Electromagnetic wave analyzer

Details Electromagnetic wave analyzer Electromagnetic wave analyzer

2001-05-08

2003-01-07

Allen, Stephone (Department: 2877)

Optics: measuring and testing

By light interference

C356S484000

Reexamination Certificate

active

06504612

ABSTRACT:

TECHNICAL FIELD
The present invention is generally related to the analysis of electromagnetic waves and more particularly, is related to a system and method for analyzing the intensity and phase of light pulses with durations shorter than one nanosecond.
BACKGROUND OF THE INVENTION
Analysis of electromagnetic waves, such as ultrashort laser pulses, is required for fundamental research and applications, such as laser development, semiconductor characterization, combustion diagnostics, and optical coherence tomography. In addition, many material characterization techniques depend upon precise analysis of ultrashort pulses. One of the most promising applications requiring analysis of such waves is communications using intensity-shaped pulses and/or phase-shaped pulses.
For decades autocorrelators were the primary tool used to measure electromagnetic waves such as ultrashort laser pulses. However, autocorrelators are complex instruments having a large number of components, and autocorrelators yield, at best, only vague measurements of the pulse. To measure a pulse with an autocorrelator, the pulse is split into two identical copies of the pulse. The two pulses are then spatially and temporally overlapped in a carefully aligned nonlinear optical medium such as a second-harmonic-generation (SHG) crystal. The relative delay between the pulses must be scanned while maintaining alignment. The alignment involves four sensitive degrees of freedom—two spatial, one temporal, and one crystal angle. The sensitivity of the alignment increases the potential for error in autocorrelators and other pulse analysis systems.
In addition, autocorrelators require a very thin SHG crystal due to bandwidth constraints. The required thin SHG crystals can be expensive, hard to align, difficult to obtain, and troublesome to handle. The alignment and handling requirements of the SHG crystal increase the complexity of the autocorrelator. Thus, the thin SHG crystal is also a potential source of error in pulse analysis systems.
The potential source of error related to thin SHG crystals arises from the need to avoid group velocity mismatch (GVM). GVM is the walking off in time of the pulse and the second harmonic of the pulse due to different group velocities of the wavelength of the pulse and the wavelength of the second harmonic of the pulse. GVM can also be described as a crystal having a finite phase-matching bandwidth that only allows a small range of wavelengths to achieve efficient frequency doubling. The thin SHG crystal must be thin enough that the two pulses overlap throughout the entire thin SHG crystal. As an example, analysis of 100 femtosecond pulses requires an SHG crystal with a thickness of approximately 100 microns. SHG crystals approximately 100 microns thick are difficult to obtain and the thickness is difficult to verify. In addition, SHG crystal efficiency scales as the square of the SHG crystal thickness. Therefore, even when the SHG crystal is sufficiently thin, poor signal strength can limit the sensitivity of the analysis.
Autocorrelators do not measure the full intensity and phase of a pulse. One way to measure the full intensity and phase of an ultrashort laser pulse is a method known as Frequency-Resolved Optical Gating (FROG). FROG is described in U.S. Pat. No. 5,754,292 to Kane and Trebino, and U.S. Pat. No. 5,530,544 to Trebino et al. The '292 and '544 Patents are entirely incorporated herein by reference. The FROG method adds a spectrometer to an autocorrelator. Unfortunately, the addition of the spectrometer further complicates the autocorrelator and increases alignment problems. However, alternatives to FROG are more complex than the FROG apparatus and method. Some of the alternatives require one or more interferometers, or a first interferometer within a second interferometer.
FIG. 1
is a schematic illustration of a prior art ultrashort full pulse measuring device
10
using the FROG methodology. In
FIG. 1
an ultrashort light input pulse
12
is introduced to a beam splitter
14
. The beam splitter
14
produces a probe pulse
13
and a gate pulse
15
. The probe pulse
13
is directed by an optical alignment system
16
through lens
20
into a rapidly responding nonlinear optical medium such as a thin SHG crystal
22
. The gate pulse
15
is provided with a variable delay “&tgr;” by delay line
18
. The probe pulse
13
and the gate pulse
15
are focused into the thin SHG crystal
22
through lens
20
. Thus, beams having electric fields E(t) and E(t-&tgr;) intersect in the thin SHG crystal
22
.
The interaction of the two beams in the thin SHG crystal
22
can occur via many processes and geometries, and many are treated in the prior art including Laser-Induced Dynamic Gratings, by H. J. Eichler et al., Springer-Verlag, New York (1988), which is entirely incorporated herein by reference. Several such geometries are shown in the '544 Patent. In the geometry shown in
FIG. 1
, the thin SHG crystal
22
is phase-matched for noncollinear second harmonic generation. With the thin SHG crystal
22
, neither the probe pulse
13
nor the gate pulse
15
alone achieves significant second harmonic generation in the direction of the signal pulse. However, the probe pulse
13
and the gate pulse
15
together do achieve efficient second order harmonic generation. The gate pulse
15
gates the probe pulse
13
(i.e., gates a temporal slice of the probe pulse
13
). The roles of the probe pulse
13
and the gate pulse
15
may be reversed. It does not matter which pulse is considered as gating the other.
Still referring to
FIG. 1
, a signal pulse (E
sig
(t,&tgr;))
17
is directed to a wavelength-selection device, such as a spectrometer
24
, to resolve the frequency components in the signal pulse
17
. The signal pulse
17
includes selected temporal slices of the probe pulse
13
. A camera
26
records the spectrum of the input pulse
12
as a function of the time delay of the probe pulse
13
to produce an intensity plot vs. frequency (or wavelength) and delay, i.e. the “trace” of the input pulse
12
. The input pulse
12
may be a femtosecond pulse; a negatively chirped pulse (i.e., a pulse with decreasing frequency with time); an unchirped pulse (i.e., a constant frequency pulse); a positively chirped pulse (i.e., a pulse with increasing frequency with time); or any other pulse. The camera
26
records traces corresponding to the input pulse
12
to uniquely determine the intensity and phase characteristics of the input pulse
12
. The trace is a plot of intensity vs. frequency and delay (i.e., a type of spectrogram of the pulse) that is familiar to those of ordinary skill in the art. The trace contains all of the information necessary to reconstruct the intensity and phase characteristics of the input pulse
12
. Without the spectrometer
24
, the ultrashort pulse measuring device
10
of
FIG. 1
is an autocorrelator in which the signal field's energy is measured vs. delay.
The trace is recorded and provided to a processing unit
28
to carry out processing calculations such as those described in the '544 Patent. Such a processing unit
28
may be a digital computer operating in accordance with a stored program, a neural net which is trained to recognize the output of the spectrometer
24
, or numerous other calculating devices known to those skilled in the art, some of which are shown in the '544 Patent.
The operation and features of the prior art analysis systems have been described in various articles including, “Measuring Ultrashort Laser Pulses in the Time-Frequency Domain Using Frequency-Resolved Optical Gating,”
Rev. Sci. Instr
., vol. 68, pp. 3277-3295, 1997 by R. Trebino, K. W. DeLong, D. N. Fittinghoff, J. N. Sweetser, M. A. Krumbügel, and D. J. Kane, which is entirely incorporated herein by reference. Prior art analysis systems are also described in U.S. Pat. No. 5,648,866, and U.S. Pat. No. 6,008,899, both to Trebino et al, which are entirely incorporated herein by reference. In addition, prior art analysis systems are described in U

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