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BACKGROUND OF THE INVENTION
The applicant and associates recently described and demonstrated a new
modulation system, termed a TDHM (i.e., Time Domain Holography
Modulation), refer (1) Characteristics of and Requirements for
Subnanometer-Wavelength Multiplexing, Technical Digest of the 1981
International Conference on Integrated Optics and Optical Fiber
Communication (IOOC '81), Apr. 27-29, 1981, San Francisco, Calif. by C. S.
Ih and Bing-Yan Chen; (2) A Direct RF Compatible Optical Fiber
Transmission System, Proceedings of the International Conference on LASER
'80, Dec. 15-19, 1980, New Orleans, La. by C. S. Ih, Bing-Yan Chen and A.
Mallya; (3) Subnanometer-Wavelength Multiplexing for Fiber-Optical
Communication, Journal of the Optical Society of America (JOSA), 70, 1569
(Dec. 1980) by C. S. Ih and Bing-Yan Chen; (4) C. S. Ih, JOSA, 68, 1384
(October, 1978); and (5) U.S. Pat. No. 4,210,803 issued July 1, 1980 to
Charles S. Ih.
Basically, TDHM can be achieved with a single frequency laser and with
apparatus similar to a Mach-Zender interferometer, such as described by W.
E. Martin in his article (6) entitled A New Waveguide Switch/Modulator for
Integrated Optics, Appl. Phys. Lett., 26, 562 (May 1975), modified by the
inclusion of an AO (Acousto Optical) or SAW (Surface Acoustic Wave)
modulator.
However, by incorporating an AO or SAW modulator in one of the arms, The
modulated laser beam is automatically put on a (sub) carrier. The laser
beam can then be effectively Amplitude, Frequency, Phase or Single Side
Band (SSB) modulated (with respect to the subcarrier) through the input of
the Acoustic Optical Modulation (AOM) as described in references (4) and
(5) supra.
By using frequency up-shifting and down-shifting alternatively, it appears
that high orders (up to the third) fiber dispersions (for single mode
fibers) can be compensated (7) C. S. Ih, Feasibility and Requirements for
Dispersion Compensation in Coherent FOC, submitted to the International
Symposium on Optical Waveguide Sciences, June 20-23, 1983, Guilin, China.
As described in the Ih patent, reference (5) supra, in TDHM the laser beam
is first split into two beams, one of which is reserved as a reference
beam. The other beam is the information beam and is modulated and
frequency-shifted by an acousto-optical (AO) or surface-acoustic wave
(SAW) modulator. SAWs are used for implementing TDHM in an
integrated-optics (IO) form. The information and reference beams are then
combined and coupled into an optical fiber for transmission. A photo
diode, which is a square-law detector, detects the beat frequency between
the reference and information beams and thus reproduces the original
signal (on a carrier) at the receiving end, all as detailed in reference
(5) supra. The carrier F.sub.c can be a frequency anywhere in the range
from several tens of mHz to tens of GHz.
The TDHM system can be used for subnanometer wavelength multiplexing (refer
references (1) and (3) supra) and for direct RF compatible transmission
(refer reference (2) supra). Despite these advantages, because the Bragg
angle conditions must be satisfied for the AO and SAW modulators, and also
because of their finite transition time, the information bandwidth for
each channel is limited. Large information bandwidth can be attained only
by using a large number of channels. In order to minimize cross-modulation
between channels, a strong reference beam must be used. Optical power
channeled into the reference beam is largely wasted, since it carries no
information. For these reasons, the TDHM per se is not well-suited for
either long distance digital transmission or for high bit rate.
This invention provides an improved TDHM whereby both digital (and analog)
information can be efficiently transmitted. Since the digital (and some
analog) information is automatically put on a "carrier", the new system is
suitable for increasing digital rate through Frequency Division
Multiplexing (FDM). The potential advantages of my FDM system over the
conventional TDHM system are as follows: (a) the noise is reduced due to
the reduction of the effective bandwidth(for each channel), (b) the
capacitance of the photo diode can be "neutralized" with inductors or can
be made part of a resonant cavity, since the information is modulated on a
carrier with a "moderate bandwidth", (c) all of the electronic circuits
and lasers are operated in parallel at lower speeds as compared with high
speed serial operation, and (d) a further reduction in noise may be
possible by using a new optoelectronic heterodyne technique (8), R. I.
MacDonald and K. O. Hill, Avalanche Optoelectronic Downconverter, Optics
Letters, 7, 83 ( Feb. 1982.) In at least some instances, the low speed
parallel-operated FDM channels are more economical and more reliable than
the high speed serially operated TDM (Time-Division-Multiplexing) system.
