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Claims  |
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What is claimed is:
1. An optical frequency analyzer for measuring frequency characteristics of
an object on the basis of incident light, said optical frequency analyzer
comprising
means for applying incident light to an object;
an optical frequency sweeping means for frequency sweeping light and for
outputting frequency swept light of a first frequency;
an optical heterodyne detector means for heterodyning said frequency swept
light of said first frequency from said optical frequency sweeping means
and light of a second frequency indicative of said incident light, and for
outputting an electrical signal having a frequency corresponding to the
difference between said first and second frequencies;
first filter means for filtering said electrical signal from said optical
heterodyne detector means and for producing an electrical output
indicative of the filtered results; and
a signal processing/displaying means for processing and displaying said
electrical signal from said first filter means.
2. The analyzer of claim 1, further comprising a polarization control means
for controlling the polarization plane of said incident light and for
producing an output light indicative thereof; and photo amplifying means
for amplifying said output light from said polarization control means and
for producing an output light indicative thereof.
3. The analyzer of claim 2, wherein said photo amplifying means comprises a
photo amplifier for producing an amplified output light; a source for
generating a wavelength stabilizing light; and an optical frequency mixer
for mixing said wavelength stabilizing light from said source and said
amplified output light from said photo amplifier.
4. The analyzer of claim 1, further comprising detecting means for
detecting said electric output from said first filter means and for
producing an output; means for measuring an optical frequency spectrum of
said incident light; and means for causing said output from said detecting
means to be an optical power input to said signal processing/display means
and for causing an electrical signal indicative of said swept light from
said optical frequency sweeping means to be a frequency axial input to
said signal processing/display means.
5. The analyzer of claim 4, further comprising means for generating a
periodic pulse light; means for causing the frequency of said swept light
from said optical frequency sweeping means to be swept in a step
configuration in response to a signal synchronized with said period pulse
light so as to measure the frequency spectrum of said periodic pulse
light.
6. The analyzer of claim 1, wherein said optical frequency sweeping means
comprises a tunable laser for producing output light corresponding to said
swept light; and a marker light source for outputting a marker light at a
given wavelength; wherein said signal processing/display means comprises
means for outputting frequency characteristics of said object together
with said marker light.
7. The analyzer of claim 6, further comprising a light detector means for
converting said marker light into an electrical signal; and means for
causing said electrical signal from said detector means to be applied as a
marker signal to said signal processing/display means.
8. The analyzer of claim 6, wherein said marker light source comprises an
absorption cell comprising a standard substance, means for causing output
light from said tunable laser to strike said absorption cell, and means
for transmitting said marker light to undergo absorption at a specified
wavelength corresponding to said standard substance.
9. The analyzer of claim 6, wherein said marker light source comprises a
Fabry-Perot resonator, means for applying said output light from said
tunable laser to said Fabry-Perot resonator, and means for outputting
light from said Fabry-Perot resonator as said marker light.
10. The analyzer of claim 9, wherein said Fabry-Perot resonator comprises
an electro-optic element and means for controlling said electro-optic
element to thereby control the effective length of said Fabry-Perot
resonator.
11. The analyzer of claim 6, wherein said marker light source comprises a
source for producing light of consecutive spectrums, a Fabry-Perot
resonator, means for applying light from said source to said Fabry-Perot
resonator, and means for causing light output from said Fabry-Perot
resonator to be said marker light.
12. The analyzer of claim 11, wherein said Fabry-Perot resonator comprises
an electro-optic element and means for controlling said electro-optic
element to thereby control the effective length of said Fabry-Perot
resonator.
13. The analyzer of claim 6, wherein said marker light source comprises a
semiconductor laser and an external resonator.
14. The analyzer of claim 6, wherein said tunable laser comprises a laser
resonator and an ultrasonic modulator provided in said laser resonator.
15. The analyzer of claim 6, wherein said tunable laser comprises a laser
resonator, and an electro-optic element provided in said laser resonator.
16. The analyzer of claim 8, wherein said marker light source comprises a
reference wavelength laser for producing a light having a given
wavelength.
17. The analyzer of claim 16, wherein said reference wavelength laser
comprises a laser diode having an oscillation wavelength controlled to the
absorption spectrums of atoms of said standard source.
18. The analyzer of claim 1, wherein said optical frequency sweeping means
comprises an optical synthesizer/sweeper comprising a reference wavelength
light source for producing an output light; an optical phase locked loop
for producing a light output having a wavelength which corresponds to the
oscillation wavelength of said light from said reference wavelength light
source; and means for varying the wavelength of light from said optical
phase locked loop.
