Method and system for measuring optical scattering characteristics

• US 20040027560A1
• Filed: 08/07/2003
• Published: 02/12/2004
• Est. Priority Date: 04/23/2001
• Status: Active Grant

• Current Assignee: Sensortran Incorporated (Halliburton Company)
• Original Assignee: Systems & Processes Engineering Corporation
• Inventors: Chin, Robert, Hallidy, William, Fredin, Leif
• Granted Patent: US 7,057,714 B2
• Time in Patent Office: Days
• US Class Current: 356/73.1
• CPC Class Codes: G01K 11/32 using changes in transmittance, scattering or luminescence in optical fibres
• CPC Class Codes: G01M 11/3109 Reflectometers detecting the back-scattered light in the time-domain, e.g. OTDR

Abstract

A method and system for measuring optical scattering characteristics includes coupling a continuous wave laser excitation signal to an optical fiber. Radiation backscattered by the optical fiber in response to the coupled excitation signal is detected to produce a backscattered radiation signal. The backscattered radiation signal is mixed with the excitation signal to produce a mixed signal. The mixed signal is filtered to reduce the magnitude of frequencies other than conjugate mixing frequencies relative to the conjugate mixing frequencies. The filtered signal is digitized and the magnitude of backscattered radiation from a specific portion of the fiber is calculated based on the digitized signal. The temperature of a specific portion of the fiber can be determined from the magnitude of the backscattered radiation.

Claims

1. A system for measuring optical characteristics comprising:

a laser producing an excitation signal;

a single-mode optical fiber coupled to the laser so that a coupled excitation signal is introduced into the optical fiber, wherein the coupled excitation signal is a continuous wave signal modulated at variable frequencies;

a first detector positioned to receive radiation backscattered by the optical fiber in response to the coupled excitation signal.

2. The system of claim 1 further comprising an external modulator coupled between the laser and the optical fiber and operating to modulate the intensity of the excitation signal at a frequency that varies linearly from a first frequency to a second frequency over a first duration.

3. The system of claim 1, wherein the external modulator modulates the intensity of the excitation signal over a second duration immediately following the first duration at a frequency that varies linearly from the first frequency to the second frequency.

4. The system of claim 1 further comprising a laser driver coupled to the laser and operating to modulate the intensity of the excitation signal at a frequency that varies linearly from a first frequency to a second frequency over a first duration.

5. The system of claim 4, wherein the laser driver modulates the intensity of the excitation signal over a second duration immediately following the first duration at a frequency that varies linearly from the first frequency to the second frequency.

6. The system of claim 1, wherein the coupled excitation signal is modulated at a first discrete frequency for a first duration and modulated at a second discrete frequency, different from the first discrete frequency, for a second duration by an external modulator coupled between the laser and the optical fiber.

7. The system of claim 1, wherein the coupled excitation signal is modulated at a first discrete frequency for a first duration and modulated at a second discrete frequency, different from the first discrete frequency, for a second duration by a laser driver coupled to the laser.

8. The system of claim 1, further comprising a second detector positioned to receive radiation backscattered by the optical fiber in response to the coupled excitation signal and sensitive to a different spectrum of backscattered radiation frequencies than the first detector.

9. The system of claim 1, further comprising an analog to digital converter that converts the output of the first detector from analog to digital format