Transmissive optical delay system and method

The disclosed discrete or continuous optical delay is a medium with high transmission, a high index of refraction and a low dispersion coefficient at the wavelength of light of interest. One side of the medium, orthogonal to the incident light, is fabricated to delay the light at discrete values in a periodic pattern that repeats as the optical delay rotates. The disclosed discrete or continuous optical delay enables the creation of compact interferometry equipment to be used outside a laboratory environment.

FIELD OF INVENTION

The present invention is related to a transmission optical delay system and method, and more particularly, to a transmission optical delay system and method for time-domain interferometry.

BACKGROUND

Time-domain spectroscopy is a type of spectroscopy that measures the power of electromagnetic radiation as a function of time. Normal spectroscopy is frequency-domain spectroscopy that measures radiation as a function of frequency or wavelength. Time-domain spectroscopy usually requires the use of Fourier transforms.

Generally, in time-domain spectroscopy, two monochromatic light sources of different frequencies are combined to produce interference patterns. At time t=0, the waves may interfere constructively providing a maximum in an added plot. As time continues, the waves may become more out of phase leading to destructive interference and a minimum in the added plot. From one peak maximum to the next is a cycle. Broad peaks in spectra are made of many wavelengths.

Absorptions in spectra usually look like broad peaks. This means that a peak may contain many wavelengths of radiation. These wavelengths can be plotted in the time domain producing smaller areas of constructive interference. This constructive interference is because the closely spaced wavelengths quickly become more and more out of phase.

Spectroscopy done with visible or infrared radiation involves wavelengths around 1000 Hz (1 KHz). Detectors are limited in collecting measurements with that timing. Therefore, in order to reduce the frequency of the signals, a Michelson interferometer may be used. Generally, monochromatic light traveling through a Michelson interferometer is split into two beams by a beam splitter. These beams travel to mirrors, reflect, and then recombine at the beam splitter. One of the mirrors may be adjusted. When the two mirrors are the same distance apart, the light from each mirror interacts at the beam splitter in-phase to produce constructive interference. As the one mirror is shifted, the beams may be out of phase. When the movable mirror has moved a distance equal to one-quarter of the wavelength of light, the extra distance the light reflecting from that movable mirror travels is one-half the wavelength of light causing the two beams to combine at the beam splitter producing destructive interference. Shifting the mirror a total distance of one-half the wavelength of light causes the spectra to complete one cycle. A plot of the power of radiation as a function of distance traveled by the movable mirror may be provided and, with knowledge of the speed of the movable mirror, the power of radiation may be plotted as a function of time.

However, moving a mirror to create the distance of ½ the wavelength of light can cause complications in alignment and precision of movement within the system.

SUMMARY

A coherent light source passes through a transparent medium which is fabricated such that, when rotated about an axis parallel to the light source, the time of flight of the transmitted light is delayed by discrete values, determined by the profile of the rotating medium, due to the difference in refractive index between the transparent medium and air. In time-domain interferometry experiments, two coherent optical signals are superimposed spatially and temporally, and modulated by a sample somewhere in the optical path to resolve small features on the sample. The measured superposition is resolved in time by modulating the length of one optical signal in space using an optical delay line. Typical optical delay lines include two mirror pairs, one stationary and one that moves away from the first, lengthening the optical path length. These delay lines are large, slow and introduce unwanted variance in the measured superposition as the movement occurs. The disclosed optical delay modulates the optical path length by passing the light through a refractive medium and modulating the length of the material in a periodic pattern. This type of optical delay is smaller, faster and introduces less variation than traditional optical delay lines.

The disclosed discrete or continuous optical delay is a medium with high transmission, a high index of refraction and a low dispersion coefficient at the wavelength of light of interest. One side of the medium, orthogonal to the incident light, is fabricated to delay the light at discrete values in a periodic pattern that repeats as the optical delay rotates. The disclosed discrete or continuous optical delay enables the creation of compact interferometry equipment to be used outside a laboratory environment.