19. The analyzer of claim 18, wherein said reference wavelength light
source comprises a laser diode for generating an output, and means for
controlling the oscillation wavelength of said output from said laser
diode to the absorption spectrum of any one of D.sub.2 beams at 780 nm of
Rb atoms and D.sub.1 beams at 795 nm of Rb atoms; and wherein said optical
phase locked loop outputs light having a wavelength band which is twice as
large as the oscillation wavelength of said output from said laser diode.
20. The analyzer of claim 18, wherein said reference wavelength light
source comprises a laser diode for producing an output; and means for
controlling the oscillation wavelength of said output from said laser
diode to the absorption spectrum of Rb atoms or Cs atoms.
21. The analyzer of claim 18, wherein said reference wavelength light
source comprises a semiconductor laser for producing an output light;
modulation means for effecting frequency modulation on a part of said
output light from said semiconductor laser and for producing an output
indicative thereof; an absorption cell comprising a standard substance
which causes absorption at a given wavelength of said output light from
said modulation means; photo detector for converting light transmitted
through said absorption cell into electrical signals; and control means
for controlling the oscillating wavelength of said semiconductor laser,
said control means comprising means for applying said electrical signals
from said photo detector to said semiconductor laser.
22. The analyzer of claim 21, wherein said control means comprises a
lock-in amplifier; means for applying electrical signals indicative of
said electrical signals from said photo detector to said lock-in
amplifier; means for effecting synchronous rectification at a modulation
frequency of said modulation means or at a frequency of odd numbered
multiples thereof; and a control circuit for controlling electric current
applied to said semiconductor laser so that the output of said lock-in
amplifier becomes a specified value.
23. The analyzer of claim 21, wherein said modulation means comprises an
acousto-optic deflector.
24. The analyzer of claim 21, wherein said modulation means comprises an
phase modulator comprising an electro-optic element.
25. The analyzer of claim 18, wherein said reference light source comprises
a semiconductor laser for producing an output light; an absorption cell
for causing absorption at a specified wavelength of said output light from
said semiconductor laser; magnetic applying means for applying a magnetic
field to said absorption cell; modulation means for varying the intensity
of said magnetic field at a fixed frequency; photo detector means for
converting light transmitted through said absorption cell into an
electrical signal; and means responsive to said electrical signal from
said photo detector means for controlling an electric current applied to
or temperature of said semiconductor laser.
26. The analyzer of claim 18, wherein said optical phase locked loop
comprises a heterodyne detector for heterodyning said output light from
said reference wavelength light source and another light and for
outputting an electric output indicative of the heterodyned results; a
variable wavelength light source for producing a variable output light;
means for controlling the oscillation wavelength of said variable output
light by using said electric output from said heterodyne detector; and an
optical frequency multiplying means for multiplying the frequency of said
variable output light to produce said another light; and means for
applying said another light to said heterodyne detector.
27. The analyzer of claim 18, wherein said optical phase locked loop
comprises a heterodyne detector for heterodyning said output light from
said reference wavelength light source and another light and for
outputting an electric output indicative of the heterodyned results; a
variable wavelength light source for producing a variable output light;
and a mixer circuit for mixing said variable output light to produce said
another light; and means for applying said another light to said
heterodyne detector.
28. The analyzer of claim 18, wherein said phase locked loop comprises a
heterodyne detector for heterodyning said output light and another light,
and for outputting an electric output representing the heterodyned
results; and a variable wavelength light source wherein oscillation
wavelength of output light is controlled by an output relative to said
electric output from said heterodyne detector, and wherein said another
light pertaining to the output light from said variable wavelength light
source is applied to said heterodyne detector.
29. The analyzer of claim 18, wherein said phase locked loop comprises a
heterodyne detector for heterodyning said output light and another light,
and for outputting an electric output representing the heterodyned
results; a variable wavelength light source wherein the oscillation
wavelength of output light is controlled by an output pertaining to said
electric output from said heterodyne detector; and an optical frequency
shifter for shifting frequency of light relative to the output light from
said variable light source; and means for applying said another light
relative to the output light from said optical frequency shifter to said
heterodyne detector.
30. The analyzer of claim 1, wherein said optical frequency sweeping means
comprises means for applying said sweeping light of said second frequency
to said object; means for generating and applying a sweeping light of said
first frequency to said optical heterodyne detector means; means for
arranging light reflected from said object to be as said incident light
upon the analyzer; and comparison means for comparing said electrical
output from said filter means with an electrical signal having a frequency
which is equal to the difference between said first and second frequencies
and for producing an electric output indicative of the compared results.
