zone of ≈ 1.4 µm that is consistent with the calculated—Gaussian optics and ABCD matrices—laser spot size of ≈ 1.2 µm. Higher laser exposure of 700 nJ energy at 60 mm/s speed and 500 nJ energy at 46 mm/s speed, respectively, were applied to increase the modification contrast. The angled cracks noted in the 100 and 250 kHz cross-section images of Fig. 1 are now strongly apparent in the 250 kHz cross-section image of Fig. 2. The extended dark lines are seen to form at an angle of ≈ 56°, which agrees well with cracking along the 55°52′ crystalline angle of the rhombohedron unit cell of lithium niobate.29 On the other hand, at the repetition rate of 700 kHz, such cracks appear greatly diminished and laser formation of a new periodic structure is reported for the first time in the bulk of a crystalline material. A micro-grating of ≈1.2 µm period is noted with alignment also matched to the 55°52′ crystal plane. The period of this structure closely matches the free space wavelength of the laser (1045 nm), and is also weakly observable with similar period in the 1,500 kHz track of Fig. 1. Because theses periodic structures follow the crystalline planes, are formed with circularly polarized light, and their period seem uncorrelated to the refractive index of the medium, they appear unrelated to the nanogratings formed in glass with a linearly polarized light,30, 31 and we are further investigating the underlying formation mechanisms.

Fig. 2. First column depicts the cross section and top views of a track formed at laser exposure conditions of 250 kHz, 700 nJ/pulse, and 60 mm/s. Cross section and top views of a track formed at 700 kHz, 500 nJ/pulse, and 46 mm/s are depicted in the second column. All images were obtained with a 100× microscope objective (0.95 NA).

3.2. Waveguiding

Fig. 3. The cross section microscope view of the 700 kHz track of Fig. 2, together with the 1300 nm TE-guided mode profile, showing its relative position with respect to the laser modified zone. The writing laser was applied from the bottom in this figure.

For orthogonal polarization (TM), guiding was observed in the regions between the three separate laser interaction zones (dark spots seen in Fig. 3), but propagation losses were too high for quantitative analysis. This TM guiding is attributed to the increase of extraordinary refractive index in the region sandwiched between two laser interaction zones, as previously reported by Burghoff et al.5 All samples were also probed by 1550 nm light, but guiding was too weak for quantitative analysis.

Fabry-Perot spectra as noted in Fig. 4 were applied extensively to assess the TE-mode propagation losses of waveguides written at 250 and 700 kHz and results are mapped in Fig. 6(a) and (b), respectively, across a widely explored energy-speed parameter subspace. At 250 kHz, two regions of exposure conditions (high energy, high speed) and (low energy, low speed) resulted in good guiding with moderate losses of several dB/cm. The lowest waveguide losses of 2.0 and 2.2 dB/cm were observed at laser exposure conditions of 700 nJ and 60 mm/s, as well as 320 nJ and 1 mm/s. Clearly, the former is the preferred process window, owing to the 60-fold faster fabrication speed.

Fig. 4. The measured transmission spectra of the lowest loss lithium niobate TE-mode waveguides, written at repetition rates of (a) 250 kHz and (b) 700 kHz (depicted in Fig. 2 and 3), showing Fabry-Perot fringes. Both waveguides were 9.2 mm long.
Fig. 5. TE polarization mode profiles recorded at 1300 nm wavelength in waveguides fabricated at 250 kHz (a) and 700 kHz (b), whose transmission spectra are depicted in Fig. 4(a) and 4(b), respectively.

Fig. 6. The 2-D maps for TE-mode waveguides indicating low-loss laser processing window for a repetition rates of (a) 250 kHz and (b) 700 kHz. Normalized pseudo color (scale on top) indicates low loss guiding with dark colors (black to dark red) and high loss or weak guiding with light colors (yellow to white).

3.3. Heat accumulation and annealing

To verify whether heat accumulation has occurred in our experiments, we examine the relevant temporal and spatial thermal conditions.32 The effective cooling time, τc=d2/D, diffusing heat out of a laser interaction volume of diameter d,18 yields values of τc=1.4-4.8 µs, for the present case of d=1.4 µm and the diffusivity values from Table 2. This suggests that a transition to heat accumulation effects is possible for laser repetition rates exceeding Rp=1/τc=200 to 700 kHz, which we have transitioned in the present experiments. In other words, the temporal condition required for the heat accumulation has been satisfied under our experimental conditions.

