Optics: measuring and testing – By light interference – Having wavefront division
Reexamination Certificate
1999-07-14
2001-03-20
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
Having wavefront division
C356S499000, C356S494000, C356S488000
Reexamination Certificate
active
06204926
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to optical correlation techniques for characterizing materials and optical waveforms.
Modern laser technology permits the routine generation of ultrashort optical pulses, i.e., pulses having a duration of less than about 1 psec. Some lasers can even generate pulses as short as about 10 fsec. More generally, modern laser systems can produce ultrafast optical waveforms that have features as short as ultrafast pulses, e.g., a terahertz train of ultrashort pulses. See, e.g., U.S. Pat. Nos. 5,682,262 and 5,719,650. Such ultrashort waveforms (including single pulse waveforms) can be used to probe chemical and physical phenomena in atoms, molecules, and materials. Unfortunately, the time scales for such measurements and for the optical waveforms themselves exceed the bandwidth of most, if not all, electronic detectors. As a result, many measurements involve optical correlation techniques in which two or more waveforms overlap on a sample or non-linear optical crystal.
SUMMARY OF THE INVENTION
The invention features methods and systems for optical correlation of ultrashort optical waveforms, e.g., pulses. The optical waveform passes through a diffractive optic, e.g., a mask or grating, to generate multiple sub-beams corresponding to different diffractive orders. At least two of the sub-beams are then imaged onto the sample to produce a desired crossing pattern. To perform the correlation, the diffracted sub-beams are variably delayed relative to one another prior to overlapping on the sample. The sample generates a signal beam in response to the overlapping sub-beams, the signal beam providing a correlation between the sub-beams for each of the variable delays.
In general, in one aspect, the invention features a method for autocorrelating an optical waveform. The method includes: passing an input beam containing the optical waveform through a diffractive mask to form at least two sub-beams; delaying one of the sub-beams relative to the other sub-beam; and imaging the two sub-beams onto a non-linear optical crystal to allow the two sub-beams to spatially overlap with one another. The diffractive mask defines the object plane and the non-linear optical crystal defines the image plane. The overlapping sub-beams are delayed relative to one another, and the non-linear optical crystal generates a signal beam in response to the overlapping sub-beams.
The method can include any of the following features. The method can further include measuring the intensity of the signal beam and repeating the measuring step for each of multiple delays between the sub-beams. The method can further include spectrally resolving the signal beam and measuring the intensity of the spectrally resolved signal beam, and repeating the resolving and measuring steps for each of multiple delays between the sub-beams. The non-linear optical crystal can generate the signal beam by second harmonic generation or by any other non-linear optical mechanism. The delaying step can include introducing material into a path of one of the sub-beams. The imaging step can include passing the sub-beams through a pair of lenses. The optical waveform can have temporal features shorter than about 1 psec, shorter than about 300 fsec, or even shorter than about 50 fsec. The optical waveform can be an optical pulse. The two sub-beams can correspond to different orders of diffraction for the diffractive mask.
In general, in another aspect, the invention features an optical autocorrelator for characterizing an an optical waveform. The autocorrelator includes: a diffractive mask which during operation diffracts an input beam carrying the optical waveform into at least two sub-beams; an optical delay assembly positioned in the path of a first of the two sub-beams, wherein during operation the optical assembly introduces a variable delay between the two sub-beams; a non-linear optical crystal; an optical imaging system which during operation images the two sub-beams onto the non-linear optical crystal to allow the two sub-beams to spatially overlap one another, the diffractive mask defining a object plane and the non-linear optical crystal defining the image plane; and an analyzer which during operation measures an intensity of a signal beam produced by the non-linear optical crystal in response to the two overlapping sub-beams.
The autocorrelator can include any of the following features. The autocorrelator can further include a controller connected to the optical delay assembly and the analyzer, wherein during operation the controller causes the optical delay assembly to introduce multiple delays between the two sub-beams and records the intensity of the signal beam for each of the multiple delays. The optical delay assembly can include an optical window positioned in the path of the first sub-beam and a rotation stage supporting and adjustably orienting the optical window, the adjustable orientation of the optical window defining the variable delay between the two sub-beams. The autocorrelator can further include a stationary optical window positioned in the path of the second of the two sub-beams to impart a fixed delay to the second sub-beam. The analyzer can include a grating and a multi-element photodetector, wherein during operation the grating spectrally resolves the signal beam on the photodetector and the photodetector records the spectrally resolved intensity of the signal beam. Alternatively, the analyzer can be a photodetector. The non-linear optical crystal can generate the signal beam by second harmonic generation or by any other non-linear optical technique. The optical imaging system can include a pair of lenses and the optical delay assembly can be positioned between the pair of lenses. Each of the pair of lenses can be a spherical lens. The two sub-beams can correspond to different orders of diffraction for the diffractive mask.
Embodiments of the invention include many advantages. For example, the correlation technique optimizes the overlap of the two sub-beams on the sample (e.g., the non-linear crystal) and thereby greatly simplifies alignment and robustness of the optical correlation system.
Other features, aspects, and advantages follow.
REFERENCES:
patent: 5734470 (1998-03-01), Rogers et al.
patent: 6043886 (2000-03-01), Bruning
Ippen et al, “Dynamic Spectroscopy and Subpicosecond Pulse Compression.”, Applied Physics Letters, p. 488-490, Nov. 1975.*
Rogers et al., “Optical System for Rapid Materials Characterization with the Transient Grating Technique: Application to Nondestructive Evaluation of Thin Films Used in Microelectronics,”Appl. Phys. Lett. 71:225-227, Jul. 14, 1997.
Crimmins Timothy F.
Maznev Alexi A.
Nelson Keith A.
Fish & Richardson P.C.
Kim Robert
Massachusetts Institute of Technology
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