A device, system and method are disclosed. The device is designed to impart a varying optical delay into an optical system based on rotation. The device includes a first surface configured to be aligned substantially perpendicular to a light beam in use, the first surface having a first surface center and a second surface angularly offset with the first surface by an angle, ϕ, the second surface having a second surface center concentric with the first surface center and defining a center of the device. The device is configured to accept a light beam outside the center of the device, and is configured to rotate in a rotation angle, Θ, about the center to create a varying optical delay in a light beam where the optical delay is defined by the rotation angle θ and angular offset ϕ. The device comprises an index of refraction greater than 1. The device comprises a transmission of approximately 1. The device may be continuous in forming the angular offset ϕ. Alternatively, the device may be segmented in forming the angular offset ϕ with the number of segments within a range from 4-12 segments. The angular offset ϕ may be between 5-50 degrees, and more particularly between 20-40 degrees, and more particularly 35-40 degrees. The device may be formed from at least one of N-BK7, UV fused silica, calcium fluoride, magnesium fluoride, zinc selenide, sapphire, barium fluoride, silicon, and germanium. The optical delay created is a function of the thickness of the device interacting with the beam.

The system utilizing an optical delay including the device may further include a light source, a beamsplitter to split the light source into two light beams and an accumulator optic designed to direct the two light beams onto a detector. The system may further include an optic designed to capture the delayed light beam as the light beam shifts as a result of the angle ϕ causing the beam to refract. The system may further include at least one aligning mirror to maintain alignment of the two light beams. The optical delay in the system is a function of the thickness of the device interacting with the beam. The light source may be coherent and may be infrared, such as centered at 785 nm or 815 nm.

The method for performing optical delay using an optic includes providing a coherent light beam from a light source, splitting the beam to provide dual light beams, interacting with one of the dual light beams using the optic that provides a varying optical delay based on the rotation angle of the optic, and converging the dual beams onto a detector to measure an interferometric signal in intensity based on rotation of the optic imparting vary optical delay.

DETAILED DESCRIPTION

A coherent light source passes through a transparent medium which is fabricated such that, when rotated about an axis parallel to the light source, the time of flight of the transmitted light is delayed by discrete values, determined by the profile of the rotating medium, due to the difference in refractive index between the transparent medium and air. In time-domain interferometry, two coherent optical signals are superimposed spatially and temporally, and modulated by a sample somewhere in the optical path to resolve small features on the sample. The measured superposition is resolved in time by modulating the length of one optical signal in space using an optical delay line. Typical optical delay lines include two mirror pairs, one stationary and one that moves away from the first, lengthening the optical path length. These typical optical delay lines are large, slow and introduce unwanted variance in the measured superposition as the mirror moves. The disclosed optical delay modulates the optical path length by passing the light through a refractive medium and modulating the length of the material in a periodic pattern. This disclosed optical delay is smaller, faster and introduces less variation than traditional optical delay lines.

The disclosed discrete or continuous optical delay is a medium with high transmission, a high index of refraction and a low dispersion coefficient at the wavelength of light of interest. One side of the medium, orthogonal to the incident light, is fabricated to delay the light at discrete values in a periodic pattern that repeats as the optical delay rotates. The disclosed discrete or continuous optical delay enables the creation of compact interferometry equipment that can be used outside a laboratory environment.

FIG.1Aillustrates a three-dimensional depiction of an optic100designed to create an optical delay in the transmission of a light source. Optic100is illustrated with eight segments1101,1102,1103,1104,1105,1106,1107,1108(collectively referred to as segments110) and a back surface130. In the exemplary eight segment110optic100, each segment110includes a 45-degree wedge of the optic. While optic100is illustrated with eight segments110, other numbers of segments may be utilized, including 2, 4, 6, 10, 12, 16, 24, and 32, as would be understood by those possessing an ordinary skill in the art. That is, the use of eight segments is for illustration purposes only with an understanding that any number of segments may be used in the present system.

Each of segments110may define a plane at a different distance from back surface130. As illustrated, segments110may have increasing distance between the defined plane of segment110and back surface130. A plurality of walls140defined between the transmissions of segments110may be substantially vertical. The vertical aspect of walls140may be defined so the walls140fail to interact, or minimally interact, with the light propagating from a light source120. As would be understood, the number of walls140is defined by the number of segments110used. Therefore, the number of walls140may vary from that illustrated inFIG.1A. In the exemplary eight segment110illustration, eight walls140exist.

