Method and apparatus for measuring polarization sensitivity of optical devices

• US 5,298,972 A
• Filed: 09/06/1991
• Issued: 03/29/1994
• Est. Priority Date: 01/22/1990
• Status: Expired due to Term

• Current Assignee: Agilent Technologies, Inc.
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Heffner, Brian L.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US07/755,931
• Time in Patent Office: 935 Days
• Field of Search: 356/364, 356/365, 356/366, 356/367, 250/225
• US Class Current: 356/364
• CPC Class Codes: G01J 4/04 Polarimeters using electric detection means G01J4/02 takes precedence
• CPC Class Codes: G01M 11/00 Testing of optical apparatus; Testing structures by optical methods not otherwise provided for

Abstract

An instrument includes a polarized optical source for producing three sequential predetermined states of polarization of a light beam and an optical polarization meter for measuring the polarization of a portion of the light beam transmitted by or reflected from an optical network by splitting it into four beams, passing three of the beams through optical elements, measuring the transmitted intensity of all four beams, and calculating Stokes parameters. The light beam enters the optical polarization meter through a single-mode optical fiber that acts as a spatial filter for controlling the position and alignment of the beam with respect to the optical elements. The distortion of the light beam polarization caused by this optical fiber is corrected by introducing two different linearly polarized light beams and measuring Stokes parameters which are used to construct a calibration matrix that is inverted and multiplied times measured Stokes parameters of subsequent measurements to yield true Stokes parameters. The three sequential predetermined states of polarization yield three corresponding Jones input vectors, and the Stokes parameters for the responses of the optical network are converted to three Jones output vectors. A Jones matrix for the optical network to within a complex constant is then computed from the Jones input and output vectors. Relative polarization sensitivity can be determined from this matrix for the optical network. The relative distortion caused by the optical network can be corrected by multiplying by the inverse of the matrix during later measurements through the optical network. Additionally, power measurements on the optical network and a substituted optical through enable absolute determinations and corrections.

Claims

1. A method for calibrating an optical polarization meter to compensate for any polarization distortion caused by optical elements, which polarization distortion leads to transmission loss that is independent of polarization, the method comprising the steps of:

transmitting a first incident polarized light beam from a polarized optical source through the optical elements to the meter;

measuring a set of Stokes parameters of the first incident beam;

computing a first normalized Stokes vector according to the Stokes parameters of the first incident beam;

transmitting a second incident polarized light beam from the polarized optical source through the optical elements to the meter, the second incident beam having a polarization different from that of the first incident beam;

measuring a set of Stokes parameters of the second incident beam;

computing a second normalized Stokes vector according to the Stokes parameters of the second incident beam;

computing a set of calibration factors according to the normalized Stokes vectors; and

correcting polarization parameters of an unknown light beam as measured by the meter according to the calibration factors to provide a calibrated polarization measurement.

2. A method according to claim 1 wherein the optical elements comprise an optical spatial filter of the optical polarization meter.

3. A method according to claim 1 wherein computing the calibration factors comprises:

computing a first calibration vector by scaling each element of the first normalized Stokes vector by the magnitude of the first normalized Stokes vector;

computing a second calibration vector by scaling each element of the second normalized Stokes vector by the magnitude of the second normalized Stokes vector; and

computing a third calibration vector by taking the vector cross product of the second calibration vector and the first calibration vector and then scaling each element of the third calibration vector by the magnitude of the third calibration vector.

4. A method according to claim 3 wherein computing the calibration factors comprises:

forming a first matrix having three columns each comprising elements that are equal to elements of one of the calibration vectors;

forming a second matrix which includes the first matrix; and

taking the inverse of the second matrix to obtain the calibration factors.

5. A method according to claim 1 wherein the first and second incident polarized light beams are linearly polarized and the relative angle between the direction of polarization of the first incident polarized light beam and the second incident polarized light beam is 45 degrees.

6. An instrument for measuring polarization sensitivity of an optical network, the instrument comprising:

polarized optical source means that sequentially generates three predetermined states of polarization of a light beam and impinges the light beam having each of the three predetermined polarization states onto the optical network;

optical polarization measurement means that receives a portion of each of the three predetermined polarization states of the light beam that is one of a) transmitted by and b) reflected from the optical network and measures the polarization states produced by the effect of the optical network on each of the three predetermined polarization states of the beam; and

means for computing a ratio of maximum and minimum values from the measured polarization states produced by the effect of the optical network on each of the three predetermined polarization states of the beam to thereby provide a measurement of the polarization sensitivity of the optical network.

7. The instrument as in claim 6 wherein the polarized optical source means comprises an optical source for generating a beam light, the optical source having an output, and a polarization synthesizer having an input connected to the output of the optical source, the polarization synthesizer comprising optical elements for producing the three predetermined states of polarization of the light beam in response to the beam of light received from the optical source.

8. The instrument as in claim 7 wherein the optical elements of the polarization synthesizer comprise a zero-degree linear polarizer, a 60-degree linear polarizer, and a 120-degree linear polarizer sequentially inserted into a path of the beam of light received from the optical source.

