Optics: measuring and testing – By polarized light examination
Reexamination Certificate
1999-12-14
2004-09-07
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
C356S073100
Reexamination Certificate
active
06788410
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a delay time measurement apparatus for an optical element and, more particularly, to a delay time measurement apparatus for an optical element, which includes a wavelength dispersion measurement apparatus for measuring wavelength dispersion that occurs when light passes through an object to be measured such as an optical fiber or the like, and a polarization dispersion measurement apparatus for measuring polarization dispersion that occurs when light passes through the object to be measured.
As is well known, the velocity at which an optical signal propagates in an optical element, for example, an optical fiber, varies depending on the wavelength of the optical signal.
Hence, the pulse width (time duration) of a pulse waveform in an optical pulse signal output from a light source having a wavelength spread is broadened in the optical fiber.
Since the propagation frequency band of the optical fiber is inversely proportional to the pulse width, it finally influences the limitation on the propagation velocity of an optical signal.
Hence, the measurement of the propagation velocity (wavelength dispersion) in the optical fiber in units of wavelengths is a very important test item for the optical fiber.
Especially, since an ultra-fast optical signal beyond 100 Gbits/s, which will be used in the next generation large-capacity optical network, has a pulse width as narrow as several ps (pico seconds), and a large wavelength spread, wavelength dispersion of the optical fiber considerably influences optical transmission.
In a pulse generation technique as well, the generation ratio of high-quality pulses, i.e., transform-limited optical pulses largely depends on the wavelength dispersion of the optical fiber and, hence, wavelength dispersion measurement becomes a more important item.
AS the wavelength dispersion measurement methods, (a) time-resolved spectrometry, (b) a pulse method, (c) an interference method, (d) a difference method, (e) a phase difference method, and the like have been proposed.
Of these methods (a) to (e), the pulse method (b) and interference method (c), which are relatively frequently implemented, will be explained below.
The pulse method proposed by Jpn. Pat. Appln. ROKAI No. 6-174592 will be explained first using FIG.
12
.
As shown in
FIG. 12
, white pulses which are output from a white pulse light source
1
and have a broad wavelength range are wavelength-limited to a specific wavelength by a tunable optical bandpass filter
2
, and are divided into an input optical pulse
4
and reference optical pulse
5
by an optical power divider
3
.
The input optical pulse
4
enters one end of an optical power coupler
7
via a fiber
6
to be measured.
On the other hand, the reference optical pulse
5
directly enters the other end of the optical power coupler
7
.
The optical power coupler
7
outputs a combined optical signal
8
obtained by combining the input optical pulse
4
and reference optical pulse
5
to a delay time detection means
9
.
The delay time detection means
9
calculates a delay time t
D
of the input optical pulse
4
with respect to the reference optical pulse
5
on the basis of the combined optical signal
8
.
More specifically, since the input optical pulse
4
is delayed when it has passed through the fiber
6
to be measured, two peaks form in the signal waveform of the combined optical signal
8
upon combining the input optical pulse
4
and the reference optical pulse
5
free from any time delay.
The time difference between these two peaks is the delay time t
D
detected by the delay time detection means
9
.
By changing a wavelength &lgr; of the tunable optical bandpass filter
2
, the delay time detection means
9
can calculate delay times t
D
(&lgr;) at individual wavelengths &lgr;.
The wavelength dependence of these delay times t
D
(&lgr;) defines wavelength dispersion characteristics.
The pulse method proposed by Jpn. Pat. Appln. KOKAI No. 4-177141 will be explained below using FIG.
13
.
As shown in
FIG. 13
, optical pulses output from an ultra short pulse generation device
11
pass through an optical fiber
12
to be measured, and are then divided into two optical pulses A and B by an optical power divider
13
.
Only a specific wavelength component of one optical pulse A passes through a tunable bandpass filter
14
as a first optical pulse.
On the other hand, the other optical pulse B passes through a delay line
15
as a second optical pulse.
These first and second optical pulses are combined by an optical power coupler
16
, and the combined pulse is converted into an electrical signal by a photosensor
17
.
The electrical signal is input to a pulse waveform observation device
18
to measure the relative delay time difference between the first and second optical pulses as a function of the wavelength, thus obtaining the aforementioned wavelength dependence of the delay time.
A wavelength dispersion measurement apparatus which uses the interference method (c) and is specified by JIS c6827, as shown in
FIG. 14
, is known as an apparatus that implements wavelength dispersion measurement with high precision.
As shown in
FIG. 14
, white light which is output from a white light source
20
and has a broad wavelength range is input to a spectroscope
21
having predetermined spectrum characteristics to extract the component of a specific wavelength &lgr;
C
.
The spectrum characteristics of the spectroscope
21
have a predetermined wavelength spread having a center wavelength &lgr;
C
, as shown in FIG.
15
.
In the spectrum characteristics of this spectroscope
21
, the width 1/e (e: base of natural logarithm) below the peak value is called a half-width.
In this case, the half-width is set to fall within the range from 2 to 10 nm.
Light output from the spectroscope
21
is split into input light A
1
and reference light B
1
by a beam splitter
22
.
This input light A
1
enters an object
23
to be measured such as an optical fiber or the like.
The input light A
1
via the object
23
to be measured is launched into one end of a optical power coupler
26
comprising a half mirror as output light A
2
.
On the other hand, the reference light B
1
is delayed a predetermined period by an optical path delay element
24
, and then passes through a variable optical delay device
25
comprising a corner cube mirror, which is controlled to move in the direction in which light travels. The light output from the delay device
25
is launched into the other end of the optical power coupler
26
as reference light B
2
.
The optical power coupler
26
outputs combined light C obtained by combining the output light A
2
and reference light B
2
to a photosensor
27
.
Note that a lock-in amplifier
28
is provided to amplify with high S/N only a signal output from the photosensor
27
, which is synchronized with an optical chopper incorporated in the spectroscope
21
.
In this case, if the output light A
2
and reference light B
2
have an equal optical path length, the light intensity of the combined light C, i.e., an interference intensity I, increases, and a large signal is output from the photosensor
27
.
Therefore, the delay amount in the variable optical delay device
25
is adjusted to maximize the signal output from the photosensor
27
, i.e., to match the optical path lengths of the output light A
2
and reference light B
2
.
In this case, the delay amount of the reference light B
2
from the reference light B
1
, i.e., the input light A
1
in the optical path delay element
24
and variable optical delay device
25
is known.
Hence, the delay amount of the reference light B
1
at that time is that of the output light A
2
, and the delay time of the object
23
to be measured can be measured from this delay amount.
FIG. 16
is a graph showing the relationship between the interference intensity I between the output light A
2
and reference light B
2
, and an optical path difference L (=|L
1
−L
2
|) between the output light A
2
and reference li
Otani Akihito
Otsubo Toshinobu
Anritsu Corporation
Font Frank G.
Frishauf Holtz Goodman & Chick P.C.
Lee Andrew H.
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