Method for writing arbitrary index perturbations in a wave-guiding structure

• US 6,035,083 A
• Filed: 07/06/1998
• Issued: 03/07/2000
• Est. Priority Date: 10/02/1997
• Status: Expired due to Term

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Company (3M Company)
• Inventors: LaBrake, Dwayne L., Brennan, James F. III
• Primary Examiner(s): Sanghavi, Hemang
• Application Number:
  US09/110,495
• Time in Patent Office: 610 Days
• Field of Search: 385/10, 385/31, 385/33, 385/37, 385/123, 359/558, 359/566, 359/569, 359/573, 372/26, 372/28
• US Class Current: 385/37
• CPC Class Codes: G02B 2006/12107 Grating
• CPC Class Codes: G02B 2006/12109 Filter
• CPC Class Codes: G02B 2006/12138 Sensor
• CPC Class Codes: G02B 6/02095 Long period gratings, i.e. transmission gratings coupling light between core and cladding modes
• CPC Class Codes: G02B 6/02138 based on illuminating a phase mask
• CPC Class Codes: G02B 6/02147 Point by point fabrication, i.e. grating elements induced one step at a time along the fibre, e.g. by scanning a laser beam, arc discharge scanning G02B6/02133 and G02B6/02142 take precedence
• CPC Class Codes: G02B 6/02152 involving moving the fibre or a manufacturing element, stretching of the fibre G02B6/02147 takes precedence
• CPC Class Codes: G02B 6/124 Geodesic lenses or integrated gratings
• CPC Class Codes: G02B 6/29317 Light guides of the optical fibre type
• CPC Class Codes: G02B 6/29394 Compensating wavelength dispersion G02B6/29376 takes precedence; dispersion compensated optical transmission systems H04B10/2507
• CPC Class Codes: H01S 3/0057 Temporal shaping, e.g. pulse compression, frequency chirping soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108

Abstract

A method and an apparatus for writing arbitrary refractive index perturbations along an optical waveguide. The method includes the steps of providing a waveguide having a photosensitive region and providing a writing beam of actinic radiation having a nominal diameter D, the writing beam defining an optical axis. A waveguide is translated relative to the optical axis of the writing beam at a known velocity v(t). A modulator modulates the radiation intensity I(t) of the writing beam as a function of time t to deliver a fluence of radiation, Φ(x), directly to the waveguide, wherein ##EQU1## The refractive index change Δn.sub.(x) at a position x along the waveguide length is related to the fluence delivered to that position as, Δn.sub.(x) =C·Φ.sub.(x), wherein C is a scaling factor that accounts for the photosensitivity of the waveguide.

Claims

1. A method for writing arbitrary refractive index perturbations along an optical waveguide, the method comprising the steps of:

providing a waveguide having a photosensitive region;

providing a writing beam of actinic radiation having a nominal diameter D, the writing beam defining an optical axis;

translating the waveguide relative to the optical axis of the writing beam at a known velocity v(t);

modulating the radiation intensity I(t) of the writing beam as a function of time t;

delivering a fluence of radiation, Φ

(x), directly to the waveguide, wherein ##EQU17## wherein the refractive index change Δ

n.sub.(x) at a position x along the waveguide length is related to the fluence delivered to that position as, Δ

n.sub.(x) =C·

Φ

.sub.(x), wherein C is a scaling factor that accounts for the photosensitivity of the waveguide.

2. The method of claim 1, further comprising the step of modulating the intensity of the writing beam in a periodic manner as a function of time t at a frequency f(t) to deliver a fluence of radiation to the waveguide, wherein:

##EQU18## where I₀ is the maximum intensity and A_n, are weighting components for the n^th term in the Fourier series representation of the function Φ

.sub.(x).

3. The method of claim 1, further comprising the step of writing a periodic index perturbation of period Λ

into the waveguide, where ##EQU19##

4. The method of claim 2, further comprising the step of modulating the peak intensity I₀ of the periodically-modulated writing beam.