9. An instrument for measuring polarization sensitivity of an optical network, the instrument comprising:

polarized optical source means that sequentially generates three predetermined states of polarization of a light beam and impinges the light beam having each of the three predetermined polarization states onto the optical network;

optical polarization measurement means that receives a portion of each of the three predetermined polarization states of the light beam that is one of a) transmitted by and b) reflected from the optical network and measures the polarization states produced by the effect of the optical network on each of the three predetermined polarization states of the beam, the optical polarization measurement means comprising;

an optical spatial filter which receives and filters each of the three predetermined polarization states of the portion of the light beam from the optical network;

means for splitting the filtered portion of the light beam into four partial beams;

a first optical element located in the path of a first one of the partial beams and imparting a first polarization thereto;

a second optical element located in the path of a second one of the partial beams and imparting a second polarization thereto;

a third optical element located in the path of a third one of the partial beams and imparting a third polarization thereto;

four photodetectors, each photodetector receiving a different one of the four partial beams and providing a signal indicative of the intensity of that partial beam; and

means for computing the polarization of the incident beam of light from the signals provided by the photodetectors; and

means for computing a ratio of maximum and minimum values from the measured polarization states produced by the effect of the optical network on each of the three predetermined polarization states of the beam to thereby provide a measurement of the polarization sensitivity of the optical network.

10. The instrument as in claim 9 wherein the polarized optical source means comprises an optical source for generating a beam of light, the optical source having an output, and a polarization synthesizer having an input connected to the output of the optical source, the polarization synthesizer comprising optical elements for producing the three predetermined states of polarization of the light beam in response to the beam of light received from the optical source.

11. The instrument as in claim 10 wherein the optical elements of the polarization synthesizer comprise a zero-degree linear polarizer, a 60-degree linear polarizer, and a 120-degree linear polarizer sequentially inserted into a path of the beam of light received from the optical source.

12. A method for measuring the polarization sensitivity of an optical device under test based on one of a) transmission and b) reflection responses of said optical device under test, the method comprising:

producing a polarized light beam having three sequential predetermined states of polarization, corresponding to three input optical electric field Jones vectors;

impinging the light beam on the optical device under test;

measuring one of the a) transmission and b) reflection responses of the optical device under test to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical device under test;

computing Stokes vectors from the Stokes parameters;

converting the Stokes vectors to output optical electric field Jones vectors;

computing a Jones matrix to within a complex constant for the optical device under test from the Jones input and output vectors; and

computing a ratio of the maximum and minimum values of the one of the a) transmission and b) reflection responses to provide a measurement of the polarization sensitivity of the optical device under test in relative terms from the matrix.

13. A method according to claim 12, further comprising the step of computing states of polarization corresponding to the at least one of the maximum and minimum values of the one of the a) transmission and b) reflection responses of the optical device under test.

14. A method for calibrating an instrument to correct for distortion of polarization state caused by an optical network which is not completely polarizing, comprising the steps of:

producing a polarized light beam having three sequential predetermined states of polarization, corresponding to three input optical electric field Jones vectors;

impinging the light beam on the optical network;

measuring one of the a) transmission and b) reflection responses of the optical network to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical network;

computing Stokes vectors from the Stokes parameters for the optical network;

converting the Stokes vectors correlated to the responses of the optical network to output optical electric field Jones vectors;

computing a Jones matrix to within a complex constant for the optical network from the Jones input and output vectors;

computing the inverse of the matrix for the optical network; and

correcting parameters of an unknown light beam as measured by the instrument according to the inverse of the matrix for the optical network to provide a calibrated measurement of said parameters whereby said parameters are correctly measured notwithstanding any distortion of the state of polarization caused by the optical network.

15. A method according to claim 14, further comprising the steps of:

inserting an optical device under test into the path of the light beam;

measuring one of the a) transmission and b) reflection responses of the optical network and the optical device under test to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical network and the optical device under test;

computing second Stokes vectors from the Stokes parameters for the optical network and the optical device under test;

converting the second Stokes vectors correlated to the responses of the optical network and the optical device under test to second output optical electric field Jones vectors;

computing a Jones matrix to within a complex constant for the optical device under test from the Jones input vectors and second Jones output vectors after multiplication of the second Jones output vectors by the inverse of the matrix for the optical network; and

computing a ratio of maximum and minimum values of the one of a) transmission and b) reflection responses of the optical device under test to provide a measurement of the polarization sensitivity of the optical device under test in relative terms from the matrix for the optical device under test.

16. A method according to claim 15, further comprising the step of computing states of polarization corresponding to the at least one of the maximum and minimum values of the one of the a) transmission and b) reflection responses of the optical device under test.

