Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1998-12-22
2001-01-30
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S484000
Reexamination Certificate
active
06181429
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to interferometers, and is directed toward an interferometer which is useful for measuring optically induced changes of the optical properties in samples of materials under study, e.g., temporally resolved optical nonlinearities. More specifically, the invention is directed to a hybrid interferometer which uses optical guides in the reference path and a combination of optical guides and free space propagation in the measurement arm.
BACKGROUND OF THE INVENTION
The role of nonlinear materials in high-speed applications such as optical switching, amplification, limiting and frequency conversion has created a need for an efficient method of characterizing nonlinear parameters. Many of these parameters can be characterized by the analysis of the index of refraction of a material. In particular, semiconductor materials exhibit a broad range of nonlinear effects with response times that span several orders of magnitude, owing to electronic nonlinearities, free-carrier effects, and thermal nonlinearities. Other materials may also exhibit properties which change over time, e.g., due to optical interaction or to environmental factors, and which also change the materials' index of refraction.
The presence of two or more nonlinear mechanisms can complicate the interpretation of optical nonlinearities because many techniques cannot distinguish between them. Quantitative information concerning the nonlinear index of refraction for optical materials is essential for the development of all-optical devices, such as opto-optical switches. Several techniques have been proposed for conducting this measurement, most of which are based on a direct interferometric measurement that uses a pump and probe technique.
One technique is to analyze temporal interference fringes to obtain the nonlinear index of refraction, as described in “Nonlinear-Index-Of-Refraction Measurement In A Resonant Region By The Use Of A Fiber Mach-Zehnder Interferometer”, Applied Optics, Vol. 35, No. 9, Mar. 20, 1996, pages 1485-88. This technique uses fiber light guides in both the reference and measurement arms. Also included in each arm is an adjustable delay unit (AD) based on an optical fiber pigtailed graded index rod-lens pair, to vary the optical length of each arm.
The inventors have found that this technique is difficult to use to do measurements of bulk sample properties because of the difficulty in preparing an interface between the light guides in the measurement path and the sample to be measured. Often, it is possible that installing connecting light guides to the sample will result in some shift of its electrical properties. In addition, some samples cannot be connected directly to optical light guides.
Furthermore, according to this technique the pump pulses propagate in the optical fibers comprised in the interferometer arms; the inventors have observed that this sets a limit to the maximum pump power available for the measurements.
Another technique is disclosed in “Time-Resolved Absolute Interferometric Measurement Of Third-Order Nonlinear-Optical Susceptibilities”, Journal of the Optical Society of America B, Vol. 11, No. 6, June 1994, pages 995-999. This technique, as illustrated in
FIG. 1
of the paper, uses free space propagation of optical signals to measure nonlinear optical properties of bulk materials. A Mach-Zehnder interferometer compares the two beams (probe and reference) in amplitude and phase. The sample is located in the probe arm and interacts with the stronger collinear pump beam. The time delay &tgr; between the pump and probe pulses provides the basis for a sampling interferometry.
The inventors have observed that the above techniques has disadvantages linked with using an optical measurement system wherein the light propagates completely in free space; in particular it is bulky and it needs careful alignment of all the optical components, what renders this technique difficult to use.
Other discussions of measurement of nonlinear properties can be found in “Femtosecond Time-Resolved Interferometry For The Determination Of Complex Nonlinear Susceptibility”, Optics Letters, Vol. 16, No. 21, Nov. 1, 1991, pages 1683-1685 and “Interferometric Measurement Of The Nonlinear Index Of Refraction n
2
Of CdS
x
Se
1−x
-Doped Glasses”, Applied Physics Letters, Vol. 48, No. 18, May 5, 1986, pages 1184-1186.
U.S. Pat. No. 5,268,739 discloses a laser apparatus for measuring the velocity of a fluid. In the system disclosed, a laser beam is fed into a pipe through which a fluid is flowing. Particles in the fluid interfere with the light. The velocity of the fluid is calculated from this interference.
SUMMARY OF THE INVENTION
Applicant has found that the optical properties of a sample can be measured without the need to attach light guides to the sample, while taking advantage of the beneficial properties of using light guides in a measurement apparatus, by using a hybrid interferometer which has a combination light guide and free space light path in its measurement arm. This arrangement greatly simplifies the testing of nonlinear optical properties of the samples under consideration.
More specifically, the inventors have developed a hybrid interferometer with a reference arm comprised of light guide paths and a measurement arm comprised of a combination of light guide paths and a free space area where the sample under test is located, and where coupling of a pump beam to the sample is conducted in free space.
According to a first aspect the present invention is related with an interferometer comprising:
a first optical source for use as a source of a probe beam;
a reference arm comprised of one or more optical guides for guiding a light signal from the first optical source to an output detector,
a measurement arm comprised of a plurality of optical guides, a lens system, and a free space area for mounting a sample under test, so that the probe beam is guided through the sample;
a second optical source for use as a source of a pump beam to be provided to the sample in the free space area; and
a photodetector for detecting the changes in the optical properties of the sample by means of comparing the signal received from the reference arm with the signal received from the measurement arm.
In a preferred embodiment the optical guides are single mode optical fibers. A polarization controller is preferably included along one of said measurement or reference arm. Alternatively, the optical guides can be polarization maintaining optical fibers.
The interferometer preferably includes a coupler for combining the signal received from the reference arm and the signal received from the measurement arm into an interference signal and for coupling said interference signal to the photodetector.
According to preferred embodiments, the probe and pump beam are collinear within the sample, and the interferometer includes a selective reflector in the free space area for reflecting the pump beam to the sample and for transmitting the probe beam.
Possible embodiments for the selective reflector are a dichroic mirror or a polarizer.
A selective transmission device is preferably included in the free space area, for transmitting the probe beam and for preventing the pump beam from entering the optical guides. Possible embodiments for the selective transmission device are a dichroic mirror or a polarizer.
The interferometer can have a feedback circuit which includes a piezo controller for maintaining the interferometer in its quadrature condition. Also, the interferometer can have means for periodically modulating the phase of the signal along one of said reference or measurement arms.
According to a second aspect the present invention is related with a method of measuring the optical properties of a sample, comprising:
generating a probe laser beam;
propagating a portion of the probe beam in a first optical fiber and another portion of the probe beam in a second optical fiber;
mounting the sample in a free space area along said second fiber;
illuminating the sample in free
Barberis Angelo
Caselli Stefano
Martinelli Mario
Pietralunga Silvia Maria
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kim Robert
Pirelli Cavi E Sistemi S.p.A.
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