Microstrip line

Details Microstrip line Microstrip line

2000-03-14

2003-02-04

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C385S129000, C359S328000, C359S332000

Reexamination Certificate

active

06516127

ABSTRACT:

The invention concerns a rib waveguide, a process for manufacturing it, its utilization as well as a light source containing this rib waveguide in accordance with the generic terms of the independent claims.
BACKGROUND
Frequency conversion of laser light by non-linear optical interactions has been accorded some attention since the sixties. Non-linear optical processes enable the generation of coherent laser light at optical frequencies (wavelengths), which cannot, or else only with difficulty, be generated by a direct laser process. In general, for such a frequency conversion process a laser is utilized as pumping source, the light beam of which propagates through a non-linear optical material. The non-linear optical interaction between the laser beam and the material leads to the effect, that a part of the pumped light is converted into light of a higher or lower frequency. Among the non-linear optical processes, the second harmonic generation (SHG), sum frequency generation (SFG), difference frequency generation (DFG) and optical parametric amplification (OPA) are of particular significance. These processes enable the generation of coherent laser radiation in the ultraviolet, visible, near—and intermediate infrared spectral range, i.e., between 0.1 &mgr;m and 10 &mgr;m wavelength. Lasers which emit light at these wavelengths find applications in spectroscopy, optical data storage, medicine, biology, etc.
A further important non-linear optical process is the electro-optical modulation of laser light. Hereby an electrical field is applied to the non-linear optical crystal and with this the intensity or propagation velocity of the laser light is influenced. This effect can be utilized for transferring electronic signals to the optical beam. This makes possible the transmission of information with the help of optical systems, which today is finding widespread use in communications technology. Apart from this, the electro-optical effect is exploited for various other applications, such as in optical switches or in Q-switches in lasers for the generation of very short laser pulses.
A great number of crystalline materials which are suitable for non-linear optical interactions have been investigated. Among these, especially the class of the ferro-electric oxides has found attention, e.g., potassium niobate (KNbO
3
), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), barium titanate (BaTiO3) and KTP (KTiOPO4). In general, crystals of these materials manifest great non-linear optical susceptibilities, a material characteristic, which is a necessary prerequisite for efficient frequency conversion and electro-optical modulation. In particular potassium niobate on the basis of its outstandingly good characteristics has proved to be an excellent material for non-linear optical applications.
A further class of materials which has interesting non-linear optical characteristics are crystals based on borate compounds, such as &bgr;-BaB2O4 (BBO), LiB3O5 (LBO), CsB3O5 (CBO) and CsLiB6O10 (CLBO). This group of non-linear optical crystals is distinguished by the fact, that its optical transparency in contrast to most of the ferro-electric oxides reaches far into the ultraviolet spectral range. On the basis of this characteristic, borate crystals are interesting for frequency conversions, in which ultraviolet laser radiation is generated.
The degree of conversion in the case of a non-linear process increases with the length of interaction of pumped beam and crystal and with the intensity of the pumped beam. In order to achieve a high degree of conversion, therefore frequently additional measures have to be undertaken, in order to increase the intensity of the pumped beam, this in particular, if the laser is operated in the continuous wave (cw) mode. Investigated as such measures were above all resonant processes in optical cavities and conversion processes in optical waveguides. Resonant conversion processes, in the case of which the non-linear optical crystal is placed in a cavity, provide the possibility of achieving very high degrees of conversion. However, they have the disadvantage, that they are very sensitive with respect to the mechanical adjustment of the optical components and to small fluctuations of the pumped beam wavelength. Therefore they normally require a complicated active feedback system for stabilization. In contrast, the utilization of waveguides for frequency conversion has the advantage of, solely on the basis of the very small cross sectional surface area of the waveguide and of the lateral guiding of the laser beam, assuring a high intensity over a long interaction length and of thus achieving a high degree of conversion. For this, already a single pass of the pumped beam through the waveguide is sufficient, which substantially reduces the demands of the mechanical and frequency-related stability in comparison with optical cavities.
Waveguides also offer advantages for electro-optical applications compared with volume crystals. By the restriction of the light to a very small surface over a long distance, the electrical voltage applied for the modulation can be kept very low, whereby the required electrical power is significantly reduced. Waveguides are in addition compatible with the fibreglass technology, which is utilized in today's communications systems.
Of particular significance for applications are rib waveguides, i.e., channel-shaped waveguides, which guide the light in two directions and limit it to a very small surface.
The above explanations emphasize the significance of waveguides, in particular of rib waveguides, for non-linear optical applications. The manufacturing processes of such waveguides, however, are frequently technologically difficult and have to be adapted to the corresponding material characteristics. This invention is based on a process, which permits the manufacture of waveguides of good optical quality in non-linear optical crystals, while maintaining the advantageous characteristics of these materials.
For the manufacture of optical waveguides in non-linear optical crystals, different methods have been investigated, both chemical—as well as physical ones. By means of ion diffusion—or ion exchange processes in the non-linear optical crystal, one, for example, succeeded in manufacturing waveguides in LiNbO3, LiTaO3 and KTP. Proving to be successful, e.g., was the diffusion or implantation of titanium (Ti) in LiNbO3. Within the Ti-doped zone, the refraction index is increased, while simultaneously the desired optical characteristics of the LiNbO3 are maintained. Almost all other non-linear optical crystals, however, are not accessible for this process, because the outside ion diffusion constants and the thermal stability are insufficient. Also the forced doping with the help of ion implantation does not achieve the objective, because the ion implantation of heavy ions such as Ti damages the host lattice structure through atom impacts and creates defects, so that no usable waveguides are produced. Among the physical methods, above all the implantation of light ions such as H+ or He+ have found applications. With these processes, the crystalline material is subjected to a bombardment of high-energy ions. This leads to the formation of a buried optical barrier, i.e., to a zone with a lowered refraction index, and to a wave-guiding layer underneath the surface of the crystal.
Fluck and others, in the publication “Low-loss optical channel waveguides in KNbO3 by multiple energy ion implantation” (J. Appl. Phys. 72 (5), 1671 (1992)) have demonstrated, that the manufacture of rib waveguides by ion implantation, e.g., into ferro-electric oxides such as KNbO3, is possible with a process, which uses several implantation steps. The rib waveguides manufactured in this manner, however, have the disadvantage, that they only conduct light of one polarization direction, while light with a polarization vertical to that does not propagate in the waveguide. In order, however, to make a frequency conversion in KNbO3 possible, the waveguide must ne

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