Title: Optical characteristic measuring apparatus, method and recording medium
Abstract: Disclosed is an apparatus for enlarging the range of modulation frequencies that modulate the variable wavelength light generated by the light source without prejudice to the measurement of optical characteristics. A modified modulation frequency computing section computes modified modulation frequencies by multiplying by the initial modulation frequency fmin by the value obtained by dividing the given phase value by the phase difference between the first phase of the transmitted light resulting from the transmission through the DUT of the incident light of the first wavelength modulated by the initial modulation frequency fmin and the second phase of the transmitted light resulting from the transmission through the DUT of the incident light of the second wavelength modulated by the initial modulation frequency fmin. A modified modulation frequency setting section sets the modified modulation frequency as the frequency of the modulating signal so that the frequencies for modulating the incident light may be wider in range than the initial modulation frequency fmin and that the phase difference may be kept at a value equal to or below the given phase value, and the precision of measuring phase differences can be enhanced.
Patent Number: 6,954,263 Issued on 10/11/2005 to Nakamura, et al.
| Inventors:
|
Nakamura; Kenichi (Tokyo, JP);
Kimura; Eiji (Tokyo, JP);
Tomi; Takahisa (Tokyo, JP)
|
| Assignee:
|
Advantest Corporation (Tokyo, JP)
|
| Appl. No.:
|
297204 |
| Filed:
|
June 1, 2001 |
| PCT Filed:
|
June 1, 2001
|
| PCT NO:
|
PCT/JP01/04666
|
| 371 Date:
|
May 22, 2003
|
| 102(e) Date:
|
May 22, 2003
|
| PCT PUB.NO.:
|
WO01/94906 |
| PCT PUB. Date:
|
December 13, 2001 |
Foreign Application Priority Data
| Jun 06, 2000[JP] | 2000-169027 |
| Current U.S. Class: |
356/73.1 |
| Intern'l Class: |
G01N 021/00 |
| Field of Search: |
356/731
385/24-27,39-46,147,123,122
250/227.18,227.23,551
|
References Cited [Referenced By]
U.S. Patent Documents
| 5991477 | Nov., 1999 | Ishikawa et al.
| |
| 6154273 | Nov., 2000 | Suzuki.
| |
| 6594003 | Jul., 2003 | Horiuchi et al.
| |
| Foreign Patent Documents |
| 19724676 | Jan., 1999 | DE.
| |
| 0280328 | Aug., 1988 | EP.
| |
| 6-34447 | Feb., 1994 | JP.
| |
| 9-264814 | Oct., 1997 | JP.
| |
Primary Examiner: Nguyen; Tu T.
Attorney, Agent or Firm: Lowe Hauptman & Berner, LLP
Claims
1. An apparatus for measuring the characteristics of device under test that transmits
light comprising:
a variable wavelength light source for generating a variable wavelength light;
a wavelength setting means for setting said variable wavelength light at a first
wavelength and a second wavelength;
an initial modulation frequency setting means for setting the initial modulation
frequency for modulation;
a modulating signal generating means for generating a modulating signal of a
set modulation frequency;
an optical modulating means for receiving the input of said modulating signal
and modulating said variable wavelength light with the frequency of said modulating
signal;
a phase measuring means for measuring a first phase of a transmitted light, which
is obtained by the transmission through the device under test of an incident light
having the first wavelength and a second phase of said transmitted light, which
is obtained by he transmission through the device under test of an incident light
having the second wavelength;
a modified modulation frequency computing means for computing a modified modulation
frequency by multiplying a value, which is obtained by dividing a give phase value
by the phase difference between the first phase and the second phase, by said initial
modulation frequency; and
a modified modulation frequency setting means for setting said modified modulation
frequency as the frequency of said modulating signal,
wherein the characteristics of device under test are measured on the basis of
the transmitted light resulting from the transmission through said device under
test of the incident light modulated by a frequency set by said modified modulation
frequency setting means.
2. The optical characteristic measuring apparatus according to claim 1, wherein
said initial modulation frequency setting means sets a minimum initial modulation
frequency and said initial modulation frequencies other than said minimum initial
modulation frequency;
said modified modulation frequency computing means computes a modified modulation
frequency by multiplying by said minimum initial modulation frequency a value obtained
by dividing said given phase value by the phase difference between said first phase
and said second phase of said transmitted light resulting from the transmission
through said device under test of said incident light modulated by said minimum
initial modulation frequency; and
said modified modulation frequency setting means sets a maximum said initial
modulation frequency among said initial modulation frequencies equal to or below
said modified modulation frequencies as the frequency of said modulating signal.
3. The optical characteristic measuring apparatus according to claim 1, wherein
there are a plurality of first wavelengths and a plurality of second wavelengths.
4. The optical characteristic measuring apparatus according to claim 3 wherein,
the intervals between said first wavelength and said second wavelength are equal, and
said second wavelength is taken as said first wavelength and furthermore another
second wavelength is taken so that the intervals between said first wavelength
and said second wavelength are equal.
5. The optical characteristics measuring apparatus according to claim 1, wherein
after completing the setting of said first wavelength and said second wavelength,
said modified modulation frequency setting means sets said modified modulation
frequency as the frequency of said modulating signal.
6. The optical characteristics measuring apparatus according to claim 1, wherein
every time when said first wavelength and said second wavelength are set, said
modified modulation frequency setting means sets said modified modulation frequency
as the frequency of said modulating signal.
7. The optical characteristics measuring apparatus according to claim 1, comprising
an optical/electrical conversion means for outputting electrical signals obtained
by optical/electrical conversion of said transmitted light to said phase measuring means.