To verify the satisfaction of the spatial condition, we estimate the spatial extent of the heat accumulation zone. The 2-D thermal diffusion length associated with the time interval between two consecutive laser pulses is obtained from

LD=4DRp,
(1)

with predicted values listed in Table 2 for the 100 kHz to 1.5 MHz range of repetition rates, studied here. The table further provides values for the distance traveled by the sample between two consecutive laser pulses, calculated from

h=vsRp,
(2)

Table 1. Thermal parameters of lithium niobate33

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for the case of vs=46 mm/s scan speed, which matches the experimental conditions in Fig. 1. The total heat loading also depends on the effective number of laser pulses overlapping within the laser focal diameter, which we estimate from

N=dRpvs.
(3)

Values for N are also presented in Table 2 for vs=46 mm/s.

Examination of the data in Table 2 clearly shows the calculated diffusion length exceed the laser focal diameter (d=1.4 µm) for repetition rates of 250 kHz and lower, therefore suggesting that no significant heat accumulation is expected for this range of repetition rates. In contrast, at 700 kHz and beyond, the calculated diffusion lengths are comparable with the size of the laser interaction zone, and one therefore expects the onset of cumulative thermal effects to be between these two rates. Further driving this thermal scaling is a decreasing step size, h, and a commensurate increase in the number of effective overlapping pulses, N, that is expected to strongly heat the sample at increasing repetition rate, while reducing the overall degree of temperature cycling.

The evidence for such cumulative heat effects is given in the optical microscope images of Fig. 2, for the transitional zone of 250 to 700 kHz repetition rates. The top view of the 250 and 700 kHz tracks both clearly show a thin central region of 1.4 µm width, associated with the original laser focal spot size, and the formation of an extended laser affected zone of 4.3 µm width. The size of these two regions are equal, despite a factor of almost 3 increase in total fluence for the higher repetition rate case. However, an extended heat affected zone is also faintly visible, with diameter increasing from 6.1 to 9.0 µm for 250 and 700 kHz, respectively. Although the absolute size of these extended melt zones are different from previously reported measurements in glass,11, 15, 19 the observations are in qualitative agreement, with visible evidence of an extended heat affected zone that grows in size with increasing fluence and increasing repetition rate.

The results of Fig. 1 and Table 2 suggest that high repetition rates in the range of 250 to 700 kHz mark an important transition in lithium niobate, where laser pulses are interacting with media heated above the melt temperature, and undergoing little thermal cycling due to the rapid arrival of pulses faster than the effective cooling time calculated above. Such nearly continuous heating avoids the shock-induced damage apparent at lower repetition rates in Fig. 1, thereby providing final structures with much reduced damage through processes that possibly resemble a combination of epitaxial regrowth and rapid thermal annealing (RTA). At this stage, it is not known whether the poor quality of waveguides formed at higher repetition rates of 1 and 1.5MHz is due to weakened refractive index contrast resulted from overly extended heat zones, or the lack of enough energy in individual interacting laser pulses. More extensive thermal modeling, together with temporally and spatially resolved spectroscopy of laser interactions are underway to better elucidate this physics.

Table 2. Thermal effects in LN waveguides for 46 mm/s scan speed

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3.4. Trends and comparison

On the other hand, the gradual onset of heat accumulation effects at 250 kHz and beyond (Sec. 3.3) opens a new processing domain, where thermal annealing appears to reduce the formation of scattering centers and microcracks that otherwise contribute to linear loss in the waveguides. At 250 kHz repetition rate, the laser dissipation transitions from diffusion-only formation of heat affected zones at lower rates to the cumulative regime at higher rates, where continuous heat and reduced temperature cycling manifest in lower loss waveguides. Following this transition is a transformation of the low loss processing window from a diagonal band of constant net laser exposure for 250 kHz, as seen in Fig. 6(a) (low energy and low scan speed to high energy and high scan speed), to a flat onset threshold of 400 nJ energy per pulse at 700 kHz in Fig. 6(b) that, surprisingly, is largely independent of scan speed over a wide 1 to 60 mm/s range. That is the characteristic of material structuring in the high repetition rate regime, where past certain energy threshold, the fluence loses its significance to some extent, because of the nature of processing heated material. Although, in this regime, the combination of pulse energy and scan speed still plays a key role in the characteristics of the waveguides, especially its loss figures, the overall behavior of guiding is almost independent of them.