17. A method for measuring the polarization sensitivity of an optical device under test based on one of a) transmission and b) reflection responses of the optical device under test, the method comprising the steps of:

producing a polarized light beam having three sequential predetermined states of polarization, corresponding to three input optical electric field Jones vectors;

impinging the light beam on an optical through;

measuring one of the a) transmitted and b) reflected power of the optical through to the light beam for at least one of the three sequential predetermined polarization states;

substituting the optical device under test for the optical through;

impinging the light beam on the optical device under test;

measuring one of the a) transmission and b) reflection responses of the optical device under test to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical device under test;

measuring one of the a) transmitted and b) reflected power of the optical device under test to the light beam for at least one of the three sequential predetermined polarization states;

computing Stokes vectors from the Stokes parameters for the optical device under test;

converting the Stokes vectors correlated to the responses of the optical device under test to output optical electric field Jones vectors;

computing a Jones matrix for the optical device under test from the Jones input and output vectors and the power measurements obtained with the optical through and the optical device under test; and

providing a measurement of the polarization sensitivity of the optical device under test in absolute terms from the Jones matrix by computing a ratio of the maximum and minimum values of the one of the a) transmission and b) reflection responses.

18. A method according to claim 17, further comprising the step of computing states of polarization corresponding to the at least one of the maximum and minimum values of the one of the a) transmission and b) reflection responses of the optical device under test.

19. A method for calibrating an instrument to correct for distortion of polarization state caused by an optical network which is not completely polarizing, comprising the steps of:

producing a polarized light beam having three sequential predetermined states of polarization, corresponding to three input optical electric field Jones vectors;

impinging the light beam on an optical through;

measuring one of the a) transmission and b) reflection responses of the optical through to the light beam for each of the three sequential predetermined polarization states by splitting the light beam from the optical through to be measured into four beams, passing three of the beams through optical elements, and measuring the intensities of all four beams;

measuring one of the a) transmitted and b) reflected power of the optical through to the light beam for each of the three sequential predetermined polarization states;

substituting the optical network for the optical through;

impinging the light beam on the optical network;

measuring one of the a) transmission and b) reflection responses of the optical network to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical network;

measuring one of the a) transmitted and b) reflected power of the optical network to the light beam for at least one of the three sequential predetermined polarization states;

computing Stokes vectors from the Stokes parameters for the optical network;

converting the Stokes vectors correlated to the responses of the optical network to output optical electric field Jones vectors;

computing a Jones matrix for the optical network from the Jones input and output vectors and the power measurements obtained with the optical through and the optical network;

determining the inverse of the Jones matrix for the optical network; and

correcting responses represented by additional output optical electric field Jones vectors obtained during subsequent polarization state measurements according to the inverse of the Jones matrix for the optical network to provide calibrated absolute measurements of said subsequent polarization states notwithstanding any distortion of the state of polarization caused by the optical network.

20. A method according to claim 19, further comprising the steps of:

inserting an optical device under test into the path of the light beam;

measuring one of the a) transmission and b) reflection responses of the optical network and the optical device under test to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the responses of the optical network and the optical device under test;

computing second Stokes vectors from the Stokes parameters for the optical network and the optical device under test;

converting the second Stokes vectors correlated to the responses of the optical network and the optical device under test to second output optical electric field Jones vectors;

computing a Jones matrix to within a complex constant for the optical device under test from the Jones input vectors and the second Jones output vectors after multiplication of the second Jones output vectors by the inverse of the matrix for the optical network; and

computing a ratio of maximum and minimum values of the one of a) transmission and b) reflection responses of the optical device under test to provide a measurement of the polarization sensitivity of the optical device under test in relative terms from the matrix for the optical device under test.

21. A method according to claim 20, further comprising the step of computing states of polarization corresponding to the at least one of the maximum and minimum values of the one of the a) transmission and b) reflection responses of the optical device under test.

22. A method according to claim 19, further comprising the steps of:

connecting the optical through to the optical network;

measuring one of the a) transmitted and b) reflected power of the optical through and the optical network to the light beam for at least one of the three sequential predetermined polarization states;

substituting an optical device under test for the optical through;

impinging the light beam on the optical device under test;

measuring one of the a) transmission and b) reflection responses of the optical network and the optical device under test to the light beam for each of the three sequential predetermined polarization states by measuring the Stokes parameters of the optical network and the optical device under test;

measuring one of the a) transmitted and b) reflected power of the optical network and the optical device under test to the light beam for at least one of the three sequential predetermined polarization states;

computing second Stokes vectors from the Stokes parameters for the optical network and the optical device under test;

converting the second Stokes vectors correlated to the responses of the optical network and the optical device under test to second output optical electric field Jones vectors;

computing a Jones matrix for the optical device under test from the Jones input vectors and the second Jones output vectors after multiplication of the second Jones output vectors by the inverse of the matrix for the optical network and the power measurements obtained with the optical through, the optical network, and the optical device under test; and

providing a measurement of the polarization sensitivity of the optical device under test in absolute terms from the matrix for the optical device under test by computing a ratio of the maximum and minimum values of the one of a) transmission and b) reflection responses.

23. A method according to claim 22, further comprising the step of computing states of polarization corresponding to the at least one of the maximum and minimum values of the one of the a) transmission and b) reflection responses of the optical device under test.