8. The optical characteristics measuring apparatus according to claim 1, wherein
said phase measuring means measures the phase difference between said modulating
signal and said transmitted light.
9. The optical characteristics measuring apparatus according to claim 1, comprising
a characteristic computing means for computing the group delay or the wavelength
dispersion of said device under test by means of said phase difference measured
by said phase measuring means.
10. A method for measuring the characteristics of device under test that transmits
light comprising:
a variable wavelength light generating step for generating a variable wavelength
light;
a wavelength setting step for setting said variable wavelength light at a first
wavelength and a second wavelength;
an initial modulation frequency setting step for setting the initial modulation
frequency for modulation;
a modulating signal generating step for generating a modulating signal of a set
modulation frequency;
an optical modulating step for receiving the input of said modulating signal
and modulating said variable wavelength light with the frequency of said modulating
signal;
a phase measuring step for measuring the first phase of said transmitted light,
which is obtained by the transmission through the device under test of an incident
light having the first wavelength and the second phase of said transmitted light,
which is obtained by the transmission through the device under test of an incident
light having the second wavelength;
a modified modulation frequency computing step for computing a modified modulation
frequency by multiplying a value, which is obtained by dividing a given phase value
by the phase difference between the first phase and the second phase, by said initial
modulation frequency; and
a modified modulation frequency setting step for setting said modified modulation
frequency as the frequency of said modulating signal,
wherein the characteristics of device under test are measured on the basis of
the transmitted light resulting from the transmission through said device under
test of the incident light modulated by a frequency set by said modified modulation
frequency setting step.
11. The optical characteristic measuring method according to claim 10, wherein
said initial modulation frequency setting step sets a minimum initial modulation
frequency and said initial modulation frequencies other than said minimum initial
modulation frequency;
said modified modulation frequency computing step computes a modulation modified
frequency by multiplying by said minimum initial modulation frequency a value obtained
by dividing said given phase value by the phase difference between said first phase
and said second phase of said transmitted light resulting from the transmission
through said device under test of said incident light modulated by said minimum
initial modulation frequency; and
said modified modulation frequency setting step sets a maximum said initial modulation
frequency among said initial modulation frequencies equal to or below said modified
modulation frequencies as the frequency of said modulating signal.
12. A computer-readable medium having a program of instructions for execution
by the computer to perform a characteristics measuring process for measuring characteristics
of device under test that transmits light, said characteristics measuring process comprising:
a variable wavelength light generating process for generating a variable wavelength
light;
a wavelength setting process for setting said variable wavelength light at a
first wavelength and a second wavelength;
an initial modulation frequency setting process for setting the initial modulation
frequency for modulation;
a modulating signal generating process for generating a modulating signal of
a set modulation frequency;
an optical modulating process for receiving the input of said modulating signal
and modulating said variable wavelength light with the frequency of said modulating
signal;
a phase measuring process for measuring the first phase of said transmitted light,
which is obtained by the transmission through the device under test of an incident
light having the first wavelength and the second phase of said transmitted light,
which is obtained by the transmission through the device under test of an incident
light having the second wavelength;
a modified modulation frequency computing process for computing a modified modulation
frequency by multiplying a value, which is obtained by dividing a given phase value
by the phase difference between the first phase and the second phase, by said initial
modulation frequency; and
a modified modulation frequency setting process for setting said modified modulation
frequency as the frequency of said modulating signal,
wherein the characteristics of device under test are measured on the basis of
the transmitted light resulting from the transmission through said device under
test of the incident light modulated by a frequency set by said modified modulation
frequency setting process.
13. The computer-readable medium according to claim 12,
wherein said initial modulation frequency setting process sets a minimum initial
modulation frequency and said initial modulation frequencies other than said minimum
initial modulation frequency;
said modified modulation frequency computing process computes a modified modulation
frequency by multiplying by said minimum initial modulation frequency a value obtained
by dividing said given phase value by the phase difference between said first phase
and said second phase of said transmitted light resulting from the transmission
through said device under test of said incident light modulated by said minimum
initial modulation frequency; and
said modified modulation frequency setting process sets a maximum said initial
modulation frequency among said initial modulation frequencies equal to or below
said modified modulation frequencies as the frequency of said modulating signal.
Description
TECHNICAL FIELD
The present invention relates to the measurement of the dispersion characteristic
of optical fibers and other optical devices, and in particular to the determination
of the frequency of modulating the incident light to optical devices.
BACKGROUND ART
FIG. 12 is a block diagram showing the configuration of an optical characteristic
measuring apparatus according to the prior art. As shown in FIG. 12, the measuring
system is divided into a light source system 10 and a characteristic measurement
system 20. A variable wavelength light source 12 of the light source
system 10 varies the wavelength to generate a light (variable wavelength
light) having wavelengths of λi and λi+1. The variable wavelength light
will be modulated by a light modulator 14. The light modulator 14
includes LN (lithium niobate). The light modulator 14 receives an electrical
signal having a frequency of fi from a modulation power supply 16 and modulates
the variable wavelength light with the frequency fi.
The light outputted from the light modulator 14 is introduced into an
optical fiber or other DUT (device under test) 30. The transmitted light
having transmitted the DUT 30 will be supplied to an optical/electrical
converter 22 of the characteristic measuring system 20. The optical/electrical
converter 22 proceeds to an optical/electrical conversion of the transmitted
light and outputs to a phase comparator 24. The phase comparator 24
measures the phase of the output signal of the optical/electrical converter 22
with reference to the electrical signal produced by the modulation power supply
16. Here, the phase when the incident light wavelength is λi will
be represented by φi and the phase when the incident light wavelength is
λi+1 will be represented by φi+1. The characteristic computing section
26 will compute the wavelength dispersion characteristic and other characteristics
of the DUT 30 from φi and φi+1.