All waveguide samples in this study were examined more than two months after waveguide

Table 3. Comparison of laser-formed waveguides in lithium niobate

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4. Conclusions and outlook

In conclusion, we presented what is to our knowledge the first study of high repetition rate bulk ultrafast laser matter interaction in lithium niobate.

The heat accumulation effects associated with these higher repetition rates are definitely observed for the first time in a non-glass material, and appear responsible for annealing of laser modified tracks, so that morphologically smooth waveguides were attained.

We also presented the first time observation of periodic structures in bulk interaction of an ultrafast laser with a non-glass material.

Using high repetition rate (250 to 700 kHz) ultrafast lasers to form optical waveguides in bulk lithium niobate, we have achieved propagation losses of 0.6 dB/cm, that match the best kHz results in the telecom band, while reducing the fabrication time by almost a factor of 50, and offering improved transversal mode symmetry. Megahertz rate femtosecond lasers henceforth present a maskless and chemical free process, well suited for rapid fabrication of large size waveguide arrays, therefore, possibly enabling a new generation of previously unexplored quadratic nonlinear experiments, which has been hindered so far by the complexity of guided wave optics formation in lithium niobate.

Similar to our low repetition rate study, circular polarization and stretched pulses resulted in extending low loss waveguiding intoMHz repetition rates. Further studies are required to better understand the mutual effects between polarization and pulse width as well as their interplay with the fabrication laser wavelength, and their aggregate effects on the morphology and optical characteristics of the formed structures.

Acknowledgement

This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Photonic Innovation. Helpful discussions with Joachim Meier, Abbas Hosseini, Ladan Abolghasemi, Tariq Rafique, Haibin Zhang and Shane Eaton, and laboratory support by Sergey Reznik are gratefully acknowledged.

References and links

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R. R. Thomson, S. Campbell, I. J. Blewett, A. K. Kar, and D. T. Reid, “Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime,” Appl. Phys. Lett. 88, 111109 (2006). [CrossRef]

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J. Burghoff, C. Grebing, S. Nolte, and A. Tunnermann, “Efficient frequency doubling in femtosecond laser-written waveguides in lithium niobate,” Appl. Phys. Lett. 89, 081108 (2006). [CrossRef]

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J. Burghoff, H. Hartung, S. Nolte, and A. Tunnermann, “Structural properties of femtosecond laser-induced modifications in LiNbO3,” Appl. Phys. A 86, 165–170 (2007). [CrossRef]

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Y. L. Lee, N. E. Yu, C. Jung, B. A. Yu, I. B. Sohn, S. C. Choi, Y. C. Noh, D. K. Ko, W. S. Yang, H. M. Lee, W. K. Kim, and H. Y. Lee, “Second-harmonic generation in periodically poled lithium niobate waveguides fabricated by femtosecond laser pulses,” Appl. Phys. Lett. 89, 171103 (2006). [CrossRef]

8.

A. H. Nejadmalayeri and P. R. Herman, “Ultrafast laser waveguide writing: lithium niobate and the role of circular polarization and picosecond pulse width,” Opt. Lett. 31, 2987–2989 (2006). [CrossRef] [PubMed]

9.

R. Iwanow, R. Schiek, G. I. Stegeman, T. Pertsch, F. Lederer, Y. Min, and W. Sohler, “Observation of discrete quadratic solitons,” Phys. Rev. Lett. 93, 113902 (2004). [CrossRef] [PubMed]

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A.M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26, 42–43 (2001). [CrossRef]

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C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26, 93–95 (2001). [CrossRef]

12.

