TE-TM mode converter

Abstract: A TE-TM mode converter utilizes a Y-cut lithium niobate crystal (10) which includes a straight titanium-diffused channel (12) extending along the Z-axis of the crystal (10). A parallel pair of electrodes (14,16) lie symmetrically on opposite sides of the channel (12). A.c. and d.c. voltage supplies (22,20) are connected to the electrodes (14,16) to supply voltages so as to effect complete conversion from one direction of polarization to the other direction of polarization of a wave applied as an input to one end of the channel (12) for abstraction at the opposite end.

Description: Field of the Invention This invention relates to a method for operating an optical TE-TM mode converter. Background of the Invention An electro-optic polarization converter is important for a variety of applications. For example, in a single-mode optical fiber system, it may serve as a polarization-controlling device for typical single mode fiber transmission systems in which the propagating waves normally do not preserve their initial linear polarization so that periodically restoration to a linearly polarized state of the waves becomes important. In the January 1986 issue of Optics Letters, Vol. II, No. 1, pps. 39-41, there is described a TE-TM mode converter which employs an X-cut lithium niobate (LiNbO₂) crystal substrate in which there is provided a titanium in-diffused channel waveguide which supports one TE and one TM mode. The waveguide propagation direction is along the optic Z-axis of the crystal. The electro-optically induced linear polarization rotation is accomplished through the off-diagonal r₆₁ electro optic coefficient. The electrode structure for applying the electronic field for inducing the polarization conversion consists of three separate electrodes symmetrically located with respect to the waveguide and including a center electrode overlying the waveguide and a pair of outer electrodes, one on each side of the center electrode. One of the outer electrodes is grounded, and two independent voltages V₁ and V₂ are applied to the center electrode and other outer electrode, respectively. Voltage V₁ contributes-a mainly vertical electric field component and V₂ a mainly horizontal field component inside the waveguide. By varying V₁ and V₂ appropriately, conversion between the TE and TM modes of a single mode optical wave applied to one end of the waveguide is effected. In this arrangement, the vertical electric field provided by V₁ provides the electro-optically induced TE-TM mode coupling through the off diagonal r₆₁ electro optic coefficient, and the electric field provided by the voltage V₂ is used to provide the necessary electro-optically induced phase shifts between the TE and TM modes through the complementary r₂₂ and r₁₂ electro optic coefficients. Since the wavelength propagation direction is along the optic axis, both TE and TM modes see the same ordinary index and are therefore already nearly phase-matched. This device has several drawbacks. Firstly, optical isolation needs to be provided between the central electrode and the underlying waveguide. This typically involves use of a buffer layer between the LiNbO₃ substrate and the overlying electrodes which makes for processing complexity. Additionally, the performance is extremely sensitive to the electrode alignment. Accurately aligning the three electrodes, typically separated by a few microns, makes for manufacturing complexity. An electrooptic TE/TM-mode convertion device as specified in the preamble of claim 1 is disclosed in EP-A1-0200594 and the article 'A simple and wide optical Bandwidth TE/TM converter using Z propagating LiNbO₃ Waveguides', C. Marilles and M. Papuchon in 'Integral Optics', H.-P. Nolting and R. Ulrich ed., Springer Berlin (1985) pp 174-176. Last mentioned document recognices that the response is not the same for positive or negative values of the applied voltages. This is explained by the residual difference between the two propagation constants of the TE and TM modes in addition with a voltage induced birefringence due to a slight misalignment of the electrodes. The present invention has for its object to operate an electro-optic TE/TM-mode convertion device of the kind as specified in the above documents so that the residual birefringe problem is solved. The above object is achieved by the steps of the characterizing clause of claim 1. The invention therefore is based on the recognition that even the 'residual birefringence' can be effectively eliminated by applying the DC bias voltage to eliminate any differences between the propagation constants of the two orthogonal modes so that, when the AC voltage is applied, coupling will be complete, i.e., substantially 100 percent. In addition, the availability of the DC bias technique greatly relaxes the need for precision fabrication because now tolerances can be broadened and any differences between phase velocities due to variations in fabrication can be adjusted for by setting the correct DC bias level. Brief Description of the Drawings The invention will be better understood from the following detailed description taken with the accompanying drawings wherein: FIG. 1 is a perspective view of an TE/TM-mode converter used in connection with the method of the invention; FIG. 2 shows a cross-section of the converter of FIG. 1, connected to illustrative control circuitry; and FIG. 3 is a plot of the relative magnitudes of the TE and TM components measured at the output end of the embodiment of FIG. 2 as the drive voltage is varied. Detailed Description FIG. 1 shows a Y-cut LiNbO₃ crystal 10 in which in known fashion titanium has been thermally diffused to form the waveguiding channel 12 along the length of the crystal in the direction of the optic Z axis of the crystal. Typically, the titanium strip deposited was about 480 Angstroms thick and about 3 microns wide and its in-diffusion carried out in flowing oxygen at about 1100oC for about six hours. Ramp time to the diffusion temperature was about five hours as was the ramp time back to the ambient. There were then deposited symmetrically on either side of the channel 12, parallel electrodes 14 and 16, typically of gold about 20 millimeters in length and separated by a gap of about 7 microns. The widths of the electrodes advantageously are chosen to match the impedance of the electrodes to the drive voltage supply. In a typical embodiment, electrode 16 was between 5 and 10 microns wide and electrode 14, which served as the ground plane, was about 1 millimeter wide. The crystal length was about 24 millimeters. The input wave, illustratively of a wavelength of 0.632 or 0.820 micrometers, was applied at one end of the channel by way of an optical fiber (not shown) and the converted output wave abstracted at the other end similarly by way of an optical fiber (not shown). Significantly, neither of electrodes 14 or 16 overlaps the channel 12 which is uncovered and need not be buried in the interior of the crystal. FIG. 2 depicts a cross-section of the crystal 10 showing the channel 12 at the top surface and the electrode 14 and 16, disposed on opposite sides of the channel. Electrode 14 is connected to ground 18 by way of the variable d.c. power supply source 20 and electrode 16 by way of a.c. voltage source 22. The frequency of the source may vary widely depending on the application. For use as a d.c. polarization switch, the frequency typically would be quite low; for use in a high-frequency modulator, the frequency might be as high as gigahertzs. The specific values of d.c. bias and a.c. drive for a single essentially complete exchange between modes for a given length of electrodes is best determined empirically. For the structure described, using a twenty millimeter interaction length, it was found advantageous for 95% exchange at a wavelength of .632 microns to operate with an a.c. drive of about five volts peak-to-peak and a d.c. bias of between fifteen and twenty volts. With an input of .820 microns, a peak-to-peak drive voltage of about 12 volts was found suitable for a bias between fifteen to twenty volts. The relative magnitudes of the d.c. bias and a.c. drive are chosen to insure an operating point of high conversion efficiency. As will be discussed below in connection with FIG. 3, for high conversion efficiency, it is desirable to maintain continuously at least a several volts difference between the two electrodes. It is feasible to work with d.c. biases as low as five volts if the a.c. drive is adjusted appropriately. It usually is of little advantage to operate with biases greater than twenty volts. In FIG. 3, there is plotted the relative magnitudes of the TE component shown by the solid line and the TM component shown by the broken line, measured at the output end of the waveguiding channel of a fixed length as perturbing voltage is varied. As is indicated, for values of perturbing voltage below about four volts the conversion between modes is substantially incomplete while at higher voltages a high degree of conversion is obtained. A variety of applications are know for a mode converter of the kind described. For example, it can be used simply to convert an input wave of one linear polarization essentially completely to the opposite polarization by using an interaction length matching a complete conversion length. Alternatively by using an interaction length one half that for complete transfer, a linearly polarized input wave can be converted to a nearly circularly or elliptically polarized wave. Additionally, the interaction length can be chosen to be of sufficient length only to effect a desired degree of conversion, for example, just enough to provide a desired degree of compensation in a wave which was initially linearly polarized, but whose polariation was degraded in transmission, as often occurs to a wave during transmission along an optical fiber. Similarly, it is well known to follow a mode converter with a linear polarizer to form an efficient modulator. Alternatively, mode conversion can be used in a modulator to convert a guided mode of one polarization into a lossy mode of orthogonal polarization. It is to be understood that various modifications can be made in the specific illustrative embodiment. For example, it is feasible to form the waveguiding channel in other fashions, for example, by the electro-diffusion of copper. Additionally, other materials may be substituted for the lithium niobate of the crystal, for example, lithium tantalate or other suitable materials which permits a guided structure which essentially supports only one TE mode and one TM mode. The suitability of a particular material can be determined empirically.