The operation of the characteristic computing section 26 will be described
with reference to the phase-wavelength diagram shown in FIG. 13. When φi+1-φi
is represented by Δφ, the group delay time is computed from Δφ
and the modulation frequency fi, and then the wavelength dispersion is computed therefrom.
Here, the range of phase difference that can be measured from the phase comparator
24 extends from -π to π. Therefore, it is preferable that φi+1-φi
would be within the range extending from -π to π. This is because any
large modulation frequency fi can easily exceed the range of -π to π.
In other words, when the same time difference is expressed by the phase difference,
the bigger the frequency is, the cycle is shorter, and when it is expressed by
the phase difference, the cycle will be longer. For example, when the time difference
is 1/50 secs., if the frequency is 1 Hz, the range of phase difference is only
0.04π, but if the frequency is 50 Hz, it will be 2π. Therefore, the
modulation frequency fi should be lowered to the minimum possible, and the wavelength
λ of the incident light should be varied.
However, in order to measure Δφ with a high precision, it is
preferable that the modulation frequency fi has a high value. This is due to the
fact that, when a same time difference is expressed with a phase difference, the
larger the frequency is, the shorter the cycle becomes, and when it is expressed
with a phased difference, the cycle will be greater.
Therefore, the present invention has an object of providing devices enabling
to enlarge the modulation frequency that modulates a variable length wavelength
generated by the light source without making problem with respect to the measurement
of the optical characteristic.
DISCLOSURE OF INVENTION
According to the present invention an apparatus for measuring the characteristics
of device under test that transmits light includes: a variable wavelength light
source for generating a variable wavelength light; a wavelength setting unit for
setting the variable wavelength light at a first wavelength and a second wavelength;
an initial modulation frequency setting unit for setting the initial modulation
frequency for modulation; a modulating signal generating unit for generating a
modulating signal of a set modulation frequency; an optical modulating unit for
receiving the input of the modulating signal and modulating the variable wavelength
light with the frequency of the modulating signal; a phase measuring unit for measuring
a first phase of a transmitted light, which is obtained by the transmission through
the device under test of an incident light having the first wavelength and a second
phase of the transmitted light, which is obtained by the transmission through the
device under test of an incident light having the second wavelength; a modified
modulation frequency computing unit for computing a modified modulation frequency
by multiplying the value, which is obtained by dividing the given phase value by
the phase difference between the first phase and the second phase, by the initial
modulation frequency; and a modified modulation frequency setting unit for setting
the modified modulation frequency as the frequency of the modulating signal, wherein
the characteristics of device under test are measured on the basis of the transmitted
light resulting from the transmission through the device under test of the incident
light modulated by a frequency set by the modified modulation frequency setting unit.
The initial modulation frequency is limited to a small value to insure that the
phase difference between the first phase and the second phase will be less than
the given phase value, for example, π. However, the modified modulation frequency
computing unit enables to compute a modified modulation frequency that causes the
phase difference between the transmitted light corresponding to the first wavelength
and the transmitted light corresponding to the second wavelength to coincide with
the given phase value. Therefore, if the frequency modulating the incident light
is chosen as the modified modulation frequency by the modified modulation frequency
setting unit, the phase difference between the transmitted light corresponding
to the first wavelength and the transmitted light corresponding to the second wavelength
will be the given phase value. Therefore, it is possible to measure the phase difference.
And further as the frequency for modulating the incident light can be increased,
the measurement precision of the phase difference can be improved.
The present invention is the optical characteristic measuring apparatus, wherein
the initial modulation frequency setting unit sets the minimum initial modulation
frequency an the initial modulation frequencies other than the minimum initial
modulation frequency; the modified modulation frequency computing unit computes
a modified modulation frequency by multiplying by the minimum initial modulation
frequency the value obtained by dividing the given phase value by the phase difference
between the first phase and the second phase of the transmitted light resulting
from the transmission through the device under test of the incident light modulated
by the minimum initial modulation frequency; and the modified modulation frequency
setting unit sets the maximum the initial modulation frequency among the initial
modulation frequencies equal to or below the modified modulation frequencies as
the frequency of the modulating signal.
The initial modulation frequency is limited to a small value to insure that the
phase difference between the first phase and the second phase will be less than
the given phase value, for example, π. However, the modified modulation frequency
computing unit enables to compute a modified modulation frequency that causes the
phase difference between the transmitted light corresponding to the first wavelength
and the transmitted light corresponding to the second wavelength to coincide with
the given phase value. Therefore, if the maximum initial modulation frequency among
the initial modulation frequencies below the modified modulation frequency is set
as the modified modulation frequency by the modified modulation frequency setting
unit, the phase difference between the transmitted light corresponding to the first
wavelength and the transmitted light corresponding to the second wavelength will
be the given phase value. Therefore, it is possible to measure the phase difference.
And further as the frequency for modulating the incident light can be increased,
the measurement precision of the phase difference can be improved.
The present invention is the optical characteristic measuring apparatus, wherein
there are a plurality of first wavelengths and a plurality of second wavelengths.
The present invention is the optical characteristic measuring apparatus wherein
the intervals between the first wavelength and the second wavelength are equal,
and the second wavelength is taken as the first wavelength and further another
second wavelength is taken so that the intervals between the first wavelength and
the second wavelength are equal.
The present invention is the optical characteristics measuring apparatus, wherein
after completing the setting of the first wavelength and the second wavelength,
the modified modulation frequency setting unit sets the modified modulation frequency
as the frequency of the modulating signal.