The material of optic100may have a refractive index larger than 1 (n>1) and high transmission (T˜1) of the light propagating from the light source120. By way of example, optic100may be fabricated from N-BK7, UV fused silica, calcium fluoride, magnesium fluoride, zinc selenide, sapphire, barium fluoride, silicon, and germanium. Optic100may include a wavelength-specific anti-reflection (AR) coating (not shown) centered around the wavelength of the light propagating from light source120, or in the configuration where light source120is a broader spectrum light source, a center wavelength. Such a coating may be included on back surface130and/or on front segments110, and even on the walls140if a benefit can be provided from such a coating.

FIG.1Billustrates an isometric view150of the optic100ofFIG.1Aallowing illustration of the surfaces of the optic. View150is provided to illustrate additional detail of optic100not visible or obscured inFIG.1A. Optic100is illustrated with eight segments1101,1102,1103,1104,1105,1106,1107,1108(collectively referred to as segments110) and a back surface130. As illustrated, segments110may have increasing distance between the defined plane of segment110and back surface130. A plurality of walls140defined between the transmissions of segments110may be substantially vertical.

Optic100may be designed to provide a transmissive optical delay of the light propagating from light source120. Using the exemplary eight segments, optic100may delay the light propagating from the light source120(which generally in operation may be a coherent light source) by discrete values defined by the segments110as optic100rotates. By rotating optic100, each of the segments110may be placed to interact with the light propagating from light source120for a period of time defined by the speed of rotation of optic100. As the thickness of optic100varies based on the segment110interacting with the light propagating from the light source120, the optical delay may vary as the optic is rotated.

FIG.2Aillustrates a plot200of the optical delay plotted against the rotation angle for the optic100ofFIG.1A. The optical delay is defined in seconds (s). The rotation angle of optic100is defined in degrees with zero degrees at the transition from the thickest segment1108and thinnest segment1101. The optical delay associated with a given segment110is provided in the plot of optical delay, collectively referred to as optical delay210, with like subscript. Segment1101provides an optical delay2101, segment1102provides an optical delay2102, segment1103provides an optical delay2103, segment1104provides an optical delay2104, segment1105provides an optical delay2105, segment1106provides an optical delay2106, segment1107provides an optical delay2107, and segment1108provides an optical delay2108.

As may be seen in plot200, matching each of the segments110is a flattened optical delay210associated with each of the respective segments110. The exemplary discrete transmission optical delay210includes 8 steps matching the 8 segments110. As illustrated in the plot200, once the rotation of optic100reaches the next segment110, a new optical delay210is achieved. This piecewise stepping continues for each of the segments110of optic100.

FIG.2Billustrates a plot250of the intensity plotted against time for the interferometer signal using the optic100ofFIG.1A. Plot250illustrates the sampled time-domain interferometry signal, where the 8 dots represent discrete values220of the signal corresponding to the segments110at discrete times. The curve in plot250is represented as a spline to show how the signal may be interpolated in post-processing. More specifically, segment1101provides an optical delay2101that results in intensity2201, segment1102provides an optical delay2102that results in intensity2202, segment1103provides an optical delay2103that results in intensity2203, segment1104provides an optical delay2104that results in intensity2204, segment1105provides an optical delay2105that results in intensity2205, segment1106provides an optical delay2106that results in intensity2206, segment1107provides an optical delay2107that results in intensity2207, and segment1108provides an optical delay2108that results in intensity2208.

Based on the parallel aspect of the segments110and the back surface130, the beam may experience negligible deviation (likely caused only by reflection, refraction) in beam propagating through optic100. However, any notable deviations in the light source120may be accounted for, as would be understood by those possessing an ordinary skill in the art. Such methods to account for deviations may include wave plates and beamsplitters and other optics generally used to correct the beam angle. One method to account for the deviations may include imparting a slight wedge in optic100to shift the beam propagation as needed. A ray trace is provided for the continuous optic described below to further the understanding of light collection and sensing.

FIG.2Cillustrates a depiction of the optic100ofFIG.1Aillustrating the eight sample points of the discrete optic as discussed with respect toFIG.1A.

FIG.3illustrates a system300utilizing the discrete delay provided by the optic ofFIG.1A. System300provides an example application of the discrete transmission optical delay. A single coherent light source310is split into two coherent beams via beamsplitter320. One beam is focused through a lens330using a right-angle mirror340(to enable alignment) onto a detector350. The other beam is first passed through the optic100providing the transmission optical delay and then focused through lens330onto a detector350. Both beams may transmit through the lens330at the same point and may be incident on detector350also at the same location. The superposition of the two beams are spatially (and therefore temporally) convolved as the transmission optical delay rotates. A ray trace is provided for the continuous optic described below to further the understanding of light collection and sensing. Similar techniques may be used in system300for optic100.

