Optical sampler

Details Optical sampler Optical sampler

1999-07-07

2001-10-16

Lee, John R. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C359S264000, C324S076360, C324S096000

Reexamination Certificate

active

06303926

ABSTRACT:

This patent application claims priority based on a Japanese patent application, H10-191276 filed on Jul. 7, 1998, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical sampler that produces a low speed signal based on a high speed electric signal so that the high speed electric signal can be analyzed using the low speed electric signal.
2. Description of the Related Art
FIG. 1
schematically shows the structure of a typical optical sampler. In the figure, the signal source
1000
generates: a signal Vin, an electric signal changing at high speed, which is measured by the optical sampler. The optical sampler has an optical modulator
2000
, a laser pulse source
3000
, a polarizer
4000
, an analyzer
5000
, an optical detector
6000
, a detecting circuit
7000
, ad a holding circuit
6000
. The optical modulator
2000
is of a bulk type, including, for example, an electrooptical effect crystal
2100
, which can be made from LiNbO
3
, and a pair of electrodes
2200
A and
2200
B opposing to each other.
FIG. 2A
shows a waveform of an optical pulse PF output by the laser pulse source
3000
,
FIG. 2B
shows a waveform of the signal Vin output by the signal source
1000
, and
FIG. 2C
shows a waveform of the low speed signal LO output by the holding circuit
8000
.
The optical pulse PF provided by the laser pulse source
3000
is polarized by the polarizer
4000
, which feeds a polarized optical signal Pin to the optical modulator
2000
. The sampling frequency of the optical pulse PF is set to be higher than the signal frequency of the signal Vin. Meanwhile, the signal Vin generated by the signal source
1000
is fed to the optical modulator
2000
, whereby an electric field is applied to the electrooptical effect crystal
2100
using the pair of electrodes
2200
A and
2200
B.
The direction of the electric field is right-angled to the direction in which the optical signal Pin advances. Hence, by providing the polarization plane electric field to the electrooptical effect crystal
2100
, the polarization plane of the optical signal Pin advancing through the electrooptical effect crystal
2100
is rotated according to the electric field. In other words, the angle of the polarization plane of the optical signal Pin is defined by the electric field, that is to say, by the signal Vin. In this way, the signal Vin is sampled using the polarized optical signal Pin, that is, the polarized optical signal Pin is modulated by the signal Vin. After being rotated, the optical signal Pin is input to the analyzer
5000
which subsequently outputs an optical signal Pout.
FIG. 3
shows a relationship between the signal Vin and the a ratio of the signal Pin to the signal Pout. The relationship is represented by a sine curve. In
FIG. 1
, it is assumed that the polarizer
4000
and the analyzer
5000
are set so that the difference between the angle of the polarization plane of the polarizer
4000
and that of the analyzer
5000
is 45 degrees. If there is no an electric field, that is to say, there is no signal Vin, a signal Pout corresponding to Vin=0 is fed from the analyzer
5000
.
In
FIG. 3
, the characteristic near Vin=0 is as follows. If the signal Vin is positively applied, the polarization plane of the optical signal Pin is rotated clockwise by +&thgr;. Thus, the optical signal Pout is increased according to the sine curve. For example, an optical signal Pout corresponding to Vin=Vp is output. On the contrary, if the signal Vin is negatively applied, the polarization plane of the optical signal Pin is rotated counterclockwise by &thgr; (clockwise by −&thgr;) . Thus, the optical signal Pout is decreased according to the sine curve. For example, the optical signal Pout corresponding to Vin=Vn is output. In summary, the ratio of the optical signal Pin to the optical signal Pout depends upon the angle of the polarization plans given by the optical signal Pin, and therefore depends upon the signal Vin.
The optical signal Pout fed by the analyzer
5000
undergoes optical/electric conversion in the optical detector
6000
, whereby an electric signal corresponding to the optical signal Pout is produced and fed into the detecting circuit
7000
. After receiving the electric signal, the detecting circuit
7000
amplifies it to output the amplified electric signal to the holding circuit
8000
. The holding circuit
8000
carries out sampling/holding on the electric signal to provide the low speed signal LO. The frequency of the low speed signal LO is a beat frequency. In other words, the frequency is equal to the difference between the frequency of the signal Vin and the frequency of the optical pulse PF or the optical signal Pin. Accordingly, the change in the signal Vin can be represented by the low signal LO.
FIG. 4
shows the structure of an optical interferometer type optical modulator
9000
. Unlike the optical modulator
2000
of
FIG. 1
, the optical modulator
9000
comprises a plate
9100
made of electrooptical effect crystal such as LiNbO
3
. The optical modulator
9000
further includes an input port
9000
A, an output port
9000
B, a division port
9000
C, a combination port
9000
D, and optical paths
9000
E-
1
and
9000
E-
2
. The input port
9000
is formed on a side of the plate
9100
while the output port
9000
B is formed on the opposite side thereof. The division port
9000
C and the combination port
9000
D are formed between the input port
9000
A and the output port
9000
B, wherein the optical paths
9000
E-
1
and
9000
E-
2
are formed in parallel with each other therebetween.
An electrode
9200
A is formed along the optical path
9000
E-
1
, an electrode
9200
B is formed between the optical path
9000
E-
1
and the optical path
9000
E-
2
, and an electrode
9200
C is formed along the optical path
9000
E-
2
. The signal Vin generated by the signal source
1000
is applied across the electrodes
9200
A and
9200
B, while both the electrodes
9200
B and
9200
C are grounded. The laser pulse source
3000
is connected to the input port
9000
A, and the optical detector
6000
is connected to the output port
9000
B.
The optical pulse PF generated by the laser pulse source
3000
Is fed into the input port
9000
A to be divided into two components at the division port
9000
C. One component advances along the optical path
9000
E-
1
, while the other component advances along the optical path
9000
E-
2
. The former component changes in propagation velocity through the electric field formed by the signal Vin, while the velocity of the latter component remains unchanged. Hence, the components interfere with each other according to the change of the former component in the propagation velocity at the combination port
9000
D. The light that is phase-modulated along the optical path
9000
E-
1
is combined at the combination port
9000
D to be an amplitude modulation light by interference. Consequently, the amplitude modulation light, that is, an intensity modulation light is output from the output port
9000
B.
FIG. 5
shows a relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. Here, unlike the above bulk-type optical modulator accompanied by the polarizer, the optical signal PF is identical with the optical signal Pin. The relationship is given on assumption that the length of the optical path
9000
E-
1
and that of the optical path
9000
E-
2
are equivalent to each other. In the figure, with respect to the characteristic when the signal Vin is zero or near zero, Vin=0 provides the maximum ratio while both Vin>0 and Vin<0 provides other ratios smaller than the maximum ratio.
FIG. 6
shows another relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. The relationship is given based on an assumption that the length of the optical path
9000
E-
1
differs from that of the optical path
9000
E-
2
by &lgr;/4 where &lgr; denotes the wavelength of ligh

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