Also, one of the carriers can be utilized as a clock for digital reception
or for synchronized detection.
BRIEF DESCRIPTIONS OF THE DRAWINGS
The following partially schematic drawings constitute part of this
disclosure, as to which:
FIG. 1 is an overall representation of a preferred embodiment of apparatus
according to this invention, showing only a single signal transmitter;
FIG. 2 is a representation, in integrated optics form, of a plurality of
signal transmitters as they would be employed in this invention;
FIG. 3 is an oscilloscopic trace of a feasibility demonstration of the
double beam modulated signal obtained with this invention;
FIG. 4 is a representation of an apparatus auxiliary according to this
invention utilizing two lasers, injection-locked, to produce highly
monochromatic pulsed radiation for long distance single mode fiber
transmission;
FIG. 5 is a representation of a split beam embodiment of apparatus
according to this invention incorporating a time delay element in a
preselected radiation beam adapted to effect signal (dispersion)
compensation;
FIG. 6 is a representation of an embodiment of apparatus according to this
invention effecting signal (dispersion) compensation utilizing two lasers
injection-locked to a third laser; and
FIG. 6A is a schematic Driving Current v. Time representation of a pair of
time-coordinated electrical signals illustrating the optical delay
compensation achieved by the apparatus of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 2, the conventional laser input sub-assemblies
incorporate parallel-connected units 100, 200 and 300 as to which
individual laser input units LR.sub.1, LR.sub.2 and LR.sub.n,
respectively, deliver pulsed radiation outputs of wavelengths
.lambda..sub.1, .lambda..sub.2 and .lambda..sub.n, respectively.
Individual laser sub-assemblies 100, 200 and 300 are preselected to
deliver radiation wavelengths .lambda..sub.1, .lambda..sub.2 . . .
.lambda..sub.n, (corresponding to frequencies .gamma..sub.1,
.gamma..sub.2, . . . .gamma..sub.n,) spaced one nanometer or more apart,
which is easily achieved because of the relatively wide wavelength
separations existing between lasers as normally produced in manufacture.
Laser sub-assemblies 100, 200 and 300 are similar to Integrated-Optic
Spectrum Analyzers (IOSA) of designs described by D. Mergerian and E. C.
Malarkey in their publication WH2 Integrated-Optic Spectrum Analyzer:
Current Status, p. 114, Technical Digest of the Third International
Conference on Integrated Optics and Optical Fiber Communication (IOOC
'81), Apr. 27-29, 1981, and references cited therein, and by WH3 T. R.
Ranganath, Integrated-Optic Spectrum Analyzer: A First Demonstration, p.
114, Technical Digest of the Third International Conference on Integrated
Optics and Optical Fiber Communication (IOOC '81), Apr. 27-29, 1981, San
Francisco, Calif.
Each channel is provided with a Standing Wave Surface Acoustic Wave
Modulator (SWSAWM), only that denoted 101 for sub assembly unit 100 being
shown in FIG. 2, which preferably employs two sets of interdigital
fingers. If desired, a two-phase driving configuration similar to that
described by H. A. Haus, Picosecond Optical Sampling, Proceedings of the
joint meeting of the NSF Grantee-User Group in Optical Communication and
the National Telecommunications and Information Administration Task Force
on Optical Communications, May 27-29, 1981, p. 90, St. Louis, Mo., can be
used.
A peferred embodiment of the detailed apparatus is described (refer FIG. 1)
in conjunction with the operation, which is as follows. The pulsed laser
output at wavelength .lambda..sub.1 is first collimated by lens 10, which
can conveniently be a lens similar to a microscope objective or a geodesic
lens used for integrated optics. A wavefront correction optics (WCO)
element 12, typically an anamorphic lens, may be required to correct the
wavefront distortion of laser diode LR.sub.1, and this is followed by
Standing Wave Acousto-Optic Modulator 14, typically a commercially
available acousto-optic modulator such as Model SWM-40 made by
Intra-Action Corporation, or a double SAW used in integrated-optics
provided with sub-carrier frequency input. Sub-carrier frequency F.sub.c
(introduced into the SWAOM as the F.sub.c /2 in this embodiment) can be,
typically, several tens of mHz up to several GHz as compared with laser
diode frequency at 3.times.10.sup.14 Hz, typically. The collimated laser
beam is passed through SWAOM 14 at the Bragg angle. The diffracted beam
from 14 is focused by lens 15 which is similar to lens 10, and coupled to
an optical fiber 16, typically a commercially available fiber, for
transmission.