The definition of optical delay is generalized as follows:

τdelay=z⁡(θ)⁢1c⁢nmn0,
where τdelayis the optical delay in seconds, z is the thickness of optic100(medium) traveled by the light from light source310in meters, θ is the angle of rotation of optic100in radians, c is the speed of light in meters per second, nmis the refractive index of optic100(medium), which is unitless, and n0is the refractive index outside optic100(medium), which is also unitless.

For example, if the optic100is formed of fused silica optic100having an optical thickness z=0.03 meters (3 cm) may have nm=1.5 and where the system is operating in the air at sea level n0=1.0. given this example, the optical delay may be reduced to:

FIG.4Aillustrates a three-dimensional depiction of an optic400designed to create a continuous optical delay in the transmission of a light source.FIG.4Billustrates a three-dimensional depiction of the optic400designed to create a continuous optical delay in the transmission of a light source shown from the opposite side as the view presented inFIG.4A. Optic400provides a transmissive optical delay delaying the coherent light by continuous values as optic400rotates. Collectively,FIGS.4A and4Bdepict optic400which depictions include a back surface430and a front surface410. Optic400, instead of the having eight segments110of optic100, is formed with a continuous wedge created by an angle φ between back surface430and front surface410. The angle φ may be any angle from 0 to 60 or more degrees. By way of example, a 10°, 20°, 30°, 40°, or 50° wedge may be used. While the available wedge angle is described above as selecting an angle from a list in 10° increments, other values to create the list of available wedge angles may also be used. These other increments include 5° increments for angles between 0 and 60. Other angles between 0 and 60 degrees, such as angles from a list that includes increments of 1° may also be used.

When the light from light source420interacts with the surfaces of optic400, such as front surface410and back surface430, the light may refract. This refraction is depicted inFIG.4Ausing an incident light beam4200from light source420. After interacting with front surface410, incident light beam4200refracts along a path defined by first refracted light beam4201which beam then refracts at back surface430along a path defined by second refracted light beam4202. The beams, incident light beam4200and second refracted light beam4202may be parallel and offset by a distance440defined by the thickness and refractive index of the optic. Other optics in the system may be used to account for this beam displacement by distance440.

The material of optic400may be similar to that of optic100. Optic400may designed to have a refractive index larger than 1 (n>1) and high transmission (T˜1) of the light propagating from the light source120. By way of example, optic400may be fabricated from N-BK7, UV fused silica, calcium fluoride, magnesium fluoride, zinc selenide, sapphire, barium fluoride, silicon, and germanium. Optic400, similar to optic100, may include a wavelength-specific anti-reflection (AR) coating (not shown) centered around the wavelength of the light propagating from light source420, or in the configuration where light source420is a broader spectrum light source, a center wavelength. Such a coating may be included on back surface430and/or on front surface410.

FIG.4Cillustrates an isometric view450of the optic400ofFIGS.4A,4Ballowing illustration of the surfaces of the optic400. Since the incident surface410of optic400is no longer normal to the light from light source420, the light may refract and emerge from the back surface430with an offset, roffset, which will vary proportionally to the distance traveled with in the medium. The offset, roffset, may be defined according to the following equation:
roffset∝z(Θ,φ,nm),
where nmis the index of the optic400. By rotating optic400, optic400interacts with the light propagating from the light source420and the thickness of optic400involved in the interaction varies and is defined by the speed of rotation of optic100. As the thickness of optic400involved in the interaction varies based on the thickness of optic400interacting with the light propagating from the light source120, the optical delay may vary as the optic is rotated. This deviation in the light propagating from the light source120may be accounted for as would be understood by those possessing an ordinary skill in the art. Such methods may include wave plates and beamsplitters and other optics generally used to correct the beam angle.