31. The analyzer of claim 29, further comprising a second optical
heterodyne detector means, means for applying said first and second
frequency sweeping light from said optical frequency sweeping means to
said second optical heterodyne detector means, said second optical
heterodyne detector means heterodyning said first and second frequency
sweeping light, and outputting an electric output indicative of the
heterodyned results; and second filter means for filtering said electric
output from said second optical heterodyne detector means and for
producing an electrical output indicative of the filtered results; and
wherein said comparison means compares said electrical output from said
second filter means with said electrical output from said first filter
means.
32. The analyzer of claim 30, wherein said comparison means comprises an
amplitude comparing circuit.
33. The analyzer of claim 30, wherein said comparison means comprises a
phase comparing circuit.
34. The analyzer of claim 30, wherein said first filter means comprises a
band pass filter having a transmissive frequency band corresponding to the
difference between two output frequencies of said swept light from said
optical frequency sweeping means. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an optical frequency analyzer having high
accuracy and high resolving power.
2. Description of the Prior Art
A conventional optical frequency spectrum analyzer may be of the following
types. (A) One utilizing a diffraction grating or a prism as a
spectroscope. (B) One utilizing a Fabry-Perot resonator as a spectroscope.
As illustrated in FIG. 1, two half mirrors HM are disposed to form a
resonator. Let the light velocity be c, and let the distance between the
half mirrors be L. This resonator has a resonant frequency (see FIG. 2) at
a frequency interval of c/2L. When light to be measured is rendered
incident upon the left half mirror HM, light having the frequency
identical to the resonant frequency is transmitted through the half mirror
HM and falls on receiving device PD. When half mirror HM is oscillated by
means, for example, of a PZT or the like, in order to sweep the resonant
frequency, the spectrum of the measurement light can be observed from the
output from light receiving device PD.
In the optical frequency spectrum analyzer described in (A), however,
wavelength resolving power becomes 0.1 nm (equivalent to about 30 GHz) or
thereabout; and an absolute accuracy is about 2 nm (equivalent to about
600 GHz). These results are not favorable. On the other hand, the optical
frequency spectrum analyzer described in (B) shows results of the limit of
frequency resolving power to about several tens of MHz. If the measurement
if effected by inputting light having a reference wavelength, the absolute
wavelength can be measured. The treatment is, however, very difficult, and
accuracy is deteriorated (in connection with degreee of parallelism of
mirrors and addjustment of perpendicular incidence, or error in frequency
caused by fluctuations of an interval at which the mirrors are disposed).
Furthermore, there is a defect in that it is impossible to simultaneously
measure laser beams which are being oscillated in a plurality of modes.
Frequency measurement with high accuracy of 1 MHz or less and with high
resolving power is required in the field of optical communication s and
photo applied mesurements. Hence, the above types of optical frequency
spectrum analyzers are unsatisfactory.
FIG. 3 is a block diagram depicting a conventional optical fiber loss
wavelength characteristic measuring device. Output light from a variable
wavelength light source VL enters a fiber MF to be measured, and the
subsequent emergent light is detected by a photo detector PD. The detected
light is outputted as an electric signal to an amplifying/displaying
circuit DP. The characteristics of wavelength are measured from the
variations of light power obtained when sweeping the output wavelength of
the variable wavelength light source VL.
FIG. 4 is a block diagram showing a conventional optical fiber wavelength
dispersion characteristic measuring device. The variable wavelength light
source VL and a reference wavelength light source SL are amplitude
modulated by a modulation signal source Ef having a frequency f. The photo
detector member PD detects output optical powers both of measurement fiber
Mf to which the output light of variable wavelength light source VL is
applied and of reference fiber SF to which the output light of source SL
is applied. The phase differences in component of the frequency f between
the two outputs are detected by a phase measuring device PS, thereby
measuring a propagation delay time with respect to the wavelength of the
measurement fiber MF.
However, the measuring devices depicted in FIGS. 3, 4 are deficient in many
respects, such as, the optical phase propagation characteristics cannot be
measured in a highly accurate manner. The only acceptable measurement
becomes possible with use of a long light path as in the case of optical
fibers. A short waveguide path is not acceptable for obtaining accurate
measurements. The measurement in regard to the propagation characteristic
(e.g. loss, gain, phase, delay) and reflection characteristics is of
importance to testing performance of such devices as the optical fiber,
light waveguide path, wavelength branching filters, optical switches and
the OEIC which are all essential components in any communication system or
photo applied measurement systems. The above described conventional
devicees and systems are not sufficiently adequate.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to overcome the aforementioned
and other deficiencies and defects of the prior art.