If SWAOM 14 is driven in continuous wave (CW) mode at a frequency F.sub.c1
/2, the diffracted beam contains two frequency components, one at
.gamma..sub.1 +(1/2) F.sub.c1 and the other at .gamma..sub.1 -(1/2)
F.sub.c1. A square law detector 17, typically a semiconductor high
frequency photodiode, will detect the beat frequency of these two
frequency components, which is F.sub.c1. Therefore, this system is
effectively a double side band (DSB) modulation with the carrier
suppressed.
Since the described modulator, which can be termed double beam modulation
(DBM), automatically and effectively put the original optical pulse on a
carrier, a FDM system for digital transmission can be devised in
integrated optic form such as shown in FIG. 2.
The output (optical pulses) of lasers LR.sub.1, LR.sub.2, . . . LR.sub.n at
wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n (at
frequencies (.gamma..sub.1), (.gamma..sub.2) . . . (.gamma..sub.n),
respectively) are modulated on sub-carriers F.sub.C1, F.sub.c2, . . .
F.sub.cn, respectively. The modulated optical beams are combined to form a
single output coupled to optical fiber 16. Since the individual channels
are separated in the radio frequency range vhf, uhf to microwave) at
frequency F.sub.c1, F.sub.c2, . . . F.sub.cn, the exact wavelengths and
separations of .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n of the
lasers are not as important, as long as they do not overlap into the
F.sub.cn 's. This condition can be easily satisfied. Thus, if the
wavelengths are assured of a separation of 1 nm at a wavelength of 1
.mu.m, this corresponds to a frequency separation of 300 Ghz, which is
much higher than what the current state of technology can provide for the
F.sub.cn 's. This would have the same effect of the subnanometer
wavelength multiplexing proposed and investigated previously, as described
in references 1, 3 supra.
I have demonstrated the concept of this invention using a He-Ne laser
LR.sub.1. The laser beam is first modulated by a traveling wave
acousto-optic modulator, not shown in FIG. 1, at 2 mbit (1 m Hz square
wave). The modulated beam is then passed through a Standing Wave AOM 14
driven (CW) at approximately 40 mHz. The diffracted beam is coupled into a
1 km optical fiber 16. The receiver 17 incorporates a square law detector,
typically a high frequency photo diode, and a modified commercial TV
receiver tuned to channel 5 (76-82 mHz). The load of the diode is a
parallel resonant circuit tuned to 80 mHz., so that the capacitance of the
diode was effectively neutralized by the inductor. The output of the diode
is then matched to the input impedance of the TV receiver. The TV receiver
was modified so that its video output could be observed on an
oscilloscope. The received signal is shown in FIG. 3. The slow risetime
and falltime were mainly due to the limited bandwith of the TV receiver.
The apparatus of this invention can be used with multi-mode (longitudinal)
laser service as long as the modal spacing is much larger than, and the
frequency spectrum of each mode is much smaller than, the sub-carrier
frequencies, F.sub.c1, F.sub.c2, . . . F.sub.c n. These conditions, can be
satisfied for injection lasers. The DBM modulated beam can be transmitted
through a multi-mode optical fiber 16 within its bandwidth (-length ),
i.e., BWL. The optical fiber used for the demonstration was a step-indexed
fiber with a BWL of 50 mHz-km. Applicant's signal, which was equivalently
modulated on an 80 mHZ carrier, was successfully transmitted through the 1
km fiber length. The He-Ne laser had an output of 2 mw. The pulse
modulated beam had only 1 mw. The attenuation of the optical fiber 16 was
10 db/km at 632.8 nm.
The potentially very large information bandwidth system hereinabove
described can be most efficiently utilized in a coherent transmission
system in which single mode lasers and single mode optical fibers are
employed. In this case, the general transmission characteristic of the DBM
system can be analyzed.
The complex field of the modulated beam, E.sub.1, can be represented by,
E.sub.1 =g(t)exp[j(w.sub.o +w.sub.c /2)t]+g(texp[j(w.sub.o -w.sub.c
/2)t](1)
where w.sub.o =2 .pi.f.sub.o and w.sub.c =2 .pi.f.sub.c.
