Wavelength conversion by quasi phase matching and the manufacture and use of optical articles therefor

• US 5,157,754 A
• Filed: 07/18/1991
• Issued: 10/20/1992
• Est. Priority Date: 04/25/1990
• Status: Expired due to Term

• Current Assignee: E. I. du Pont de Nemours and Company (Corteva, Inc.), Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: E. I. du Pont de Nemours and Company (Corteva, Inc.)
• Inventors: Brown, Joseph B. III, van der Poel, Carolus J., Bierlein, John D.
• Primary Examiner(s): Lee, John D.
• Application Number:
  US07/732,028
• Time in Patent Office: 460 Days
• Field of Search: 359/326-332, 385/122, 385/129, 385/130, 385/131, 385/141, 385/142
• US Class Current: 385/122
• CPC Class Codes: G02F 1/3548 Quasi phase matching [QPM], e.g. using a periodic domain inverted structure
• CPC Class Codes: G02F 1/3553 having the formula MTiOYO4, where M=K, Rb, TI, NH4 or Cs and Y=P or As, e.g. KTP
• CPC Class Codes: G02F 1/37 for second-harmonic generation G02F1/3532 takes precedence
• CPC Class Codes: G02F 1/3775 with a periodic structure, e.g. domain inversion, for quasi-phase-matching [QPM] G02F1/383 takes precedence

Abstract

Articles and process for wavelength conversion are disclosed which use a series of aligned sections of optical materials for wavelength conversion selected from materials having the formula K_1-x Rb_x TiOMO₄ where x is from 0 to 1 and M is selected from P and As and materials of said formula wherein the cations of said formula have been partially replaced by at least one of Rb⁺, Tl⁺ and Cs⁺, and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺. The series of sections is characterized with regard to a change in nonlinear optical coefficient, section length, and section Δk (i.e., the difference between the sum of the propagation constants for the incident waves and the sum of the propagation constants for the waves generated). The sections are selected such that the sum for the series of the product of the length of each section with the Δk is equal to about 2πN (where N is an integer other than zero) and the nonlinear optical coefficient for at least one section is changed relative to the nonlinear optical coefficient for at least one adjacent section. Also disclosed is a process for preparing a channel waveguide for wavelength conversion systems wherein areas along a portion of a crystal substrate surface used for forming the desired channel are alternately masked and unmasked during cation replacement by immersion in a molten salt.

Claims

1. A process for wavelength conversion comprising the step of directing the incident optical waves for wavelength conversion through a single crystal containing a series of aligned sections of optical materials for wavelength conversion selected from (a) materials having the formula K_1-X Rb_X TiOMO₄ where x is from 0 to 1 and M is selected from P and As and (b) materials of said formula wherein the cations of said formula have been partially replaced by at least one of Rb⁺, Tl⁺ and Cs⁺, and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺, with the provisos that at least one section is of optical materials selected from (b) and that for optical materials selected from (b) wherein x is greater than 0.8, the cations of said formula are partially replaced by at least one of Tl⁺ and Cs⁺ and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺, said sections being selected such that the sum for the series of sections of the product of the length of each section in the direction of alignment and the Δ

k for that section is equal to about 2π

N where N is an integer other than zero, and such that the nonlinear optical coefficient of at least one section is changed relative to the nonlinear optical coefficient of at least one adjacent section;

wherein the Δ

k for each section is the difference between the sum of the propagation constants for the incident waves for the wavelength conversion system in that section and the sum of the propagation constants for the generated waves for the wavelength conversion system in that section.

2. A process for wavelength conversion in accordance with claim 1, wherein incident waves of frequencies ω

₁ and ω

₂ are used to generate waves of frequency ω

₃ ; and

wherein ω

₃ is equal to the sum of ω

₁ and ω

₂.

3. A process for wavelength conversion in accordance with claim 1, wherein incident waves of frequency ω

₄ are used to generate waves of frequencies ω

₅ and ω

₆ and wherein ω

₄ is equal to the sum of ω

₅ and ω

₆.

4. A process for wavelength conversion in accordance with claim 1, wherein second harmonic waves of the incident optical waves are generated.