The present invention is the optical characteristics measuring apparatus, wherein
every time when the first wavelength and the second wavelength are set, the modified
modulation frequency setting unit sets the modified modulation frequency as the
frequency of the modulating signal.
According to the present invention, the optical characteristics measuring
apparatus includes an optical/electrical conversion unit for outputting electrical
signals obtained by optical/electrical conversion of the transmitted light to the
phase measuring unit.
The present invention is the optical characteristics measuring apparatus, wherein
the phase measuring unit measures the phase difference between the modulating signal
and the transmitted light.
According to the present invention, the optical characteristics measuring
apparatus includes a characteristic computing unit for computing the group delay
or the wavelength dispersion of the device under test by unit of the phase difference
measured by the phase measuring unit.
According to the present invention, a method for measuring the characteristics
of device under test that transmits light includes: a variable wavelength light
generating step for generating a variable wavelength light; a wavelength setting
step for setting the variable wavelength light at a first wavelength and a second
wavelength; an initial modulation frequency setting step for setting the initial
modulation frequency for modulation; a modulating signal generating step for generating
a modulating signal of a set modulation frequency; an optical modulating step for
receiving the input of the modulating signal and modulating the variable wavelength
light with the frequency of the modulating signal; a phase measuring step for measuring
the first phase of the transmitted light, which is obtained by the transmission
through the device under test of an incident light having the first wavelength
and the second phase of the transmitted light, which is obtained by the transmission
through the device under test of an incident light having the second wavelength;
a modified modulation frequency computing step for computing a modified modulation
frequency by multiplying the value, which is obtained by dividing the given phase
value by the phase difference between the first phase and the second phase, by
the initial modulation frequency; and a modified modulation frequency setting step
for setting the modified modulation frequency as the frequency of the modulating
signal, wherein the characteristics of device under test are measure on the basis
of the transmitted light resulting from the transmission through the device under
test of the incident light modulated by a frequency set by the modified modulation
frequency setting step.
The present invention is the optical characteristic measuring method, wherein
the initial modulation frequency setting step sets the minimum initial modulation
frequency and the initial modulation frequencies other than the minimum initial
modulation frequency; the modified modulation frequency computing step computes
a modified modulation frequency by multiplying by the minimum initial modulation
frequency the value obtained by dividing the given phase value by the phase difference
between the first phase and the second phase of the transmitted light resulting
from the transmission through the device under test of the incident light modulated
by the minimum initial modulation frequency; and the modified modulation frequency
setting step sets the maximum initial modulation frequency among the initial modulation
frequencies equal to or below the modified modulation frequencies as the frequency
of the modulating signal.
The present invention is a computer-readable medium having a program of instructions
for execution by the computer to perform a characteristic measuring process for
measuring characteristics of device under test that transmits light, the characteristics
measuring process including: a variable wavelength light generating process for
generating a variable wavelength light; a wavelength setting process for setting
the variable wavelength light at a first wavelength and a second wavelength; an
initial modulation frequency setting process for setting the initial modulation
frequency for modulation; a modulating signal generating process for generating
a modulating signal of a set modulation frequency; an optical modulating process
for receiving the input of the modulating signal and modulating the variable wavelength
light with the frequency of the modulating signal; a phase measuring process for
measuring the first phase of the transmitted light, which is obtained by the transmission
through the device under test of an incident light having the first wavelength
and the second phase of the transmitted light, which is obtained by the transmission
through the device under test of an incident light having the second wavelength;
a modified modulation frequency computing process for computing a modified modulation
frequency by multiplying the value, which is obtained by dividing the given phase
value by the phase difference between the first phase and the second phase, by
the initial modulation frequency; and a modified modulation frequency setting process
for setting the modified modulation frequency as the frequency of the modulating
signal, wherein the characteristics of device under test are measured on the basis
of the transmitted light resulting from the transmission through the device under
test of the incident light modulated by a frequency set by the modified modulation
frequency process.
The present invention is the computer-readable medium, wherein the initial modulation
frequency setting process sets the minimum initial modulation frequency and the
initial modulation frequencies other than the minimum initial modulation frequency;
the modified modulation frequency computing process computes a modified modulation
frequency by multiplying the minimum initial modulation frequency the value obtained
by dividing the given phase value by the phase difference between the first phase
and the second phase of the transmitted light resulting from the transmission through
the device under test of the incident light modulated by the minimum initial modulation
frequency; and the modified modulation frequency setting process sets the maximum
the initial modulation frequency among the initial modulation frequencies equal
to or below the modified modulation frequencies as the frequency of the modulating signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example, and not limitation, in
the figures of the accompanying drawings, wherein elements having the same reference
numeral designations represent like elements throughout and wherein:
FIG. 1 is a block diagram showing the configuration of an optical characteristic
measuring apparatus related to the first preferred embodiment of the present invention.
FIG. 2 is an illustration describing the principle of how the modified modulation
frequency computing section 44 computes the modified modulation frequency fi.
FIG. 3 is a flowchart showing the operation of the first preferred embodiment
of the present invention.
FIG. 4 is a block diagram showing the configuration of an optical characteristic
measuring apparatus related to the second preferred embodiment of the present invention.
FIG. 5 is a flowchart showing the operation of the second preferred embodiment.
FIG. 6 is a phase-wavelength diagram showing the operation of the second preferred embodiment.
FIG. 7 shows the relationship between the phases measured by the phase comparator
24 and the wavelengths of the variable wavelength light when there are three
or more wavelengths of the variable wavelength light in the third preferred embodiment
through the sixth preferred embodiment.