Another object is to provide an optical frequency analyzer wherein light
relative to incident light is made to fall upon an optical heterodyne
detecting member together with frequency swept light outputted from an
optical frequency sweeping member; and an electrical signal with a
frequency corresponding to the difference between two frequencies is
outputted and is then signal processed by a signal processing/displaying
member through the intermediary of a filter, thereby making it possible to
measure frequency characteristics of a measurement object with high
accuracy, high resolving power and high stability.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1, 2 are diagrams depicting the principles of a conventional optical
frequency spectrum analyzer.
FIG. 3 is a block diagram depicting a conventional optical fiber loss
wavelength characteristic measuring device.
FIG. 4 is a block diagram depicting a conventional optical fiber wavelength
dispersion characteristic measuring device.
FIG. 5 is a block diagram depicting a first illustrative embodiment of the
invention.
FIGS. 6(a) and 6(b) is a time chart depicting operation of the embodiment
of FIG. 5.
FIG. 7 is a graph depicting operation of the embodiment of FIG. 5.
FIG. 8 is a block diagram depicting a photo amplifying device.
FIG. 9 is a block diagram depicting a second illustrative embodiment of the
invention.
FIG. 10 is a block diagram depicting details of a marker attached variable
wavelength light source used in the embodiment of FIG. 9.
FIG. 11 is a spectrum chart depicting a marker signal output Em on the
basis of frequency regions of the arrangement of FIG. 10.
FIG. 12 is a block diagram depicting a second example of the marker
attached variable wavelength light source of FIG. 10.
FIG. 13 is a block diagram depicting a third example of the marker attached
variable wavelength light source of FIG. 9.
FIG. 14 is a block diagram depicting a fourth example of a marker light
source used in the arrangement of FIG. 9.
FIGS. 15(A), 15(B) and 15(C) are graphs depicting characteristic curves of
the device of FIG. 14.
FIG. 16 is a block diagram depicting a fifth example of a marker light
source used in the arrangement of FIG. 9.
FIG. 17 is a block diagram depicting a sixth example of a marker light
source used in the arrangement of FIG. 19.
FIG. 18 is a graph depicting a characteristic curve of the device of FIG.
17 .
FIG. 19 is a block diagram depicting one example of a tunable laser user in
the arrangement of FIG. 10.
FIG. 20 is a block diagram depicting a second example of a tunable laser.
FIG. 21 is a block diagram depicting a third example of a tunable laser.
FIG. 22 is a perspective view depicting a fourth example of a tunable
laser.
FIG. 23 is a block diagram depicting one example of a reference wavelength
laser light source used in the arrangement of FIG. 10.
FIG. 24 is a block diagram of a third illustrative embodiment of the
invention.
FIG. 25 is a block diagram depicting a fourth illustrative embodiment of
the invention.
FIG. 26 is a block diagram depicting one example of an optical frequency
synthesizer/sweeper used in the embodiments of FIGS. 5 and 24.
FIG. 27 is a block diagram depicting a second example of the arrangement of
FIG. 26.
FIGS. 28(A), 28(B) are wave charts showing characteristic curves of the
arrangement of FIG. 27.
FIG. 29 is an energy diagram depicting energy levels of Rb gas.
FIG. 30 is a block diagram depicting a third, partially varied, example of
the arrangement of FIG. 26.
FIG. 31 is a block diagram depicting a fourth example of the arrangement of
FIG. 26.
FIG. 32 is a block diagram depicting another example of the optical
frequency synthesizer/sweeper wherein a multiple light source is formed.
FIG. 33 is a graph depicting characteristic curves of the frequency
spectrum of the output light of the arrangement of FIG. 32.
FIG. 34 is a block diagram depicting an example of a frequency stabilized
semiconductor laser used as a reference wavelength light source.
FIG. 35 is an energy diagram showing a hyperfine structure of the energy
level of a Cs atom.
FIG. 36 is a graph depicting optical absorption caused by the Cs atoms.
FIG. 37 is a diagram depicting operation of the arrangement of FIG. 34.
FIG. 38 is a graph depicting the characteristic curve of the arrangement of
FIG. 34.
FIG. 39 is a block diagram depicting the principal portions of a second
example of a frequency stabilized semiconductor laser.
FIG. 40 is a block diagram depicting the principal portions of an optical
system of a third example of a frequency stabilized semi-conductor laser.
FIG. 41 is a block diagram depicting a fourth example of a frequency
stabilized semiconductor laser.
FIG. 42 is a graph depicting an output signal from a lock-in amplifier used
in the arrangement of FIG. 41.
FIG. 43 is a block diagram depicting principal portions of a fifth example
of a frequency stabilized semiconductor laser.
FIG. 44 is a block diagram depicting principal portions of a sixth example
of a frequency stabilized semiconductor laser.