FIG.4Dillustrates a plot490of the optical delay for an example material for optic400as a function of angle of rotation. This optical delay may occur in the system described inFIGS.6A and6B, for example. The example material is CaF2and the exemplary plot490is illustrated for 785 nm light. Plot490illustrates different angles φ of the optic400in the respective curves for 5 degree, 10 degrees, 20 degrees, 30 degrees, and 40 degrees wedge angles. At 40 degrees for φ, the optical delay is 0 at an angle of rotation of zero degrees, increases to approximately 13 ps at an angle of rotation of 180 degrees, and back to an optical delay of 0 at an angle of rotation of 360 degrees (fully rotated and back to the 0 degree rotation position). The peak optical delay at 30 degrees for φ is approximately 9 ps at 180 degrees, at 20 degrees for φ is approximately 5 ps at 180 degrees, at 10 degrees for φ is approximately 3 ps at 180 degree, and at 5 degrees for φ is approximately 1 ps at 180 degree.

FIG.4Eillustrates a plot495of the chromatic effects of the exemplary material inFIG.4Dat 815 nm light as compared to 785 nm light. This optical delay and chromatic effects may occur in the system described inFIGS.6A and6B, for example. For an optic at 40 degrees for φ, the optical delay may be affected by the chromatic effects in a varying amount between 0.010 and 0.020 ps across a full 360-degree angle of rotation of optic400.

FIG.5Aillustrates a plot500of the optical delay plotted against the rotation angle for the optic400ofFIG.4A. As is illustrated in plot500as compared to plot200, the optical delay is continuous in plot500. The optical delay is defined in seconds (s). The rotation angle of optic100is defined in degrees with zero degrees at roughly the middle thickness of optic400. The optical delay associated with a given angle of rotation is provided in the plot of optical delay. The optical delay is sinusoidal in a continuous transmission delay.

FIG.5Billustrates a plot550of the intensity plotted against time for the interferometer signal using the optic400ofFIG.4A. Plot550illustrates the sampled time-domain interferometry signal of the continuous optic400rotated through a complete circle. The curve in plot550is represented as a spline to show how the signal may be interpolated in post-processing. Plot550illustrates that the temporal resolution is only limited by the sampling methods of the system, unlike with the discrete transmission delay.

FIG.5Cillustrates a depiction of the optic400ofFIGS.4A,4Billustrating the continuous sample points created by the smooth optic400.

FIG.6Aillustrates a system600utilizing the continuous transmission optical delay provided by the optic ofFIGS.4A,4B. System600provides an example application of the discrete transmission optical delay. A single coherent light source610is split into two coherent beams via beamsplitter620. One beam is focused through a lens630using a right-angle mirror640(to enable alignment) onto a detector650. The other beam is first passed through the optic400providing the transmission optical delay and then focused through lens630onto a detector650. Both beams may transmit through the lens630at the same point and may be incident on detector650also at the same location. The superposition of the two beams are spatially (and therefore temporally) convolved as the transmission optical delay rotates. The definition of optical delay is generalized as follows:

τdelay=z⁡(θ,φ,nm)⁢1c⁢nmn0,
where τdelayis the optical delay in seconds, z is the thickness of optic400(medium) traveled by the light from light source610in meters, θ is the angle of rotation of optic400in degrees, c is the speed of light in meters per second, nmis the refractive index of optic400(medium), which is unitless, and n0is the refractive index outside optic400(medium), which is also unitless. Additional optical equipment (not shown), such as a converging lens, may be needed to mitigate the variation introduced by roffsetdescribed above.

FIG.6Billustrates a depiction of an optical ray trace660of a portion of the system600ofFIG.6A. Ray trace660depicts optic400and lens630and depicts a reference optical path and the delayed optical path. Optic400is depicted with the front surface410and the back surface430. The wedge angle φ of optic400is shown. Phi (φ) is the wedge angle on the delay optic. Phi may have an upper limit that depends on the material, point of incidence, delay optic length, and the like. The ray trace660depicts a rotation angle θ from 0 degree to 180 degree and back to 360 degrees. Theta (θ) is the angle of rotation of optic400. Theta may be 0-360 degrees. Focusing lens630is used to capture the light ray in the extreme traces in both 0-degree and 180-degree rotation angle for a given wedge angle.

When the light from light source interacts with the surfaces of optic400, such as front surface410and back surface430, the light may refract as discussed herein above. This refraction is depicted inFIG.6Busing an incident light beam4200from light source420. After interacting with front surface410, incident light beam4200refracts along a path defined by first refracted light beam4201which beam then refracts at back surface430along a path defined by second refracted light beam4202. The beams, incident light beam4200and second refracted light beam4202may be parallel and offset by a distance. Other optics in the system may be used to account for this beam displacement by distance.