If the fiber dispersion can be neglected, i.e., the linear case, the
complex field at the receiving end, E.sub.2, is,
E.sub.2 =g(t-T.sub.g L)exp[j(w.sub.o +w.sub.c /2)(t-T.sub.p L)]+g(t-T.sub.g
L)exp[j(w.sub.o -w.sub.c /2)(t-T.sub.p L)] (2)
where T.sub.p is the phase delay, T.sub.g group delay and L the fiber
length. The detected signal current can be represented by,
i.varies.E.sub.2 E.sub.2.sup.* [g(T-T.sub.g L)].sup.2 exp[jw.sub.c
(t-T.sub.p L)]+c.c. (3)
where c.c. and the superscript star each represent the complex conjugates,
or be written in the real form,
i.varies.[g(t-T.sub.g L)].sup.2 cos[w.sub.c (t-T.sub.p L)] (4)
Therefore the original signal, except a time delay, can be recovered
faithfully.
Next we consider the case where the fiber dispersion cannot be neglected.
The transfer function of the fiber is given by,
H(w)=exp[jB(w)L] (5)
The B(w) can be expanded in a Taylor seris around w.sub.o +(1/2)w.sub.c and
w.sub.o -(1/2)w.sub.c and are given by,
##EQU1##
where T.sub.p .+-., T.sub.g .+-., and T.sub.g .+-. are the phase delay,
group delay, and the first order dispersion at w.sub.o .+-.(1/2)w.sub.c
respectively.
The complex field of the received beam, E.sub.2 ', is given by,
E.sub.2 '=g+(t-T.sub.g+ L)exp[j(w.sub.o +w.sub.c /2)(t-T.sub.p+ L)]+.sub.g-
(t-T.sub.g- L)exp[j(w.sub.o -w.sub.c /2)(t-T.sub.p- L)] (8)
Where
##EQU2##
and F(w) is defined by the Fourier transform and given by,
##EQU3##
The received signal current is given by,
i.varies.E.sub.2 'E.sub.2.sup.* '[g+(t-T.sub.g+ L)][g-(t-T.sub.g-
l)]exp[jw.sub.c t+.theta.]+c.c. .varies.[g+(t-T.sub.g+ L)][g-(t-T.sub.g-
L)]cos(w.sub.c t+.theta.) (12)
Where .theta.=w.sub.o (T.sub.p- -T.sub.p+)L-(1/2)w.sub.c (T.sub.p+
-T.sub.p-)L.
Since w.sub.c <<w.sub.o, we expect that T.sub.g+ =T.sub.g-, T.sub.g+
.congruent.T.sub.g-, and T.sub.g+ -T.sub.g-=w.sub.c T.sub.go. Also,
T.sub.g 's are higher order derivates of B(w), and the difference between
the T.sub.g 's should be even smaller than that of the T.sub.g 's.
Therefore the waveforms of the g.sub.+ and g.sub.- are similar but with a
slightly different time delay which can, however, be compensated for as
hereinafter described. This would not affect the detection of digital
pulses as long as the delay is smaller than the pulse width. There is also
an extra phase term in the carrier. Since w.sub.c <<w.sub.o, we expect
that T.sub.p+ +T.sub.p- .congruent.2T.sub.po, where T.sub.po is the phase
delay at w.sub.o. The detected signal current can then be written as,
i'.varies.[g.sub.+ (t-T.sub.g+ L][g.sub.- {t-(T.sub.g+ L-w.sub.c T.sub.go
L)}]cos[w.sub.c (t-T.sub.po L)-.theta.)] (13)
Where .theta.=w.sub.o (T.sub.p+ -T.sub.p-)L
The extra phase term in the carrier does not affect the detection of the
signal. The differential time delay in g.sub.+ and g.sub.-, which is
proportional to w.sub.c .multidot.T.sub.go.multidot. L, is a more dominant
term. One way of compensating for different time delays in the waveforms
of g.sub.+ and g.sub.- mentioned supra is to utilize the split beam
apparatus adapted from that of FIG. 5, Ih U.S. Pat. No. 4,210,803,
identified as reference (5) in this application as shown herein also as
FIG. 5. In this embodiment modulator 14" is a traveling wave
acousto-optical modulator (TWAOM) driven by sub-carrier frequency F.sub.c.
Here a planar glass plate 19, or other conventional delay element adapted
such as to increase the optical path length, can be interposed in the
necessary one of the radiation paths as to which radiation transit is to
be delayed, thereby compensating the radiations transit through the
optical fiber 16".
In FIG. 5 all of the common components shown in FIG. 1 are reproduced, and
each is denoted by the same reference characters as in FIG. 1, except
double primed. Mirrors 31 and 34 are conventional beam splitters, whereas
mirrors 32 and 33 are full reflection types. For single mode fibers and
for w.sub.c 's in the range of my interest (i.e., tens of GHz), the
differential time delay term should be very small, therefore high
frequency carriers can be used. For a zero dispersion fiber, i.e., a
linear system, this term is zero as expected. Therefore the described
system can make good use of the high information bandwidth of single mode
fibers without using high speed lasers, detectors and electronics.