K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator,” Opt. Lett. 26, 1516–1518 (2001). [CrossRef]

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K. Minoshima, A. M. Kowalevicz, E. P. Ippen, and J. G. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10, 645–652 (2002). [PubMed]

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C. B. Schaffer, J. F. Garcia, and E. Mazur, “Bulk heating of transparent materials using a high-repetition-rate femtosecond laser,” Appl. Phys. A-Mater. Sci. Process. 76, 351–354 (2003). [CrossRef]

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M. Will, J. Burghoff, J. Limpert, T. Schreiber, S. Nolte, and A. Tunnermann, “High speed fabrication of optical waveguides inside glasses using a high rep-rate fiber CPA system,” Proc. SPIE 5339, 168–174 (2004). [CrossRef]

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C. B. Schaffer, A. O. Jamison, and E. Mazur, “Morphology of femtosecond laser-induced structural changes in bulk transparent materials,” Appl. Phys. Lett. 84, 1441–1443 (2004). [CrossRef]

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A. Zoubir, M. Richardson, C. Rivero, A. Schulte, C. Lopez, K. Richardson, N. Ho, and R. Vallee, “Direct femtosecond laser writing of waveguides in As2S3 thin films,” Opt. Lett. 29, 748–750 (2004). [CrossRef] [PubMed]

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27.

R. R. Gattass, L. R. Cerami, and E. Mazur, “Micromachining of bulk glass with bursts of femtosecond laser pulses at variable repetition rates,” Opt. Express 14, 5279–5284 (2006). [CrossRef] [PubMed]

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32.

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33.

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OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(140.3390) Lasers and laser optics : Laser materials processing
(160.3730) Materials : Lithium niobate
(220.4000) Optical design and fabrication : Microstructure fabrication
(230.7370) Optical devices : Waveguides
(320.7160) Ultrafast optics : Ultrafast technology

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 4, 2007
Revised Manuscript: July 10, 2007
Manuscript Accepted: August 4, 2007
Published: August 14, 2007

Citation
Amir H. Nejadmalayeri and Peter R. Herman, "Rapid thermal annealing in high repetition rate ultrafast laser waveguide writing in lithium niobate," Opt. Express 15, 10842-10854 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-17-10842


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References

  1. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, "Writing waveguides in glass with a femtosecond laser," Opt. Lett. 21, 1729-1731 (1996). [CrossRef] [PubMed]
  2. T. Gorelik, M. Will, S. Nolte, A. Tuennermann, and U. Glatzel, "Transmission electron microscopy studies of femtosecond laser induced modifications in quartz," Appl. Phys. A 76, 309-311 (2003). [CrossRef]
  3. L. Gui, B. X. Xu, and T. C. Chong, "Microstructure in lithium niobate by use of focused femtosecond laser pulses," IEEE Photon. Technol. Lett. 16, 1337-1339 (2004). [CrossRef]
  4. R. R. Thomson, S. Campbell, I. J. Blewett, A. K. Kar, and D. T. Reid, "Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime," Appl. Phys. Lett. 88, 111109 (2006). [CrossRef]
  5. J. Burghoff, C. Grebing, S. Nolte, and A. Tunnermann, "Efficient frequency doubling in femtosecond laser written waveguides in lithium niobate," Appl. Phys. Lett. 89, 081108 (2006). [CrossRef]
  6. J. Burghoff, H. Hartung, S. Nolte, and A. Tunnermann, "Structural properties of femtosecond laser-induced modifications in LiNbO3," Appl. Phys. A 86, 165-170 (2007). [CrossRef]
  7. Y. L. Lee, N. E. Yu, C. Jung, B. A. Yu, I. B. Sohn, S. C. Choi, Y. C. Noh, D. K. Ko, W. S. Yang, H. M. Lee, W. K. Kim, and H. Y. Lee, "Second-harmonic generation in periodically poled lithium niobate waveguides fabricated by femtosecond laser pulses," Appl. Phys. Lett. 89, 171103 (2006). [CrossRef]
  8. A. H. Nejadmalayeri and P. R. Herman, "Ultrafast laser waveguide writing: lithium niobate and the role of circular polarization and picosecond pulse width," Opt. Lett. 31, 2987-2989 (2006). [CrossRef] [PubMed]
  9. R. Iwanow, R. Schiek, G. I. Stegeman, T. Pertsch, F. Lederer, Y. Min, and W. Sohler, "Observation of discrete quadratic solitons," Phys. Rev. Lett. 93, 113902 (2004). [CrossRef] [PubMed]
  10. A. M. Streltsov and N. F. Borrelli, "Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses," Opt. Lett. 26, 42-43 (2001). [CrossRef]
  11. C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, "Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy," Opt. Lett. 26, 93-95 (2001). [CrossRef]
  12. K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, "Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator," Opt. Lett. 26, 1516-1518 (2001). [CrossRef]
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