5. The method of claim 2, further comprising the step of modulating the writing beam intensity to control independently each term, A_n, of the Fourier series representation of the function defining the fluence delivered to the waveguide.

6. The method of claim 1, wherein the waveguide is an optical fiber having a photosensitive region.

7. The method of claim 1, wherein I(t) is an offset sinusoidal function expressible by a Fourier series as wherein the fluence delivered to the waveguide is expressible as ##EQU20##

8. The method of claim 1, wherein I(t) is an offset periodic square wave expressible by a Fourier series as

9. The method of claim 1, wherein the index perturbation creates a transmission loss spectrum through the waveguide that is matched to the inverted erbium gain spectrum.

10. An optical amplifier including a waveguide having an index perturbation manufactured in accordance with the method of claim 1.

11. A method for manufacturing a waveguide having a refractive index perturbation, wherein the change in the refractive index along a length x of the waveguide can be expressed as Δ

n(x), the method comprising the steps of;

providing a waveguide having a photosensitive region and a photosensitivity factor expressible as C;

providing a writing beam of actinic radiation with a nominal diameter D;

controlling the translation of the waveguide relative to the writing beam so that the velocity of the waveguide, v(t), for any given point in time t during waveguide irradiation is known;

controlling the radiation intensity I(t) of the writing beam so the waveguide location x receives a total radiation dose Φ

, wherein the fluence of radiation delivered to the waveguide along its length, Φ

(x), is related to the index perturbation as Δ

n.sub.(x) =C·

Φ

.sub.(x).

12. The method of claim 11, wherein ##EQU21##

13. The method of claim 11, further comprising the step of modulating the radiation intensity in a periodic manner as a function of time t at a frequency f to deliver a fluence to the waveguide, wherein:

where I₀ is the maximum intensity and A_n are weighting components for the n^th term in the Fourier series representation of the function Φ

.sub.(x).

14. The method of claim 11, further comprising the step of writing a periodic index perturbation of period Λ

into the waveguide, where ##EQU22##

15. The method of claim 13, further comprising the step of modulating the peak intensity I₀ of the periodically-modulated writing beam.

16. The method of claim 13, further comprising the step of modulating the writing beam intensity to control independently each term, A_n, of the Fourier series representation of the function defining the fluence delivered to the waveguide.

17. The method of claim 11, wherein the waveguide is an optical fiber having a photosensitive region.

18. The method of claim 11, wherein I(t) is an offset sinusoidal function expressible by a Fourier series as wherein the fluence delivered to the waveguide is expressible as ##EQU23##

19. The method of claim 11, wherein I(t) is an offset periodic square wave expressible by a Fourier series as

20. The method of claim 11, wherein the index perturbation creates a transmission loss spectrum through the waveguide that is matched to the inverted erbium gain spectrum.

21. An optical amplifier including a waveguide having an index perturbation manufactured in accordance with claim 11.

22. A long-period grating writing assembly for writing long-period gratings on a photosensitive waveguide, the assembly comprising:

a source of light producing a writing beam of actinic radition having a peak intensity of I₀ and a nominal diameter D;

a light modulator placed along the path of the writing beam of actinic radiation that controls the intensity I(t) of the writing beam, the modulator including electronic signal controls, the electronic signal controls having an amplitude modulation function, a frequency modulation function and a d.c. offset;

a fiber holding assembly and translation mechanism that supports the waveguide, aligns the waveguide to intersect the path of the writing beam and translates the waveguide at a relative velocity v(t) with respect to the writing beam.

23. The long-period writing assembly of claim 22, the fiber holding assembly including a dispensing spool, a velocity-controlled take-up spool, and a phase lock-loop circuit that controls the rotational velocity of the take-up spool, wherein the rotational velocity of the take-up spool is coupled to act in synchrony with the light modulator.

24. The assembly of claim 22, wherein the modulator modulates the radiation intensity of the writing beam in a periodic manner as a function of time t at a frequency f to deliver a fluence to the waveguide, wherein:

##EQU24##