5. A process for wavelength conversion in accordance with claim 1, wherein the length of each section is in the range of 0.25 μ

m to 50 μ

m;

wherein the width of each section is in the range of 0.2 μ

m to 50 μ

m; and

wherein the depth of each section wherein said cations have been partially replaced is in the range of 0.1 μ

m to 50 μ

m.

6. A process for wavelength conversion in accordance with claim 5, wherein said single crystal is a modified KTiOPO₄ single crystal and wherein said directing step comprises directing the incident optical waves for wavelength conversion through said modified KTiOPO₄ single crystal so as to produce blue light.

7. A process for wavelength conversion in accordance with claim 1, wherein said single crystal is a modified KTiOPO₄ single crystal and wherein said directing step comprises directing the incident optical waves for wavelength conversion through said modified KTiOPO₄ single crystal so as to produce blue light.

8. An optical article for use in a wavelength conversion system, said optical article comprising at least one optical conversion segment consisting of a single crystal containing a series of aligned sections of optical materials selected from (a) materials having the formula K_1-X Rb_X TiOMO₄ where x is from 0 to 1 and M is selected from P and As and (b) materials of said formula wherein the cations of said formula have been partially replaced by at least one of Rb⁺, Tl⁺ and Cs⁺, and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺, with the provisos that at least one section is of optical materials selected from (b) and that for optical materials selected from (b) wherein x is greater than 0.8, the cations of said formula are partially replaced by at least one of Tl⁺ and CS⁺ and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺, wherein said sections are selected so that the sum of the product of the length of each section in the direction of alignment and the Δ

k for that section is equal to about 2π

N where N is an integer other than zero, and so that the nonlinear optical coefficient of at least one section is changed relative to the nonlinear optical coefficient of at least one adjacent section;

wherein the Δ

k for each section is the difference between the sum of the propagation constants for the incident waves for the wavelength conversion system in that section and the sum of the propagation constants for the generated waves for the wavelength conversion system in that section.

9. An optical article in accordance with claim 8, having alternating sections of a first KTiOPO₄ material, and a second KTiOPO₄ material wherein cations in said second KTiOPO₄ material have been partially replaced by Ba⁺⁺, and at least one of Tl⁺ and Rb⁺.

10. An optical article in accordance with claim 9, which is a waveguide comprising alternating sections of a crystalline substrate having the formula K_1-X Rb_X TiOMO₄ wherein x is from zero to one and M is P or As, and sections of substrate material in which the cations of said substrate have been replaced by sufficient cations selected from the group consisting of Rb⁺, Cs⁺ and Tl⁺ and sufficient cations selected from the group consisting of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺ to change the nonlinear optical coefficient by at least 1% with respect to the nonlinear optical coefficient of said substrate.

11. An optical article in accordance with claim 8, wherein the length of each section is in the range of 0.25 μ

m to 50 μ

m, wherein the width of each section is in the range of about 0.2 μ

m to 50 μ

m, and wherein the depth of each section wherein said cations have been partially replaced is within the range of about 0.1 μ

m to 50 μ

m.

12. An optical article in accordance with claim 8, wherein there is at least about 1% difference between the nonlinear optical coefficient for at least one section and the nonlinear optical coefficient for at least one adjacent section;

and wherein there is at least about 0.00025 difference between the surface index of refraction of said at least one section and the surface index of refraction of said at least one adjacent section.

13. In an optical waveguide device comprising at least one channel waveguide, means to couple an incoming optical wave into said channel waveguide and means to couple an outgoing wave out of said channel waveguide, the improvement wherein the channel waveguide is an optical article as provided in claim 8.

