Methods of improving the retardation accuracy and stability of photoelastic modulator devices

1. A method for calibrating a system comprising a photoelastic modulator (PEM) on a circular dichroism (CD) measurement instrument, the method comprising:

providing a circular dichroism (CD) sample;

scanning the control input voltage (V in ) at a fixed wavelength (λ meas ); and

recording the CD scan of the sample,

wherein the control input voltage (V in ) determines the peak retardation (δ) at the fixed wavelength (λ meas ), and wherein the method is repeated for two or more fixed wavelengths, wherein the method further comprises the step of fitting the results measured at each wavelength to a drive function (F).

2. The method of claim 1 , wherein the drive function is an augmented drive function.

3. The method of claim 2 , wherein the augmented drive function is characterized by the equation:

V

in

=

(

c

+

m

⁢

⁢

λ

+

n

⁢

⁢

λ

-

1

)

·

2

π

·

Φ

,

wherein V in is the control input voltage, c is the intercept, m is slope, λ is wavelength.

4. The method of claim 2 , wherein a CD measurement error for the augmented drive is about less than 0.01%.

5. The method of claim 1 , wherein the fixed wavelength is selected from two or more wavelengths listed in Table 1

TABLE 1

Peak

Wavelength

Peak No.

(nm)

1

175.29

2

184.26

3

194.26

4

205.64

5

218.79

6

234.24

7

252.72

8

275.25

9

303.30

10

339.15

11

386.46

12

451.54

13

546.20

14

695.61

15

964.10

16

1577.53.

6. The method of claim 5 , wherein the fixed wavelength is selected from at least four wavelengths in Table 1.

7. The method of claim 5 , wherein the fixed wavelength is selected from at least eight wavelengths in Table 1.

8. The method of claim 5 , wherein the fixed wavelength consists of the 16 wavelengths in Table 1.

9. The method of claim 1 , wherein the CD sample is a DichOS optical standard.

10. The method of claim 1 , wherein conditions for recording the scans comprise:

a) a bandwidth of about 1 nm to about 8 nm;

b) a time per point of about 0.1 seconds to about 5 seconds; and

c) about 50 to about 500 points.

11. The method of claim 1 , further comprising a drive correction to account for temperature shifts, wherein the drive correction is characterized by the equation:

Dcorr

=

V

rate

V

ratenom

,

wherein V rate , corresponds to the actual volts per radian of retardation at the V-scan wavelength, V ratenom , corresponds to a nominal value for this parameter, and Dcorr is a function of the PEM resonance frequency shift (Δf).

12. The method of claim 11 , wherein the control input voltage V in is scaled by the drive correction to give a corrected control voltage V in,corr characterized by the equation:

V in,corr =V in ·Dcorr.

13. The method of claim 1 , wherein the system transmits light through the sample to measure its optical properties and the system includes a sample stage that carries the sample and has an aperture through which light is transmitted for each calibration of the system.

14. The method of claim 1 , wherein the system reflects light from the sample to measure its optical properties and the system includes a sample stage that carries the sample and also carries a mirror from which light is reflected for each calibration of the system.

15. The method of claim 1 , further comprising a temperature control system.

16. The method of claim 15 , wherein the temperature control system is used to stabilize the temperature of the PEM core.

17. The method of claim 16 , wherein the temperature control system is a proportional integral derivative (PID) control system.

18. A calibration system comprising a photoelastic modulator (PEM) on a circular dichroism (CD) measurement instrument, wherein the calibration system is configured to for calibrating the PEM on the CD measurement instrument, and a device or program for collecting data, wherein the calibration system comprises a mechanism for scanning the control input voltage ( V in) at a fixed wavelength ( λ meas), wherein the fixed wavelength is selected from two or more wavelengths listed in Table 1

Peak Wavelength

Peak No.

(nm)

 1

 175.29

 2

 184.26

 3

 194.26

 4

 205.64

 5

 218.79

 6

 234.24

 7

 252.72

 8

 275.25

 9

 303.30

10

 339.15

11

 386.46

12

 451.54

13

 546.20

14

 695.61

15

 964.10

16

1577.53;

and an analyzer for recording the CD scan, wherein the control input voltage V in determines the peak retardation (δ) at the fixed wavelength ( λ meas).

19. A method for calibrating a system comprising a photoelastic modulator (PEM) on a circular dichroism (CD) measurement instrument, the method comprising:

providing a circular dichroism (CD) sample;

scanning the control input voltage (V in ) at a fixed wavelength (λ meas ); and

recording the CD scan of the sample,

wherein the control input voltage (V in ) determines the peak retardation (δ) at the fixed wavelength (λ meas ) and wherein the method is repeated for the 16 fixed wavelengths in Table 1

TABLE 1

Peak Wavelength

Peak No.

(nm)

 1

 175.29

 2

 184.26

 3

 194.26

 4

 205.64

 5

 218.79

 6

 234.24

 7

 252.72

 8

 275.25

 9

 303.30

10

 339.15

11

 386.46

12

 451.54

13

 546.20

14

 695.61

15

 964.10

16

1577.53.

.

20. The method of claim 19 , further comprising the step of fitting the results measured at each wavelength to a drive function (F).