FIG. 8 is a flowchart showing the operation of the third preferred embodiment.
FIG. 9 is a flowchart showing the operation of the fourth preferred embodiment.
FIG. 10 is a flowchart showing the operation of the fifth preferred embodiment.
FIG. 11 is a flowchart showing the operation of the sixth preferred embodiment.
FIG. 12 is a block diagram showing the configuration of an optical characteristic
measuring apparatus according to the prior art.
FIG. 13 is a phase-wavelength diagram according to the prior art.
BEST MODE FOR CARRYING OUT THE INVENTION
The preferred embodiments of the present invention are described with reference
to drawings.
The First Preferred Embodiment
FIG. 1 is a block diagram showing the configuration of the optical characteristic
measuring apparatus related to the first preferred embodiment of the present invention.
The optical characteristic measuring apparatus related to the preferred embodiments
of the present invention includes a light source system 10 for introducing
light to the DUT 30, a characteristic measuring system 20 for receiving
light having transmitted the DUT 30 and measuring the characteristics of
the DUT 30, and a modulation frequency setting system 40 for setting
the modulation frequency.
The light source system 10 includes a variable wavelength light source
12, an optical modulator 14, a modulating power source 16,
and a wavelength setting section 18.
The variable wavelength light source 12 generates a variable wavelength
light. The wavelength of the variable wavelength light varies discretely by means
of the wavelength setting section 18 between the first wavelength of λi
and the second wavelength of λi+1. The light modulator 14 modulates
the variable wavelength light with the frequency of electrical signals generated
by the modulation power supply 16 and supplies the variable wavelength light
to the DUT 30. In the meanwhile, the light modulator 14 includes
LN (lithium niobate). The modulation power source 16 generates electrical
signals for modulating the frequencies set by the modulation frequency setting
system 40. The electrical signals for modulation are supplied to the light
modulator 14 and the phase comparator 24 described below. The wavelength
setting section 18 sets discretely the wavelength of variable wavelength
lights at the first wavelength λi and the second wavelength λi+1.
The DUT 30 is for example an optical fiber. The incident light supplied
to the DUT 30 transmits the DUT 30. The incident light transmitting
the DUT 30 is called "transmitted light."
The characteristic measuring system 20 includes an optical/electrical
converter 22, a phase comparator 24, a characteristic computing section
26 and a modified phase recording section 28.
The optical/electrical converter 22 converts the transmitted light through
the optical/electrical conversion process and generates electrical signals which
are outputted to the phase comparator 24. The phase comparator 24
measures the phase difference between the electrical signals obtained by converting
the transmitted light through the optical/electrical conversion process and the
electrical signals for modulation. The modified phase recording section 28
records the first modified phase φi and the second modified phase φi+1
respectively corresponding to the first wavelength λi and the second wavelength
λi+1 of the incident light when the modified modulation frequency setting
section 46 described later sets the frequency of the modulation power source
16 at fi. The characteristic computing section 26 computes the group
delay characteristic and the wavelength dispersion characteristic of the DUT 30
from the first modified phase φi and the second modified phase φi+1
recorded at the modified phase recording section 28. The group delay characteristic
can be computed from the relationship between the phase measured by the phase comparator
24 and the modulation frequency fi. The wavelength dispersion characteristic
can be obtained by differentiating the group delay characteristic by the wavelength.
The modulation frequency setting system 40 includes an initial phase recording
section 42, a modified modulation frequency computing section 44,
a modified modulation frequency setting section 46, and an initial modulation
frequency setting section 48. The initial phase recording section 42
records the first initial phase φmin_i and the second initial phase φmin_i+1
respectively corresponding to the first wavelength λi and the second wavelength
λi+1 of the incident light when the initial modulation frequency setting
section 48 described later sets the frequency of the modulation power source
16 at fmin. The modified modulation frequency computing section 44
computes the modified modulation frequency fi. The modified modulation frequency
setting section 46 sets the modified modulation frequency fi as the frequency
of the electrical signals for modulation generated by the modulation power source
16. The initial modulation frequency setting section 48 sets the
initial modulation frequency fmin as the frequency of the electrical signals for
modulation generated by the modulation power source 16. The initial modulation
frequency fmin is normally set at a small value so that the difference between
the first initial phase φmin_i and the second initial phase φmin_i+1
may be easily contained within a range of -π to π or 0 to 2π.
Here, the principle of how the modified modulation frequency computing section
44 computes the modified modulation frequency fi will be explained with
reference to FIG. 2. FIG. 2(
a) shows the relationship between
the phase and the wavelength when the modulation frequency is the initial modulation
frequency fmin. As FIG. 2(
a) shows, the difference between the first
initial phase φmin_i and the second initial phase φmin_i+1 is Δφmin_i
and is small. Here, when the modulation frequency f is replaced by the modified
modulation frequency fi (fi>fmin), as shown in FIG. 2(
b), the
difference between the first modified phase φi and the second modified phase
φi+1 is Δφi and is large. This is because, as shown in FIG. 2(
c),
Δφi and Δφmin_i are proportionate to fi/fmin. However,
there will be measurement errors unless Δφi is within the given range.
In other words, in case where the measurable range of the phase comparator 24
is between -π and π, if Δφi exceeds π, there will
errors in the measurement of the phase comparator 24. Therefore, when Δφi
must not exceed π, in the formula of FIG. 2(
c) the computation
of the modified modulation frequency fi by supposing Δφi=π will
give the modified modulation frequency fi as shown in FIG. 2(
d).
If such modified modulation frequency fi is used to modulate the incident light,
the difference between the first modified phase φi and the second modified
phase φi+1 will be approximately π and will be greater than the phase
difference of the initial phase Δφmin_i.
