Systems and methods for phase measurements
Preferred embodiments of the present invention are directed to systems for phase measurement which address the problem of phase noise using combinations of a number of strategies including, but not limited to, common-path interferometry, phase referencing, active stabilization and differential measurement. Embodiment are directed to optical devices for imaging small biological objects with light. These embodiments can be applied to the fields of, for example, cellular physiology and neuroscience. These preferred embodiments are based on principles of phase measurements and imaging technologies. The scientific motivation for using phase measurements and imaging technologies is derived from, for example, cellular biology at the sub-micron level which can include, without limitation, imaging origins of dysplasia, cellular communication, neuronal transmission and implementation of the genetic code. The structure and dynamics of sub-cellular constituents cannot be currently studied in their native state using the existing methods and technologies including, for example, x-ray and neutron scattering. In contrast, light based techniques with nanometer resolution enable the cellular machinery to be studied in its native state. Thus, preferred embodiments of the present invention include systems based on principles of interferometry and/or phase measurements and are used to study cellular physiology. These systems include principles of low coherence interferometry (LCI) using optical interferometers to measure phase, or light scattering spectroscopy (LSS) wherein interference within the cellular components themselves is used, or in the alternative the principles of LCI and LSS can be combined to result in systems of the present invention.
1. A method for imaging a sample comprising the steps of:
illuminating a sample, the light originating from a point on the sample having a low frequency spatial component and high frequency spatial components;
interfering along a common optical path the high frequency spatial components with the low frequency spatial component to produce a first intensity signal;
shifting the phase of the low frequency spatial component to produce a first phase shifted low frequency spatial component;
interfering along a common optical path the high frequency spatial components with the first phase shifted low frequency spatial component to produce a second intensity signal;
shifting the phase of the low frequency spatial component to produce a second phase shifted low frequency spatial component;
interfering along a common optical path the high frequency spatial components with the second phase shifted low frequency spatial component to produce a third intensity signal;
shifting the phase of the low frequency spatial component to produce a third phase shifted low frequency spatial component;
interfering along a common optical path the high frequency spatial components with the third phase shifted low frequency spatial component to produce a fourth intensity signal;
generating a phase image of the point on the sample based at least in part on the first intensity signal, the second intensity signal, the third intensity signal and the fourth intensity signal.
2. The method of claim 1 , further comprising the step of:
generating a phase image of the sample by repeating the steps of claim 1 for a plurality of points on the sample.
3. The method of claim 1 , wherein the step of illuminating comprises illuminating the sample using a transmission illumination.
4. The method of claim 1 , wherein the step of illuminating comprises illuminating the sample using a reflective illumination.
5. The method of claim 1 , wherein the step of illuminating comprises illuminating the sample by both transmission and reflective illumination.
6. The method of claim 1 , wherein the step of illuminating comprises illuminating the sample with a superluminescent light source.
7. The method of claim 1 , further comprising the step of controlling the amplitude of at least one of the high frequency spatial components.
8. The method of claim 1 , further comprising the step of controlling the amplitude of at least one of the low frequency spatial component and phase shifted low frequency spatial components.
9. The method of claim 1 , wherein each step of shifting the phase shifts the phase of the low frequency spatial component by substantially n/2.
10. The method of claim 1 , wherein the step of generating a phase image is based at least in part on the equation.
φ
(
x
,
y
)
=
arctan
[
β
·
sin
[
Δ
φ
(
x
,
y
)
]
1
+
β
·
cos
[
Δ
φ
(
x
,
y
)
]
]
,
where
tan
[
Δ
φ
(
x
,
y
)
]
=
I
image
(
x
,
y
;
3
π
/
2
)
-
I
image
(
x
,
y
;
π
/
2
)
I
image
(
x
,
y
;
0
)
-
I
image
(
x
,
y
;
π
)
,
and
I image (x,y,δ) is the intensity signal for a point (x,y) on the sample surface generated by interfering the high frequency spatial components with a low frequency spatial component of phase shift δ and β=I 1 /I 0 represents the ratio between the intensity associated with the high frequency spatial component, I 1 , and the intensity associated with the low frequency spatial component, I 0 .
11. The method of claim 2 , wherein the step of generating a phase image of the sample comprises generating a phase image of the sample with a phase sensitivity of greater than about λ/1000.
12. The method of claim 2 , wherein the sample comprises a biological tissue.
13. The method of claim 3 , wherein the sample comprises a semiconductor wafer.
14. A method for imaging a sample comprising the steps of:
illuminating a sample to generate a low frequency spatial light component and a high frequency spatial light component;
spatially separating, along a common optical path, the high frequency spatial light component from the low frequency spatial light component;
shifting a phase of the low frequency spatial light component; and
generating a phase image of a region of the sample.
15. The method of claim 14 , further comprising the step of:
generating a phase image of the sample by repeating the steps of claim 1 for a plurality of points on the sample.
16. The method of claim 14 , wherein the step of illuminating comprises illuminating the sample using a transmission illumination.
17. The method of claim 14 , wherein the step of illuminating comprises illuminating the sample using a reflective illumination.
18. The method of claim 14 further comprising using a spatial light modulator to shift the phase.
19. The method of claim 18 further comprising using a computer to control the spatial light modulator and adjust the phase shift.