FIG. 45 is a block diagram depicting principal portions of a seventh
example of a frequency stabilized semiconductor laser.
FIG. 46 is a cross sectional view depicting principal portions of an eighth
example of a frequency stabilized semiconductor laser.
FIG. 47 is a diagram depicting operation of the device of FIG. 46.
FIG. 48 is a block diagram depicting a ninth example of a frequency
stabilized semiconductor laser.
FIG. 49 is a diagram depicting operation of the device of FIG. 48.
FIG. 50 is a block diagram depicting principal portions of a tenth example
of a frequency stabilized semiconductor laser showing a partial
modification of FIG. 48.
FIG. 51 is a block diagram of an eleventh example of a frequency stabilized
semiconductor laser.
FIG. 52 is a diagram depicting operation of the device of FIG. 51.
FIG. 53 is a block diagram depicting principal portions of a twelfth
example of a frequency stabilized semiconductor laser.
FIG. 54 is a block diagram depicting a thirteenth example of a frequency
stabilized semiconductor laser.
FIG. 55 is a block diagram of a fourteen example of a frequency stabilized
semiconductor laser, in principal portions thereof.
FIG. 56 is a block diagram depicting principal portions of a fifteenth
example of a frequency stabilized semiconductor laser.
FIG. 57 is a block diagram depicting a sixteenth example of a frequency
stabilized semiconductor laser.
FIGS. 58, 59 are graphs showing outputs of lock-in amplifiers used in FIG.
57.
FIG. 60 is a block diagram of a seventeenth example of a frequency
stabilized semiconductor laser.
FIG. 61 is a block diagram depicting an eighteenth example of a frequency
stabilized semiconductor laser.
FIG. 62 is a block diagram depicting principal portions of a nineteenth
example of a frequency stabilized semiconductor laser.
FIG. 63, FIG. 64, and FIG. 65 are graphs showing Zeeman separation of the
energy levels of Cs atom.
FIG. 66 is a perspective view depicting a twentieth example of a frequency
stabilized semiconductor laser wherein the device is formed as an
integrated circuit.
FIG. 67 is a table depicting a method of fabricating respective components
of the device of FIG. 66.
FIGS. 68, 69 are perspective views depicting principal portions of another
example of the device of FIG. 66.
FIGS. 70, 71, 72 are cross sectional views depicting principal portions of
the device of FIG. 66.
FIG. 73 is a plan view depicting a twenty first example of a frequency
stabilized semiconductor laser as laid out on an integrated circuit.
FIGS. 74(A), 75(B), 74(C) and 75 are perspective and plan views showing
principal portions of another example of the arrangement of FIG. 73.
FIG. 76 is a block diagram depicting a twenty second example of a frequency
stabilized semiconductor laser.
FIG. 77 is a diagram showing operation of the arrangement of FIG. 76.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In the different figures, the same components having the same function will
bear the same reference symbol, in general. Where that is the case, to
improve clarity of description, repeated discussion of the component will
be generally avoided.
FIG. 5 depicts an optical frequency analyzer which can perform frequency
analysis. The belt like arrow head indicates flow of photo or optical
signals, while the solid line arrow head indicates flow of electrical
signals. The analyzer comprises a polarization control unit 1a which uses
a magnetic optical effect crystal (YIG, lead glass or the like) in which
measurement light, defining an object to be measured, is arranged to be
incident light; a photo amplifying device 2a for for inputting output
light from control unit 1a; a local oscillator 3a which constitutes an
optical frequency sweeping device; a half mirror HM1a for inputting beams
of output light from oscillator 3a and of amplifier 2a; an optical
heterodyne detector 4a comprising e.g. PIN photodiode, avalanche
photodiode, or the like, for inputting output light from half mirror HM1a;
filter 5a having a band pass property for inputting and amplifying
electric output from heterodyne detector 4a; a detector for inputting
electric output from filter 5a; and a signal processing/displaying circuit
72 for inputting electric output from detector 6a.
Local oscillator 3a comprises a sweeping signal generator 32a, reference
wavelength light source 1s, and optical phase locked loop 2s for inputting
light output from light source 1s. Loop 2s is arranged such that the
frequency sweeping thereof is controlled by signal generator 32s and the
output thereof is outputted to half mirror HM1a.
Photo amplifier 2a comprises a GaA1As laser (780 nm zone) and an ImGaAsP
laser (1500 nm zone) and may utilize the following types of amplifiers.
(A) A Fabry-Perot cavity type amplifier wherein bias electric current flows
in the vicinity of the oscillation threshold value, signal light strikes a
laser diode, and linear photo amplification is effected by inductive
emission.