And now the operation of the first embodiment will be described. FIG. 3 is a
flow chart showing the operation of the first embodiment. To begin with, the initial
modulation frequency setting section 48 sets the initial modulation frequency
fmin as the frequency of the electrical signals for modulation generated by the
modulation power source 16 (S10).
And the wavelength setting section 18 sets the frequency of the variable
wavelength light generated by the variable wavelength light source 12 at
the first wavelength λi and the second wavelength λi+1. The light modulator
14 is supplied with the electrical signals for modulation generated by the
modulation power source 16. The variable wavelength light is modulated by
the frequency fmin of the electrical signals for modulation at the light modulator
14 to be supplied to the DUT 30. The transmitted light that has transmitted
the DUT 30 is converted by the optical/electrical conversion process by
the optical/electrical converter 22 to be supplied to the phase comparator
24. The phase comparator 24 measures the phase differences between
the phase of the electrical signals outputted by the optical/electrical converter
22 and the phase of the electrical signals for modulation generated by the
modulation power source 16. These phase differences are the first initial
phase φmin_i and the second initial phase φmin_i+1.
In other words, the phase comparator 24 measures the first initial phase
φmin_i and the second initial phase φmin_i+1 (S12). The first
initial phase φmin_i and the second initial phase φmin_i+1 are recorded
at the initial phase recording section 42. The modified modulation frequency
computing section 44 reads the first initial phase φmin_i and the
second initial phase φmin_i+1 from the initial phase recording section 42
and computes a modified modulation frequency fi (S14). The modified modulation
frequency fi can be computed by using the formula shown in FIG. 2(
d)
when it is desired to limit Δφi at a value equal to or less than π.
If it is desired to keep Δφi at a given value other than π, it
is possible to compute the value of the modified modulation frequency fi by multiplying
the given value by fmin/Δφmin as shown in FIG. 2(
e).
The modified modulation frequency fi is sent from the modified modulation frequency
computing section 44 to the modified modulation frequency setting section
46. The modified modulation frequency setting section 46 sets the
modified modulation frequency fi as the frequency of the electrical signals for
modulation generated by the modulation power source 16 (S16).
Then, the wavelength setting section 18 sets the wavelength of the variable
wavelength light generated by the variable wavelength light source 12 at
the first wavelength λi and the second wavelength λi+1. The light modulator
14 is supplied with the electrical signals for modulation generated by the
modulation power source 16. The variable wavelength light is modulated with
the frequency fi of the electrical signals for modulation at the light modulator
14 to be supplied to the DUT 30. The transmitted light having transmitted
the DUT 30 is converted by the optical/electrical conversion process by
the optical/electrical converter 22 to be supplied to the phase comparator
24. The phase comparator 24 measures the phase differences between
the phase of the electrical signals outputted by the optical/electrical converter
22 and the phase of the electrical signals for modulation generated by the
modulation power source 16. These phase differences are the first initial
phase φi and the second initial phase φi+1. In other words, the phase
comparator 24 seeks the first modified phase φi and the second modified
phase φi+1 (S17). The first modified phase φi and the second
modified phase φi+1 are recorded at the modified phase recording section
28. And the characteristic computing section 26 reads the first modified
phase φi and the second modified phase φi+1 from the modified phase
recording section 28 to compute the group delay or the wavelength dispersion
of the DUT 30 (S18).
According to the first embodiment, it is possible to compute by means of
the modified modulation frequency computing section 46 a modified modulation
frequency fi that will leave the phase difference between the first modified phase
φi and the second modified phase φi+1 at a given phase value, for example
at a value equal to or less than π. Therefore, if the frequency for modulating
the incident light is set at the modified modulation frequency fi by the modified
modulation frequency setting section 46, the phase difference between the
first modified phase φi and the second modified phase φi+1 will be
the required phase value π, and thus the phase difference can be measured.
Moreover, as the frequency for modulating the incident light can be sufficiently
large, the measure precision of the phase difference can be enhanced.
The Second Preferred Embodiment
The second preferred embodiment is different from the first preferred embodiment
in that the modified modulation frequency fi itself is not chosen as the frequency
of the electrical signals for modulation.
FIG. 4 is a block diagram showing the configuration of an optical characteristic
measuring apparatus relating to the second preferred embodiment of the present
invention. The optical characteristic measuring apparatus relating to the preferred
embodiment includes a light source system 10 introducing light into the
DUT 30, a characteristic measuring system 20 for receiving the light
having transmitted the DUT 30 and measuring the characteristics of the DUT
30, and a modulation frequency setting system 40 for setting the
modulation frequency.
The light source system 10 includes a variable wavelength light source
12, a light modulator 14, a modulation power source 16 and
a wavelength setting section 18.
The variable wavelength light source 12 generates variable wavelength
light. The wavelength of the variable wavelength light varies discretely such as
the first wavelength λi and the second wavelength λi+1 by the operation
of the wavelength setting section 18. The light modulator 14 modulates
the variable wavelength light by the frequency of the electrical signals generated
by the modulation power source 16 which will be supplied to the DUT 30.
Incidentally, the light modulator 14 includes LN (lithium niobate). The
modulation power source 16 generates electrical signals for modulating the
frequencies set by the modulation frequency setting system 40. The electrical
signals for modulation is supplied to the light modulator 14 and the phase
comparator 24 described later. The wavelength setting section 18
sets discretely the frequency of variable wavelength light, for example, at the
first wavelength λi and at the second wavelength λi+1.
The DUT 30 is for example an optical fiber. The incident light supplied
to the DUT 30 transmits the DUT 30. The incident light transmitting
the DUT 30 is called "transmitted light."
