Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
1999-07-08
2001-05-29
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S005050, C356S005080
Reexamination Certificate
active
06239866
ABSTRACT:
DESCRIPTION
The present invention relates to a system (method and apparatus) for making temporal measurements of ultrashort far infrared electromagnetic pulses (hereinafter referred to as terahertz or THz pulses) which may be, for example, sub picosecond pulses, which in the frequency domain, may be in the terahertz (THz) frequency range. The term “electromagnetic” includes “optical,” whether or not visible. More particularly, the present invention provides a photoconductive sampling system for THz pulse measurement which decouples the sensitivity and bandwidth requirements of the measurement through the use of a triggered photoconductive attenuator and a photoconductive detector or receiver, both synchronously operated by optical pulses. The attenuator provides a rapid decrease in transmission (“turn-on”). The attenuator, by virtue of its rapid turn-on, provides the bandwidth requisite for terahertz pulse sensitivity, while the photoconductive receiver provides requisite measurement sensitivity. Inasmuch as the receiver is not responsible for the temporal resolution of the system, the temporal response of photoconductive receivers may be measured directly by utilizing them in the system.
Heretofore, ultrashort pulse measurements were carried out by photoconductive sampling (PCS) and electro-optic sampling. In both such techniques there is a trade off between sensitivity and bandwidth. In photoconductive sampling, bandwidth is a function of the photoconductive material carrier lifetime, which affects the mobility of the carriers in the photoconductive material. The optical antenna geometry of the receiver also controls the sensitivity and bandwidth. In electro-optical sampling, a nonlinear crystal is used, of a length, which determines the resolution and sensitivity of the measurement. For example, the signal measured by photoconductive sampling PCS is the cross correlation of the THz electric field (E) and the response of the receiver (R). The PCS signal (S) &tgr; is shown in the following equation:
S
(
τ
)
=
∫
-
∞
+
∞
E
(
t
)
R
(
t
-
τ
)
ⅆ
t
(
1
)
where &tgr; is the delay between the arrival of the THz pulse at the receiver, and the optical pulse which actuates the receiver.
THz pulse measurements have heretofore used the edges of control or sync pulses to gate the onset of photo conductivity of a photoconductive switch element (a so-called photoconductive attenuator) to provide temporal resolution. Such techniques measured only the integrated power of the THz pulse. To obtain the magnitude of the electric field, but not the phase, a numerical derivative had to be performed which tightly constrained the signal to noise ratio.
In accordance with the present invention the sync optical pulse activates the photo conductive attenuator, but the delay of the sync optical pulse to the attenuator is modulated. For example, the delay of the optical trigger pulse is modulated at a rate of from 40 to 100 Hz, for example, with an amplitude of 100 fs.
If a THz pulse is incident on such an attenuator, the transmitted electric field if given by E(t) G(t−&tgr;
G
), where E(t) is the electric field of the incident pulse, and G (t−&tgr;
G
) is the attenuator's time-dependent transmissivity or ‘edge function’, with the edge occurring at time &tgr;
G
. The transmitted THz pulse is then measured using a receiver with a temporal response R (t−&tgr;
R
) where &tgr;
R
is the delay between the time of arrival of the THz pulse and the receiver's optical gate pulse.
The photocurrent signal is given by,
S
(
τ
G
,
τ
R
)
=
∫
-
∞
∞
E
(
t
)
G
(
t
-
τ
G
)
R
(
t
-
τ
R
)
ⅆ
t
.
(
2
)
E(t) is recovered from this signal in the following way. The time at which the edge occurs, &tgr;
G
, is modulated at frequency &OHgr; with amplitude &dgr;&tgr;, so &tgr;
G
=&tgr;
0
−&dgr; cos(&OHgr;t). Also, the receiver trigger time, &tgr;
R
, is set so that the maximum of the receiver's response, R, is coincident with the rapid decrease in G, (i.e., &tgr;
R
=&tgr;
0
+&tgr;
1
). In addition, a lock-in amplifier is used to measure the component of S that is modulated at the dither frequency, &OHgr;. One can obtain an expression for this component if one replaces G with a taylor expansion in &dgr;&tgr; cos (&OHgr;t) about t−&tgr;
0
, and keeps terms modulated at dither frequency. For samll dither amplitudes this signal is
S
Ω
(
τ
0
,
τ
1
)
=
δτ
∫
-
∞
∞
E
(
t
)
G
′
(
t
-
τ
0
)
R
(
t
-
τ
0
-
τ
1
)
ⅆ
t
,
(
3
)
where G′(t) is the derivative of G(t) with respect to time. If the drop in the edge function occurs sufficiently quickly, its derivative is a narrow sampling window centered on the time &tgr;
0
. In the limit of an extremely rapid edge, G′ becomes a delta function. Then S
&OHgr;
(&tgr;
0
,&tgr;
1
) is given by,
S
&OHgr;
(&tgr;
0
, &tgr;
1
)=&dgr;&tgr;
E
(&tgr;
0
)
R
(−&tgr;
1
). (4)
To map out the electric field E, one slowly varies &tgr;
0
; to map out the receiver response, R, one slowly varies &tgr;
1
. In this way, the speed of the attenuator combined with the high sensitivity of the gated photoconductive receiver allows one to sample THz pulses with a much finer temporal resolution than is provided by the receiver alone.
Accordingly, it is the principal object of the present invention to provide an improved system for measuring optical pulses, and particularly pulses in the terahertz (100 fs) range.
REFERENCES:
patent: 5489984 (1996-02-01), Hariharan et al.
patent: 5585913 (1996-12-01), Hariharan et al.
N. Katzenellenbogen and D. Grischkowsky, in Ultra Wideband, Short Pulse Electromagnetics. Proceedings of an International Conference, p. xi+542, 1993.
G. Mourou, C.V. Stancampiano, and D. Ny Usa Blumenthal, in Applied Physics Letters 38 (6), 470-2, 1981.
F.G. Sun, G.A. Wagoner, and X.C. Zhang, in Applied Physics Letters 67 (12), 1656-8, 1995.
D. You, R.R. Jones, P.H. Bucksbaum et al., in Optical Letters 18 (4), 290-2, 1993.
Bromage Jake
Walmsley Ian
Buczinski Stephen C.
Lukacher Kenneth J.
Lukacher Martin
The University of Rochester
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