(B) An injection locking amplifier wherein signal light strikes a laser
diode which continues oscillating, and wherein the optical frequency and
the phase of the oscillation are controlled.
(C) A travelling wave type amplifier wherein both end surfaces of a laser
diode chip are coated with non-reflective material, and photo
amplification is performed by transmission of light.
Operation of the FIG. 5 embodiment is as follows. Local oscillator 3a
comprises an optical frequency synthesizer/sweeper, which will be
described hereinbelow in greater detail, and which sweeps the wavelength
of output from loop 2s by the output from generator 32a. Loop 2s, having
output frequency .omega..sub.o, controls the wavelength of the output such
as to correspond to an oscillation wavelength or reference source 1s,
having output frequency .omega.s. With this arrangement, it is possible to
output local oscillation light with high accuracy, high stability and high
spectral purity.
When measurement light having frequency .omega..sub.i enters unit 1a, a
polarization plane of incident light is arranged to be identical to
another polarization plane of the output from oscillator 3a by controlling
the impressed magnetic field while utilizing rotary polarization of the
magnetic optical effect crystal. The output from control unit 1a is
amplified by amplifier 2a and is then synthesized with the output from
oscillator 3a by half mirror HM1a . The synthesized light output is
converted by heterodyne detector 4a into an electrical signal having a
frequency equivalent to the difference obtained by .omega..sub.o
-.omega..sub.i ' (in this case, however, .omega..sub.i '=.omega..sub.i).
The electric output from heterodyne detector 4a is partially transmitted
through filter 5a by its band pass properties and is taken out as power in
detector 6a. The processor/display unit 7a inputs electric output from
detector 6a as a power signal and at the same time inputs a signal
relative to the sweeping from generator 32a as a frequency axial signal,
thereby spectrum displaying the measurement light.
Examples of operating frequencies are wavelength of .omega..sub.s is 780 nm
(wavelength of laser diode locked to absorption line of Rb); wavelength of
.omega..sub.o is 1560 nm.+-.50 nm; and wavelength of .omega..sub.i is 1560
nm.+-.50 nm. These operational frequency examples are confined to the case
where the measurement light has the most suitable wavelength for optical
communication. They are particularly effective in the measurement of
characteristic (e.g. absolute wavelength, spectrum distribution and
spectrum width) of light emitted from a laser diode in a communication
system.
FIG. 5, a pulse synchronizing signal is added to generator 32a for the
purpose of exhibiting such an example that the spectrum is, with pulse
light serving as incident light, measured. FIGS. 6(A), 6(B) time charts
are provided to explain the operations of the above case. A trigger signal
(see FIG. 6(B)) synchronized with pulse light is inputted to generator 32a
oscillator 3a. Synchronized with this, frequency .omega..sub.o of loop 2s
is, as shown in FIG. 6(A), swept in a step configuration. Simultaneously,
a signal, identical to that of FIG. 6(A), corresponding to the sweeping of
frequency .omega..sub.o is transmitted to processor/display 72. As a
result, a power spectrum of .omega..sub.o exists at one point for every
beam of pulse light and hence it is feasible to output the whole spectrum
of pulse light depicted in FIG. 7 after sweeping operation has been
completed.
In FIG. 5, the frequency resolving power of the optical frequency spectrum
analyzer is determined both by the spectrum width of output frequency
.omega..sub.o of oscillator 3a and the band width of filter 5a. The
spectrum width of frequency .omega..sub.o is likewise determined by the
variable wavelength light source of the optical frequency synthesizer. An
external resonator type laser diode, which will be discussed later with
respect to FIGS. 19-22, is further used, whereby it is possible to obtain
excellent frequency resolving power eg 100 KHz.
Moreover, it is feasible to obtain an optical frequency spectrum analyzer
which is highly precise, e.g. at 10.sup.-12, with absolute wavelength
accuracy and which is highly stable.
Furthermore, the light pulse can be easily measured. Also, a W-Ni
(tungsten, nickel) point contact diode and a Josephson element can be used
for the heterodyne detector 4a. Although, a band pass filter is used as
filter 5a, other filters can be used. For example, a low pass filter may
be used. In this case, there is detected optical power of .omega..sub.i '
so that the equation .omega..sub.i '=.omega..sub.o is established to
accompany sweeping of frequency .omega..sub.o.