The characteristic measuring system 20 includes an optical/electrical
converter 22, a phase comparator 24, a characteristic computing section
26 and a modified phase recording section 28.
The optical/electrical converter 22 converts the transmitted light by
the optical/electrical conversion process and generates electrical signals which
will then be supplied to the phase comparator 24. The phase comparator 24
measures the phase difference between the electrical signals obtained by converting
by the optical/electrical conversion process the transmitted light and the electrical
signals for modulation. The modified phase recording section 28 records
the first modified phase φi and the second modified phase φi+1 respectively
corresponding to the first wavelength λi and the second wavelength λi+1
of the incident light when the modified modulation frequency setting section 46
sets the frequency of the modulation power source 16 at any one of fai,
fbi, . . . . Incidentally, fai, fbi, . . . will be described later. The characteristic
computing section 26 computes the group delay characteristic and the wavelength
dispersion characteristic of the DUT 30 from the first modified phase φi
and the second modified phase φi+1 recorded in the modified phase recording
section 28. The group delay characteristic can be computed from the relationship
between the phase measured by the phase comparator 24 and the modified frequency
(any one of fai, fbi, . . . ). The wavelength dispersion characteristic can be
computed by differentiating the group delay characteristic by the wavelength.
The modulation frequency setting system 40 includes an initial phase recording
section 42, a modified modulation frequency computing section 44,
a modified modulation frequency setting section 46, and an initial modulation
frequency setting section 48. The initial phase recording section 42
records the first initial phase φmin_i and the second initial phase φmin_i+1
respectively corresponding to the first wavelength λi and the second wavelength
λi+1 of the incident light when the initial modulation frequency setting
section 48 described later sets the frequency of the modulation power source
16 at fmin. The modified modulation frequency computing section 44
computes the modified modulation frequency fi. The modified modulation frequency
setting section 46 sets the maximum below the modified modulation frequency
fi within fai, fbi, . . . as the frequency of the electrical signals for modulation
generated by the modulation power source 16. The initial modulation frequency
setting section 48 sets the initial modulation frequencies fmin, fai, fbi,
. . . as the frequency of the electrical signals for modulation generated by the
modulation power source 16. Incidentally, the initial modulation frequency
fmin is normally set at a small value so that the difference between the first
initial phase φmin_i and the second initial phase φmin_i+1 may be contained
with sufficient margin within the range between -π and π or between
0 and 2π. And fai, fbi, . . . are set at a larger value than fmin. For this
reason, the initial modulation frequency fmin is called the minimum initial modulation
frequency fmin.
Here, the method by which the modified modulation frequency setting section
46 sets the frequency of the electrical signals for modulation at any one
of fai, fbi, . . . based on the modified modulation frequency fi will be described
with reference to FIG. 2. FIG. 2(
a) shows the relations between
the phase and the wavelength when the modulation frequency f is the initial modulation
frequency fmin. As FIG. 2(
a) shows, the difference between the first
initial phase φmin_i and the second initial phase φmin_i+1 is Δφmin_i
and is small. When the modulation frequency f is replaced by the modified modulation
frequency fi (fi>fmin), as shown in FIG. 2(
b), the difference
between the first modified phase φi and the second modified phase φi+1
is Δφi and is large. This is because, as shown in FIG. 2(
c),
Δφi and Δφmin_i are proportionate to fi/fmin. However,
there will measurement errors unless Δφi is within a given range. In
other words, in case where the measurable range of the phase comparator 24
is between -π and π, if Δφi exceeds π, there will
errors in the measurement of the phase comparator 24. Therefore, for example,
when Δφi must not exceed π, in the formula of FIG. 2(
c)
the computation of the modified modulation frequency fi by supposing Δφi=π
will give the modified modulation frequency fi as shown in FIG. 2(
d).
Then, the modified modulation frequency setting section 46 sets the
maximum below the modified modulation frequency fi among fai, fbi, . . . as the
modulation frequency. The modulation of the incident light by means of such a modulation
frequency will produce the maximum value at or below π for the phase difference
between the first modified phase φi and the second modified phase φi+1,
which will be larger than the phase difference Δφmin_i for the initial phase.
Then, the operation of the second embodiment will be described. FIG. 5 is a
flow chart showing the operation of the second embodiment. To begin with, the initial
modulation frequency setting section 48 sets the initial modulation frequencies
fmin, fai, fbi, . . . as the frequency of the electrical signals for modulation
generated by the modulation power source 16 (S11).
And the wavelength setting section 18 sets the wavelength of the variable
wavelength light generated by the variable wavelength light source 12 at
the first wavelength λi and the second wavelength λi+1. The light modulator
14 is supplied with the electrical signals for modulation generated by the
modulation power source 16. The variable wavelength light is modulated by
the frequency fmin, fai, fbi, . . . of the electrical signals for modulation at
the light modulator 14 and is supplied to the DUT 30. The transmitted
light that has transmitted the DUT 30 is converted by the optical/electrical
conversion process by the optical/electrical converter 22 to be supplied
to the phase comparator 24. The phase comparator 24 measures the
phase differences between the phase of the electrical signals outputted by the
optical/electrical converter 22 and the phase of the electrical signals
for modulation generated by the modulation power source 16. Among these
phase differences, the phase differences corresponding to the incident light modulated
by the minimum initial modulation frequency fmin are the first initial phase φmin_i
and the second initial phase φmin_i+1. And when the frequencies of the electrical
signals for modulation are fai, fbi, . . . , the phase differences measured by
the phase comparator 24 are φai, φbi, . . . .