FIG.7Aillustrates a system700utilizing the delay provided by the optic100ofFIG.1Aor optic400ofFIGS.4A,4Bcombined with an optical chopper760and rotated synchronously. System700provides an example application of the transmission optical delay (either discrete using optic100or continuous using optic400) combined with an optical chopper and rotated synchronously. A single coherent light source710provides a single coherent output beam that is split into two coherent beams via beamsplitter720. After the output beam is split into two coherent beams, with much less separation needed, allowing one of the beams to propagate through the optical chopper blades760and the other of the two beams to propagate through the transmission optical delay. One beam, after passing through chopper blades760, is focused through a lens730using a right-angle mirror740onto a detector750. The other beam is first passed through the optic100,400providing the transmission optical delay and then focused through the lens730onto a detector750. Both beams may propagate through the lens730at the same point and may be incident on detector750also at the same location. The superposition of the two beams are spatially (and therefore temporally) convolved as the transmission optical delay rotates.

Since the important signals in time-domain interferometry are often very small, phase sensitive detection via a phase sensitive detector770is used to recover the signal. Phase sensitive detection relies on a reference signal, often supplied by an optical chopper760, which is modulating the optical input at the same frequency. Since the optical chopper760is already rotating in the system, if the transmissive optical delay optic100,400is attached to the chopper760, the experiment becomes much smaller and interferometry experiments may be made compact for field operation.

Phase sensitive detector770may use a lock-in amplifier. A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave (based on the optical chopper760) from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to 1 million times smaller than noise components, potentially fairly close by in frequency, may be reliably detected. Traditional lock-in amplifiers use analog frequency mixers and RC filters for the demodulation and some other instruments have both steps implemented by fast digital signal processing, for example, on an FPGA. Sine and cosine demodulation are performed simultaneously, referred to as dual-phase demodulation. This dual-phase demodulation allows the extraction of the in-phase and the quadrature component that may be transferred into polar coordinates, i.e., amplitude and phase, or further processing as real and imaginary parts of a complex number (e.g., for complex FFT analysis).

FIG.7Billustrates a view of the optic/chopper element used in system700ofFIG.7A. This element includes the optic, such as optic100fromFIG.1Aor optic400fromFIG.4A,4B, combined with the chopper760. Chopper760may include an open section7601, and a blocked section7602configured in an alternating pattern around the periphery of the optic. In the illustrated element the optical chopper760is arranged radially around the outer portion of the element radially encompassing the optic100,400. As would be understood, a configuration internal to the optic may be used in order to control the rotation of the element and to attach to phase sensitive detector770.

Generally, optical chopper760is a device which periodically interrupts a light beam—using open section7601and blocked section7602. Optical choppers may be variable frequency rotating disc choppers (illustrated), fixed frequency tuning fork choppers, and optical shutters. Optical chopper760operates in combination with the lock-in amplifier(s) of phase sensitive detector770. The chopper760may be used to modulate the intensity of the light beam, and the lock-in amplifier of phase sensitive detector770is used to improve the signal-to-noise ratio. Optical chopper760may be designed with a stable rotating speed. Increased frequency of rotation of chopper760increases efficiency in cases where the 1/f noise is a problem to be overcome. As would be understood, the speed of rotation of chopper760and the number of slots or pairs of open section7601and a blocked section7602may be modified as would be understood by those possessing an ordinary skill in the art. As illustrated inFIG.7B, the open section7601and a blocked section7602are similarly sized, although this configuration is just exemplary and such a configuration would not need to be maintained as would be understood by those possessing an ordinary skill in the pertinent arts.

As with the other systems, additional optics may be needed on the exit side of the rotating piece to compensate for unwanted geometric optic transformations induced by the rotating piece. For example, with the chopper, the exiting beam path may be an elliptical cone, which may be corrected to a circular cone with some beam forming optics, etc.

FIG.8illustrates a method800performing optical delay using the optics ofFIGS.1A,1B and4A,4B,4Cin systems ofFIGS.3,6A,6B and7A. Method800includes the steps of providing coherent light from a light source at step810. At step820, method800includes splitting the beam to provide dual light beams. At step830, method includes interacting with one of the dual light beams using an optic that provides a varying optical delay based on the rotation angle of the optic. At step840, method800includes converging the dual beams onto a detector to measure the interferometer signal in intensity based on rotation of the optic imparting varying optical delay.