FIG. 8 depicts another example of a photo amplifier 2a. The amplifier 2a
comprises a local oscillator OC1a generating an output having frequency
.omega..sub.L which uses a second wavelength stabilized light source; a
photo amplifier OAa for inputting output from control unit 1a; an optical
frequency mixer OX1a which uses a non-linear type optical crystal for
inputting the output from photo amplifier OAa and output from local
oscillator OC1a. Due to the non-linear optical effect, the output
frequency .omega..sub.i ' of mixer OX1a is .omega..sub.i '=.omega..sub.i
+.omega..sub.L. As the oscillator OC1a, the preferred arrangement is an
optical frequency synthesizer/sweeper, such as shown in FIG. 27, which
outputs highly accurate frequency .omega..sub.L. If such a photo amplifier
is used, the measurement frequency range will also be expanded except for
the sweeping range of .omega..sub.o. Provided that local oscillator OC1a
which is capable of outputting a plurality of frequencies .omega..sub.L1,
.omega..sub.L2 . . . is used, it is feasible to obtain an even greater
sweeping range.
FIG. 9 depicts another spectrum analyzer which differs from FIG. 5 as
follows. A marker attached variable wavelength light source 310a is
arranged such that the frequency sweeping is controlled by sweeping signal
generator 32a in local oscillator 30a that constitutes a sweeper. Half
mirror HM2a synthesizes reference wavelength light Rs and variable
wavelength light Rv of the marker attached variable wavelength light
source 310a. Beams of output light from amplifier 2a and output from half
mirror HM2a are synthesized in half mirror HM1a and are then supplied to
heterodyne detector 4a. Processor/display 7a inputs signal relative to the
sweeping from generator 32a as the frequency axial signal and concurrently
inputs electric output from detector 6a as the power signal. After this
process, processor/display 7a spectrum displays a beam of measurement
light 71a and a beam of reference light 72a and at the same time displays
a marker 73a after inputting a marker electrical signal Em outputted from
source 310a.
FIG. 10 depicts details of the source 310 of FIG. 9 which comprises an
input terminal 11a to which a sweeping electrical signal Ei for
controlling the wavelength is applied; a tunable laser 12a for inputting
the sweeping electrical signal Ei through the intermediary of input
terminal 11a; a beam splitter BS1a for bidirectionally splitting output
from tunable laser 12a; a resonator FP1a which constitutes a marker light
source, consisting of a Fabry-Perot etalon for inputting light transmitted
through beam splitter BS1a; electro-optic element EO1a provided on the
optical axis within resonator FP1a; a signal source E1a for driving
element EO1a; a light receiving element PD1a for receiving output from
resonator FP1a and for converting it to an electrical signal; and a highly
accurate and stable reference wavelength laser light source 14a for
emitting output light having a substantially constant wavelength.
Operation of the FIG. 10 embodiment is as follows. Laser 12a emits output
having a wavelength corresponding to signal Ei which is applied via input
terminal 11a. The output is reflected by beam splitter BS1a thereby to
become the variable wavelength output Rv. The rest is transmitted through
beam splitter BS1a and are then inputted to resonator FP1a. Resonator FP1a
is capable of changing its equivalent resonator length by operation of
electro-optic element EO1a, since element EO1a is present on the light
path. Thus, a beam of output light Rm from resonator FP1a produces a peak
value at a wavelength interval corresponding to the output voltage from
signal source E1a. Light receiving element PD1 detects output RM and
converts it into an electric signal which is outputted as a marker signal
Em from terminal 13a. FIG. 11 shows marker signal Em on the basis of
frequency regions. Reference source 14a emits output light Rs having a
substantially constant wavelength within the scope of the output band of
the variable light source 12a.
Operational examples of frequencies are wavelength of reference light Rs is
780 nm (wavelength of laser diode locked to absorption beam of Rb);
wavelength of variable wavelength light Rv is 780 nm.+-.50 nm; and
wavelength of .omega..sub.i is 780 nm.+-.50 nm.
In the FIG. 9 embodiment, since the reference light, marker light and
measurement data are recorded or displayed, it is feasible to readily
ascertain the absolute value of the wavelength, if the number of intervals
of marker light is counted from the wavelength of the reference light and
at the same time interpolation is effected.
The frequency resolving power of the analyzer is determined by the spectrum
width of the variable wavelength light Rv of source 310a and by the band
width of filter 5a. Since the spectrum width of the variable wavelength
light depends on tunable laser 12 of light source 310a, it is possible to
obtain excellent frequency resolving power (e.g. of 100 KHz) by utilizing
an external resonator type laser diode which will be explained with
reference to FIGS. 19-22. Also, it is feasible to produce an analyzer
which is highly accurate and stable at the absolute wavelength (e.g. in
the range of 10.sup.-12). Moreover, in the FIG. 10 embodiment, if the
effective length of the Fabry-Perot etalon FP1a can be freely varied, the
electro-optical element EO1a is not needed.