In other words, the phase comparator 24 computes the first initial phase
φmin_i, the second initial phase φmin_i+1 and the initial phases φai,
φbi, . . . (S13). The first initial phase φmin_i and the second
initial phase φmin_i+1 are recorded in the initial phase recording section
42. The initial phases φai, φbi, . . . are recorded in the modified
phase recording section 28.
Here, the method of computing the first initial phase φmin_i, the second
initial phase φmin_i+1 and the initial phases φai, φbi, . . .
will be described in greater detail with reference to FIG. 6. To begin with,
the wavelengths of the variable wavelength light are set at the first wavelength
λi and the variable frequencies are switched from fmin to fai, fbi, fci,
. . . . And the first initial phase φmin_i and the initial phases φai,
φbi, φci, . . . are measured (S13
a). Incidentally, S13
a
means the first step of S13 shown in FIG. 5. Then, the wavelength
of the variable wavelength light is set at the second wavelength of λi+1,
and the second initial phase φmin_i+1 is measured (Sl3
b). S13
b
means the last step of S13 shown in FIG. 5.
Back in FIG. 5, the modified modulation frequency computing section 44
reads the first initial phase φmin_i and the second initial phase φmin_i+1
from the initial phase recording section 42, and computes the modified modulation
frequency fi (S14). When it is desired to limit the value of Δφi
at a value equal to or below π, the modified modulation frequency fi can
be computed by means of the formula shown in FIG. 2(
d). If it is
desired to contain Δφi at a value other than π, it is possible
to compute the modified modulation frequency fi by multiplying the given value
by fmin/Δφmin as shown in FIG. 2(
e).
The modified modulation frequency fi is sent from the modified modulation frequency
computing section 44 to the modified modulation frequency setting section
46. And equally the initial modulation frequencies fmin, fai, fbi, . . .
are sent from the initial modulation frequency setting section 48 to the
modified modulation frequency setting section 46. There, the modified modulation
frequency setting section 46 sets the maximum frequency at or below the
modified modulation frequency fi among the initial modulation frequencies fmin,
fai, fbi, . . . as the frequency of the electrical signals for modulation generated
by the modulation power source 16 (S20). For example, fai<fbi<fi<fci.
In such a case, as shown in FIG. 6, the modified modulation frequency setting section
46 sets fbi as the frequency of the electrical signals for modulation generated
by the modulation power source 16.
Then, the wavelength setting section 18 sets the wavelength of the variable
wavelength light generated by the variable wavelength light source 12 at
the second wavelength λi+1. The light modulator 14 is supplied with
the electrical signals for modulation generated by the modulation power source
16. The variable wavelength light is modulated at the light modulator 14
by the frequency set by the modified modulation frequency setting section 46
(any one among fai, fbi, . . . ) of the electrical signals for modulation and is
supplied to the DUT 30. The transmitted light that has transmitted the DUT
30 is converted by the optical/electrical conversion process by the optical/electrical
converter 22 and is supplied to the phase comparator 24. The phase
comparator 24 measures the phase differences between the phase of the electrical
signals outputted by the optical/electrical converter 22 and the phase of
the electrical signals for modulation generated by the modulation power source
16. This phase difference is the second modified phase φi+1. Namely,
the phase comparator 24 computes the second modified phase φi+1 (S22).
The method whereby the phase comparator 24 computes the second modified
phase φi+1 will be described in greater details with reference to FIG. 6.
To begin with, let us suppose that the modified modulation frequency setting section
46 has set fbi as the frequency of the electrical signals for modulation
generated by the modulation power source 16. And the phase comparator 24
computes the second modified phase φi+1 (S22). In this case, since
the second wavelength λi+1 has been chosen as the wavelength of the variable
wavelength light, the phase at the time when the modulation frequency is fbi and
the wavelength of the variable wavelength light is λi+1 is measured. In this
case, for any frequencies other than f=fbi, the phase at the time when the wavelength
of the variable wavelength light is the second wavelength λi+1 is not measured.
The second modified phase φi+1 is recorded in the modified phase recording
section 28. Here, the first modified phase φi corresponds to the modulation
frequency set by the modified modulation frequency setting section 46 among
the initial phases φai, φbi, . . . . For example, if the modified modulation
frequency setting section 46 has set fbi as the modulation frequency as
shown in FIG. 6, the first modified phase φi is the initial phase φbi.
Therefore, we can assume that the first modified phase φi has already been
recorded in the modified phase recording section 28. Therefore, the characteristic
computing section 26 reads the first modified phase φi and the second
modified phase φi+1 from the modified phase recording section 28 and
computes the group delay or the wavelength dispersion of the DUT 30 (S24).
According to the second embodiment, it is possible to compute by means
of the modified modulation frequency computing section 46 a modified modulation
frequency fi so that the phase difference between the first modified phase φi
and the second modified phase φi+1 could be equal to or below the given phase
value, for example π. And if the maximum frequency equal to or below the
modified modulating frequency fi among the initial modulation frequencies fai,
fbi, . . . is adopted for the frequency for modulating the incident light by the
modified modulation frequency setting section 46, the phase difference between
the first modified phase φi and the second modified phase φi+1 is equal
to or below the given phase value π, and therefore it is possible to measure
the phase difference. Moreover, the possibility of choosing a sufficiently wide
frequency for modulating the incident light enables to enhance the precision of measurements.
In addition, the fact that the wavelength of the variable wavelength light is
changed for once can economize the time required for changing wavelength. For example,
let us suppose that it takes about 3 secs to change the wavelength of the variable
wavelength light and about 10 ms to change the modulation frequency. Then, as the
time required to change the wavelength is considerably longer than that to change
