Optical polarization sensing apparatus and method

Details Optical polarization sensing apparatus and method Optical polarization sensing apparatus and method

1999-05-04

2001-02-13

Kim, Robert H. (Department: 2877)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

C356S340000, C356S340000, C356S368000

Reexamination Certificate

active

06188477

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to optical polarization sensing apparatus and methods that are employed to monitor the change of the polarization state of light to detect and sense signals.
Optical polarization sensing devices have numerous applications, including stress, pressure and temperature sensors, for example. Typically, these devices are fairly large and employ expensive components to achieve a required level of sensitivity. For example, there have been extensive studies conducted on developing non-invasive blood glucose monitoring for diabetes patients because of the inconvenience and danger of the conventional invasive methods, which usually require the patients to cut a finger to give a blood sample. By shining light through human interstitial fluids or blood at translucent dermal positions such as fingertips, ear lobes, or ocular aqueous humor in the eyes, information about the glucose levels can be obtained optically by infrared spectroscopy, Raman spectroscopy or optical polarization changes. Because glucose rotates a polarized light beam's polarization proportionally to its concentration, a phenomenon known as optical activity, various polarimetric glucose sensors have been developed. Examples of these include devices that use Faraday rotators to directly modulate the beam's polarization state, as well as devices that use a Zeeman laser for optical heterodyne detection. However, these methods involve bulk optical components that are expensive and inconvenient.
A need therefore exists for inexpensive, small scale, high sensitivity polarization sensing devices, particularly in certain applications, such as the aforementioned detection of blood glucose levels. In this application, small size and low cost are important to enable the devices to be both portable and affordable so that they may be purchased and used by the diabetic patients without having to visit a medical facility. Another application of these devices is in a magneto-optical disk read head.
One known technique for fabrication of ultra-small micromechanical structures and devices is a polysilicon surface micromachining technique known as MEMS (Micro Electro Mechanical System). With MEMS technology, micro-sized mechanical actuators, sensors and other structures can be integrally formed on single silicon substrates or chips with integrated circuits that control, or receive signals from, the actuators or sensors. To date, however, polarization sensing devices have not been formed using MEMS technology. The necessary sensitivity of the polarization sensing systems, as small as 0.1 to 0.001 degree change in polarization rotation angle, is very hard to achieve with conventional direct detection methods. The conventional light detection schemes can not meet the sensitivity requirement without going through very sophisticated optical systems, which would be much too bulky and expensive, and involve components such as Faraday rotators that are not compatible with the MEMS technology.
SUMMARY OF THE INVENTION
The present invention solves the foregoing problem through provision of polarization sensing devices that, through use of a self-homodyne detection scheme, can employ inexpensive optical components, yet still provide the necessary polarization detection sensitivity. In particular, using the self-homodyne detection scheme enables detection of a polarization change as small as 0.001 degree without requiring use of expensive, bulky components, such as Faraday rotators.
The detection scheme is referred to as self-homodyne because only one laser source is used for the whole system and all of the light beams in the sensing system have the same fundamental optical frequency and laser phase noise, therefore the phase noise can be canceled. The sensing device phase modulates an optical signal generated thereby at a known modulation frequency, and the device uses this modulation frequency to determine light intensity values that are proportional to the change in polarization to be sensed. More particularly, in the preferred embodiments of the invention, spatially coherent light from a laser of a known frequency is incident on a polarization beamsplitter that splits the beam into a transmitted beam and a reflected beam whose electric field components are perpendicular to one another. One of the beams, e.g., the transmitted beam, is modulated by a phase modulator that changes the phase of the optical wave periodically in addition to the fundamental optical wave phase oscillation, but at a much lower frequency. The modulated beam is then recombined with the unmodulated reflected beam, and sensed by a photodetector.
If the incident laser beam is polarized, and prior to passing through the polarization beamsplitter, the polarization is rotated slightly, this rotation will cause the modulated and reflected beams to interfere with one another when they are combined. The same interference occurs if the combined beam has its polarization rotated, and is then passed through a polarizer to pass only one of the polarization states. In both cases, the intensity of the combined beam sensed by the photodetector is related to the amount of interference, which in turn is related to the value of the polarization rotation angle. If the polarization rotation angle is in proportion to the value of a variable to be sensed, e.g., temperature, pressure, stress, glucose concentration, etc., then the sensed intensity is also related to the value of the variable to be sensed. The photodetector linearly converts the laser light intensity, which is the square of the laser electrical field, into a photocurrent. Finally, the photocurrent is analyzed in the frequency domain to get a spectrum of the photocurrent. By measuring harmonics of the modulation frequency from the spectrum, the value of the polarization rotation angle, and thus the value of the sensed variable, can be determined.
With this arrangement, the intensity of the polarized beam is directly proportional to the polarization rotation angle imparted to the combined beam by the variable to be sensed. This is in contrast to other direct measurement techniques wherein the intensity of the received beam is proportional to the square of the rotation angle. As a result, the sensitivity of the device is increased by several orders of magnitude, and the device can thus be employed to measure glucose concentrations, for example, without the use of expensive bulky components. This, in turn, permits the core devices employed in the embodiments of the invention, including the polarization beamsplitter, phase modulator, lenses, etc., to be simple devices that can all implemented on a single silicon chip using MEMS, or similar technology.


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