FIG. 12 depicts a variation of the marker attached variable wavelength
light source 310a of FIG. 10 and comprises a beam splitter BS2a provided
on the output light path for reference source 14a, for causing output
light to be reflected and to strike beam splitter BS1a; a lock-in
amplifier LA1a for inputting output from light receiving element, which is
a detector , PD1a; a bias signal source E2a whose output is added to the
output from lock-in amplifier LA1a and is then applied to electro optical
element EO1a. Some beams of output light from reference source 14a are
reflected by beam splitter BS2a and fall via beam splitter S1a upon
resonator FP1a. The resonator length of resonator FP1a is controlled so
that the reference wavelength component reaches its maximum in a feedback
loop comprising lock-in amplifier LA1a, thereby making it possible to
cause the marker light to accord with the reference wavelength.
FIG. 13 depicts details of the marker attached light source 310a for use in
the arrangement of FIG. 9, and comprises an absorption cell CL1a which
comprises a standard substance, e.g. Cs, for receiving light transmitted
through beam splitter BS1a (this absorption cell CL1a constitutes a marker
light source); a light receiving element or detector PD1a for receiving
output Rm from absorption cell CL1a and converting it into an electrical
signal; a comparator CP1a for receiving electric signals from detector
PD1a; and a marker signal output terminal 13a connected to comparator
CP1a. The standard source may comprise any of Cs (having two absorption
beams in the vicinity of 852 nm), Rb (having four absorption beams in the
vicinity of 780 nm, and four absorption beams in the vicinity of 794 nm),
NH.sub.3 (having a a plurality of absorption beams, and H.sub.2 O (having
a plurality of absorption beams.).
Operation of the FIG. 13 arrangement is as follows. Part of the beams of
output from tunable laser 12a is transmitted through beam splitter BS1a
and strikes absorption cell CL1a. The incident light is subjected to
absorption at a given wavelength by the standard substance and outputted
as transmitted light Rm having peak value (i.e. the lowest point) at the
stabilized wavelength. Element PD1a converts output light Rm into
electrical signals which are waveform arranged. The arranged signal is, as
marker signal Em, outputted from terminal 13a. Since quantum standard
marker light is outputted, the wavelength can be measured with high
accuracy.
FIG. 14 depicts another marker light source used in the source 310a of
FIG. 9. The arrangement comprises a light source LL1a having consecutive
spectrums (this light source comprises an LED, a xenon lamp or the like);
a lens LS1a for causing beams of output light from light source LL1a to be
parallel; and a Fabry-Perot resonator FP2a comprising two seim-transparent
mirrors, for acting on the output light from lens LS1a.
Operation of the FIG. 14 device is as follows with reference to FIGS.
15(A), 15(B), and 15(C). Source LL1a outputs light having a large width of
spectrum as shown in FIG. 15(A). The beams of light outputted from source
LL1a are rendered parallel by lens LS1a and further enter resonator FP2a,
thereby resonating between the two semi-transparent mirrors thereof. Let
the length of the resonator (i.e. distance between the two
semi-transparent mirrors) be L.sub.1, let the light velocity be c, and let
the refractive index be n.sub.1. Then, the transmissivity of the
resonator, as shown in FIG. 15(B), has a sharp peak for every c/2n.sub.1
L.sub.1. As a result, the light outputted from the semi-transparent mirror
is as shown in FIG. 15(C). Accordingly, a reference marker can be produced
with a simple device.
FIG. 16 depicts details of another marker light source for use in source
310a of FIG. 9, wherein the effective length of the resonator of FIG. 14
is varied. The arrangement comprises an electro-optic crystal EO1a
incorporated in the light path of the Fabry-Pero resonator FP2a; and a
control signal source E1a connected to an electrode of the crystal EO1a.
When an electric field is applied to crystal EO1a by control signal source
E1a, the refractive index of crystal EO1a changes, whereby the equivalent
or effective length of the resonator is varied. Hence, the wavelength of
the output light can be readily changed. With a simple arrangement, it is
feasible to manufacture an optical frequency marker device wherein using a
simple electrical device, the frequency can be varied by varying the
effective resonator length.
In each of the foregoing embodiments, the resonator may be placed in a
constant temperature oven to control the temperature of the resonator to
be substantially constant and produce a stable frequency interval.
FIG. 17 depicts details of a further marker source comprising a
semiconductor laser LD1a comprising both ends which are AR-coated with
non-reflective material; collimator lens LS2a, LS3a for causing the beams
of output from laser LD1a to be parallel; semi-transparent mirrors HM3a,
HM4a which form an external resonator disposed outside lens LS2a, LS3a;
and an attenuator ATa through which light is outputted from
semi-transparent m | | |