Another way of achieving dispersion compensation utilizing laser injection
locking is the embodiment of FIG. 6. This embodiment utilizes a traveling
wave modulator 14'" with the driving frequency therefore being F.sub.c.
Again, all components shown in FIG. 1 are reproduced and denoted by the
same reference characters as in FIG. 1, except triple primed.
Referring to FIG. 6, the information signal is introduced via LR1A, which
is injection-locked, on one side, to laser LR3A via collimating lens 51,
full reflectance mirror 52, and focusing lens 53. LR2A is injection-locked
on the other side to laser LR3A via collimating lens 48, traveling wave
acousto-optical modulator (TWAOM) 14'" driven by sub-carrier frequency
F.sub.c, full reflectance mirror 47 and focusing lens 46. The output of
LR2A, frequency shifted relative to its input from LR3A, is routed via
collimating lens 43, anamorphic lens 42 and full reflectance mirror 41 to
beam splitter 40 where it combines with the information wave train which
is thence transmitted to coupler 15'" into optical fiber 16'" and
thereafter processed through a square law detector (not here detailed) to
the receiver 17'" as hereinbefore described.
In operation, it is now practicable to pulse lasers LR1A and LR2A with a
slight time delay A, as shown in FIG. 6A, to achieve dispersion
compensation with the same effect of optical delay as hereinbefore
described for the complete integrated optics embodiment of FIG. 5. The
embodiment of FIG. 6 is particulary advantageous for the production of
highly monochromatic radiation for long distance single mode fiber
transmission.
In the system shown in FIG. 2, each channel is on a separate carrier. The
totally (sub)carriered channel system can be made compatible for
simultaneous transmission with conventional digital signals. For every
pulse received on each channel, there is a corresponding "dc" pulse in the
baseband. Therefore, the baseband is "polluted" with these "dc" terms from
all channels. However, the waveform of the dc term pulse is nearly
identical to that of the pulse demodulated from the carrier. Therefore,
they can be cancelled out elecftronically.
Even though the system herein described is mainly for digital
transmissions, it can also be used for analog information under special
conditions. The requirement is that the signal to be DBM modulated must
have a strong dc term, such as a typical TV signal, or that if the signal
is already modulated on another carrier, the carrier must be stronger than
its sidebands. In fact, we have transmitted and received TV signals of
comparable quality under similar conditions as those obtained with TDHM
[references 2, 4 supra].
A practicable scheme for utilizing the described FDM system for a fiber
optical communication system can be the following. A baseband of
approximately up to and including 1 GHz is reserved for conventional
digital and/or analog information transmission. Five DBM-FDM channels are
operated at 2.5, 3.0, 3.5, 4.0, and 4.5 GHz at 200 mbit/sec per channel.
The five channels alone will provide a 1 Gb/s capacity. The electronic
cancellation scheme described previously may be necessary to clean up the
"pollution" between the 0-200 mHz in the baseband. The maximum operating
frequency of the AO or SAW modulators is only 2.25 GHz. Sapphire
acoustoptical cells have been reported to operate up to 13 GHz [11].
Therefore acoustooptical cells of good efficiency can be expected at 2.25
GHz. The next higher frequency range in which DBM-FDM can be used in the
same fiber system will be from 10 GHz-20 GHz. If it is also desirable for
information transmission below 1 GHz in the same fiber, frequency bands
similar to those of TV broadcasting (54-88, 174-216, 470-890 mHz) can be
used. These frequency band assignments minimize second harmonic
interferences.
It is sometimes advantageous to utilize highly monochromatic radiation,
especially for long distance single mode fiber transmission, and the
apparatus of FIG. 4 permits this. Here the laser input LR.sub.1 ' can be
identical with the LR.sub.1, . . . LR.sub.n units hereinbefore described,
except that each is backed up by a d-c laser unit LR.sub.1a, the radiation
output 9 of which is directed to the laser LR.sub.1 ' where it reinforces
the radiation output of the latter, stabilizing its output as a highly
monochromatic quality which is thereafter processed in the same manner as
described for FIGS. 1 and 2.
In summary, a new FDM system for fiber optical communication is described.
It is suitable for digital transmission and can also be used for analog
information under special conditions. The DBM system is essentially a DSB
modulation with the carrier being suppressed. The "carrier" frequencies
are generated by standing wave or traveling wave AO or SAW modulators.
This system is most effective with coherent fiber transmissions and can be
employed as an alternative to ultra high bit rate transmissions.
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