Claims: A method for operating an electro-optic TE/TM-mode conversion device comprising a Y-cut crystal (10) which includes a waveguiding channel (12) extending parallel to the optic Z-axis of the crystal (10) along one surface of the crystal (10) and which supports only one TE mode and one TM mode of an applied optical wave of a prescribed wavelength, and a pair of parallel electrodes (14, 16) symmetrically disposed on opposite sides of said channel (12) wherein a voltage is applied to said electrodes (14,16) for effecting mode conversion of a wave propagating in the channel (12), and wherein said electrodes (14, 16) are adapted to supply voltage to said crystal (10) to create an electric field across said channel waveguide (12) to change the optical properties thereof by electrooptic effect, characterized in that a DC bias voltage is applied between one (14) of said electrodes and ground (18) and an AC voltage is applied between the other (16) of said electrodes and ground (18) so as to superimpose the AC voltage on the DC voltage and to establish an electric field comprising a DC bias component operative to match the phase velocities of said TE and TM modes and an AC component for converting between said TE and TM modes. The method of claim 1 wherein for an interaction length of twenty millimeters and an input with a wavelength of 0.632 microns an AC drive of about 5V peak-to-peak and a DC bias of between 15V and 20V is applied. The method of claim 1 wherein for an interaction length of twenty millimeters and an input with a wavelength of 0.820 microns an AC drive of about 12 V peak-to-peak and a DC bias of between 15V and 20V is applied.

Inventors: DYES WILLIAM A, SANFORD NORMAN A, DYES, WILLIAM A., SANFORD, NORMAN A.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1988
Main classification: G02F1
IPC codes: G02B6, G02F1