14. A process for preparing a channel waveguide for a wavelength conversion system comprising the steps of:

(1) providing the z-cut surface of a z-cut substrate of single crystal material having the formula K_1-X Rb_X TiOMO₄ wherein x is from zero to one and M is P or As with an optically smooth surface;

(2) providing a molten salt containing cations selected from the group consisting of Rb⁺, Cs⁺ and Tl⁺ and cations selected from the group consisting of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺ with the proviso that when x is greater than 0.8, the molten salt contains cations selected from Tl⁺ and Cs⁺, in an amount effective to provide upon exposure to said optically smooth surface at a selected temperature for a selected time, sufficient cation replacement to change the index of refraction with respect to the index of refraction of said substrate and to provide a nonlinear optical coefficient which is changed with respect to the nonlinear optical coefficient of the substrate;

(3) applying a masking material on said substrate to provide a pattern of aligned areas along a portion of said optically smooth surface which are alternately masked with a material resistant to said molten salt and unmasked;

(4) immersing said masked substrate in said molten salt at said selected temperature for said selected time, thereby providing cation replacement in said unmasked areas; and

(5) removing the masking material from said substrate;

wherein the length of said masked and unmasked areas being selected such that after said cation replacement in the unmasked areas, a channel waveguide is provided at said portion of the optically smooth surface which comprises at least one optical conversion segment consisting of a series of aligned sections of nonlinear optical materials within a crystal; and

such that the sum for the series of sections of the product of the length of each section and the Δ

k for that section is equal to about 2π

N where N is an integer other than zero;

wherein the Δ

k for each section is the difference between the sum of the propagation constants for incident waves for the wavelength conversion system in that section and the sum of the propagation constants for generated waves for said wavelength conversion system in that section.

15. A method for producing a channel waveguide for a wavelength conversion system, the channel waveguide having at least one optically smooth surface which comprises at least one optical conversion segment consisting of a series of aligned sections of nonlinear optical materials within a crystal such that the sum for the series of sections of the product of the length of each section in the direction of optical propagation and the Δ

k for that section is equal to about 2π

N, where N is an integer other than zero, wherein the Δ

k for each section is the difference between the sum of the propagation constants for incident waves for the wavelength conversion system in that section and the sum of the propagation constants for the generated waves for said wavelength conversion system in that section, said method comprising the steps of;

providing the z-cut surface of a z-cut substrate of single crystal material, said material having the formula K_1-X Rb_X TiOMO₄ wherein x is from zero to one and M is P or As, with an optically smooth surface, said optically smooth surface having a masking material applied so as to provide a pattern of aligned areas along a portion of said optically smooth surface which are alternately masked with a material resistant to a molten salt and unmasked so as to provide at least one masked area and at least one unmasked area;

immersing said substrate in said molten salt at a selected temperature for a selected time so as to promote cation replacement in said at least one unmasked area;

wherein said molten salt contains cations selected from the group consisting of Rb⁺, Cs⁺ and Tl⁺ and cations selected from the group consisting of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺ with the proviso that when x is greater than 0.8, the molten salt contains cations selected from Tl⁺ and Cs⁺, in an amount effective to provide, upon exposure to said optically smooth surface at said selected temperature for said selected time, sufficient cation replacement to change the index of refraction of said at least one unmasked area with respect to the index of refraction of said substrate and to provide a nonlinear optical coefficient in said at least one unmasked area which is changed with respect to the nonlinear optical coefficient of the substrate.

16. A method for efficiently generating blue light comprising the step of applying light having wavelengths in the range of 775-950 nm to a periodically segmented non-linear optical KTP waveguide structure having a periodicity of 3-6 μ

, wherein the KTP waveguide is doped with at least one of Rb⁺ and Tl⁺ and at least one of Ba⁺⁺ Sr⁺⁺ and Ca⁺⁺.

17. The method as claimed in claim 16, wherein said light is TM polarized light.

18. Apparatus for multiplying the frequency of light comprising a non-linear optical KTP waveguide structure having a channel of alternating regions with differing linear and non-linear indexes of refraction with a periodicity of 3-6 μ

in combination with means for injecting light with a wavelength in the range of 775-950 nm into said channel, wherein the KTP waveguide is doped with at least one of Rb⁺ and Tl⁺ and at least one of Ba⁺⁺, Sr⁺⁺ and Ca⁺⁺.

19. The apparatus as claimed in claim 18, wherein said means for injecting light comprises means for polarizing said light so that its direction of polarization is perpendicular to the direction of propagation of said non-linear optical KTP waveguide structure.