Coherent light generators – Particular resonant cavity – Specified cavity component
Reissue Patent
1998-04-01
2002-07-30
Lee, John D. (Department: 2874)
Coherent light generators
Particular resonant cavity
Specified cavity component
Reissue Patent
active
RE037809
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to lasers in connection with optical devices for controlling optical beams using electric field control. In particular, the invention relates to lasers in connection with devices constructed with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes.
The invention is especially applicable to the fields of laser control, communications, flat panel displays, scanning devices and recording and reproduction devices.
Interactions with energy beams such as optical or acoustic beams can be controlled by means of applied electric fields in electro-optic (EO) or piezoelectric materials. An electrically controlled spatial pattern of beam interaction is desired in a whole class of switched or modulated devices. Patterned responses can be achieved in uniform substrates using the electro-optic or piezoelectric effect by patterning the electric field. However, Maxwell's equations for the electric field prevent sharp field variations from extending over a large range. Some materials can be poled, which means their electro-optical and/or piezoelectric response can be oriented in response to some outside influence. In these materials, is possible to create sharp spatial variations in EO coefficient over potentially large ranges. By combining slowly varying electric fields with sharply varying (poled) material, new types of patterned structures can be fabricated and used.
Polable EO materials have an additional degree of freedom which must be controlled, as compared to fixed EO crystals. Usually, the substrate must be poled into a uniformly aligned state before any macroscopic EO response can be observed. Uniformly poled substrates have been fabricated both from base materials where the molecules initially have no order, and from base materials where the molecules spontaneously align with each other locally, but only within randomly oriented microscopic domains. An example of the first type of material is the nonlinear polymer. Examples of the second type of material are sintered piezoelectric materials such as lead zirconate titanate (PZT), liquid crystals, and crystalline ferroelectric materials such as lithium niobate (LiNbO
3
). Nonlinear polymer poling is described in ♦ E. Van Tomme, P. P. Van Daele, R. G. Baets, P. E. Lagasse, “Integrated optic devices based on nonlinear optical polymers”, IEEE JQE 27 778, 1991. PZT poling is described for example in ♦ U.S. Pat. No. 4,410,823, October 1983, Miller et al, “Surface acoustic wave device employing reflectors”. (Liquid crystal poling is described in standard references, such as S. Chandrasekhar,
Liquid Crystals,
Second Edition (1992), Cambridge University Press, Cambridge.) Ferroelectric crystal poling is described in ♦ U.S. Pat. No. 5,036,220 July 1991, Byer et al., “Nonlinear optical radiation generator and method of controlling regions of ferroelectric polarization domains in solid state bodies”.
Examples of poled EO devices include:
♦ the beam diffractor in a polymer layer with interdigitated electrodes of S. Ura, R. Ohyama, T. Suhara, and H. Nishihara, “Electro-optic functional waveguide using new polymer p-NAn-PVA for integrated photonic devices,”
Jpn. J. Appl. Phys.,
31, 1378 (1992) [UOS92];
♦ the beam modulator in a polymer layer with planar electrodes of U.S. Pat. No. 5,157,541 October 1992, Schildkraut et al. “Optical article for reflection modulation”;
♦ the total internal reflection beam reflector in a lithium niobate waveguide with an electrode pair of H. Naitoh, K. Muto, T. Nakayama, “Mirror-type optical branch and switch”,
Appl. Opt.
17, 101-104 (1978);
♦ the 2×2 waveguide switch in lithium niobate with two electrodes of M. Papuchon, Am. Roy, “Electrically active optical bifurcation: BOA”,
Appl. Phys. Lett.
31, 266-267 (1977); and
♦ the wye junction beam router in a lithium niobate waveguide with three electrodes of H. Sasaki and I. Anderson, “Theoretical and experimental studies on active y-junctions in optical waveguides”,
IEEE Journ. Quant. Elect.
QE14, 883-892 (1978).
These devices use uniformly poled material with varied electrode and optical structures. Many of the advantages of patterned poled devices have not been recognized. For example, in the book by ♦ H. Nishihara, M. Haruna, T. Suhara, Optical Integrated Circuits, McGraw-Hill, New York (1989) [NHS89], many electro-optical devices activated by various electrode patterns are described, but all of these devices are fabricated on a uniformly poled substrate. The same is true of another review article, ♦ T. Suhara and H. Nishihara, “Integrated optics components and devices using periodic structures,”
IEEE J. Quantum Electron.,
QE-22, 845, (1986) [TH86], which describes the general characteristics of grating coupled devices without recognizing the advantages of a poled grating as opposed to an electrode grating.
In selected instances in the literature, certain advantages of patterned poled substrates have been pointed out.
♦ A surface acoustic wave reflector with an array of domain reversals in a piezoelectric ceramic (but no electrodes) is described in U.S. Pat. No. 4,410,823, Miller et al.;
♦ A beam steerer with triangular domain reversed regions in LiTaO
3
is described in Q. Chen, Y. Chiu, D. N. Lambeth, T. E. Schlesinger, D. D. Stancil, “Thin film electro-optic beam deflector using domain reversal in LiTaO
3
”, CTuN63, CLEO'93
Conference Proceedings,
pp 196 et. seq., Optical Society of America.
♦ A Mach-Zehnder modulator with domain reversals to compensate phase differences between microwave and optical beams is described in U.S. Pat. No. 5,278,924, January 1994, Schaffner, “Periodic domain reversal electro-optic modulator”.
♦ A Mach-Zehnder electric field sensor with one domain reversed region in an electro-optic substrate is described in U.S. Pat. No. 5,267,336, November 1993, Sriram et al., “Electro-optical sensor for detecting electric fields”.
Use of patterned poled structures offers efficiency advantages in beam control (including generation, modulation, redirection, focussing, filtration, conversion, analysis, detection, and isolation) with applications in laser control; communications; data storage; and display. What is needed in these areas are adjustable methods for beam control with high efficiency. Due to the sharp domain transitions, higher efficiency devices can generally be obtained using pattern poled substrates to create the high frequency variations; the electrodes are needed to excite the patterned poled substrate, not to create the high frequency variations.
The poling process in polymers is quite different from that of crystals, and results in poorly defined domain boundaries. In crystals, there are a discrete number of (usually two) poling directions which are stable, and poling a local region consists of flipping atoms between these alternative states. Poled regions are fully aligned, and sharp boundaries exist between oppositely aligned domains. In poled polymers, any molecule can be oriented in any direction regardless of the poling direction. The poling process produces only an average component of alignment within a random distribution of individual molecules. In polymers, the poling (and the related EO coefficients) therefore have a continuous variation in strength and orientation. The sharp domain boundaries obtained in crystals are absent. This has a profound influence on the efficiency of certain types of poled device in polymers. Since the poling strength and direction in polymers follows the strength and direction of the local applied electric field, it is not possible to obtain poling features with spatial dimensions any sharper than permitted by Maxwell's equations. In polymers, there is very little advantage to be obtained from spatially patterning the poled regions instead of the electrode
Bischel William K.
Brinkman Michael J.
Deacon David A. G.
Field Simon J.
Allen Kenneth R.
Gemfire Corporation
Lee John D.
Townsend and Townsend and Crew
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