A double-stage phase-diversity receiver divides one signal into a plurality of signals. These divided signals are mixed with first-stage local oscillation signals having predetermined phase relations to thereby provide a plurality of electrical baseband signals. These electrical signals are up-converted by using second-stage local oscillation signals having a predetermined phase relation. The up-converted IF signals are added, and are then demodulated by a heterodyne scheme.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a double-stage phase-diversity receiver 
for use in not only coherent optical fiber communications employing an 
optical fiber but also, electrical communications, and radio wave 
communications and light wave communications that use spatial propagation. 
2. Description of the Related Art 
Receivers used in coherent optical fiber communications are basically 
classified into two schemes, a heterodyne scheme and a homodyne scheme. In 
the heterodyne scheme with a very high speed of several Gbits/sec, the 
intermediate frequency (IF) becomes 10 to 20 GHz, which makes it difficult 
to realize high-performance receivers due to restriction on the frequency 
response characteristic of a photodetector or microwave circuit 
technology. In the homodyne scheme, by way of contrast, although the light 
source is required to have a narrow spectral width, the above difficulty 
can be avoided because the optical signal is converted into a baseband 
signal. In this respect, research on this homodyne scheme has recently 
been accelerated with improvement of light sources. Further, attention has 
been paid to a phase-diversity scheme in which the requirements for 
optical phase stability on laser diodes for use in a transmitter and/or 
for use in a local oscillator in a receiver are much relaxed. In this 
scheme, as in well as older homodyne schemes, however, (a) it is not 
possible to compensate in the receiver the delay distortion produced by 
group delay of optical fibers, while this compensation is possible in the 
heterodyne scheme, and (b) it is technically more difficult to realize 
coherent ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying) 
demodulators in baseband than in intermediate frequency (IF) range. Of 
these intrinsic limitations to performance of the phase-diversity scheme, 
the limitation (b) can be overcome, as recently proposed, by converting 
baseband signal into an intermediate frequency, before demodulation. Yet 
no solution has yet been proposed to the first limitation (a). 
Silica fiber has the lowest transmission loss in 1.55-.mu.m wavelength 
band, but in this wavelength band, a relatively large group delay 
distortion occurs in the signal waveform because of wavelength dispersion. 
This group delay distortion restricts the transmission speed or the 
transmission distance particularly in signal transmission at a high speed 
of several Gbits/sec. As a solution to this shortcoming, 
dispersion-shifted fibers or dispersion-flattened fibers may be employed 
to reduce the wavelength dispersion. These optical fibers, however, have 
higher transmission loss; the former type fibers have a narrow region 
where the dispersion is negligible and the latter fibers are difficult to 
manufacture. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
double-stage phase-diversity receiver which can simultaneously realize the 
merits of the homodyne scheme and heterodyne scheme, whereby a high-speed 
signal transmission can be facilitated by compensating the group delay of 
optical fibers, and utilizing the narrow band property of the homodyne 
scheme. 
According to the present invention, the communication signal is divided 
into a plurality of divided signals with which a plurality of first-stage 
local oscillator signals having a predetermined phase relation is mixed to 
provide a plurality of electrical signals which are up-converted by a 
plurality of second-stage local oscillator signals having a predetermined 
phase relation, these up-converted signals are added, and the result of 
the addition is demodulated similarly as in a heterodyne scheme. 
Fronted detection equivalent to that in the homodyne scheme and 
demodulation similar to that in the heterodyne scheme are attained, with 
only the merits of both schemes realized, and with the demerits of both 
schemes are removed. 
The narrow band property of the homodyne scheme is retained while the 
demodulation can permit compensation for the group delay distortion which 
is originated from the wavelength dispersion of optical fibers similarly 
as in the heterodyne scheme.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments will be described below referring to the accompanying 
drawings. 
FIG. 1 presents a conceptual diagram of a double-stage phase-diversity 
receiver embodying the present invention. The illustration involves 
coherent optical fiber communications. When coherent ASK or PSK modulation 
is considered, a signal light input to the double-stage phase-diversity 
receiver is expressed by: 
EQU f(t)=v(t) cos (.omega..sub.S t+.psi.) (1) 
where .omega..sub.S is an angular frequency of a carrier, and .psi. is the 
phase difference between the carrier and a local oscillation signal, which 
is constant during a bit interval T (reciprocal of a bit rate). 
The signal is divided into two signals with which first-stage local 
oscillation signals having a phase difference of 90.degree. and an angular 
frequency .omega..sub.LO are mixed at mixers. The mixed signal have 
currents i.sub.1 (t) and i.sub.2 (t). These currents are given by the 
following equations: 
EQU i.sub.1 (t)=Sv(t) cos (.omega..sub.OFF t+.psi.) (2) 
EQU i.sub.2 (t)=-Sv(t) sin (.omega..sub.OFF t+.psi.) (3) 
where S is a constant representing the mixing efficiency of the mixers and 
.omega..sub.OFF is an offset angular frequency for applying automatic 
frequency control (AFC). The latter element is given by: 
EQU .omega..sub.OFF =.omega..sub.S -.PSI..sub.LO 
Two electrical baseband signals obtained through above the mixers are 
amplified if necessary and up-converted by second-stage local oscillator 
signals having an angular frequency .omega..sub.IF, and a phase difference 
of 90.degree. and resulting intermediate frequency (IF). The currents 
i.sub.1 '(t) and i.sub.2 '(t) are given by the following equations: 
##EQU1## 
These two IF currents are inputs to an adder which in turn outputs an added 
result i.sub.O. This added output i.sub.O is given by: 
##EQU2## 
The output i.sub.O (t) is exactly the same as the current attained when the 
signal is subjected to heterodyne detection with the intermediate 
frequency .omega..sub.OFF +.omega..sub.IF. Therefore, supplying this added 
output i.sub.O (t) to an equalizer 1 having a predetermined transfer 
function can permit compensation for the group delay. The compensated 
result is demodulated by an ordinary heterodyne demodulator 2, which can 
provide a baseband signal. 
FIG. 2 is a block diagram illustrating the first embodiment of the present 
invention. 
Referring to this diagram, numeral 11 denotes an optical hybrid circuit 
which has two input ports; the first input port receives signal light from 
an optical fiber 10, and the second input port receives a optical local 
oscillation signal from an optical local oscillator 16. The optical local 
oscillation signal from the local oscillator 16 is subjected to frequency 
control by a frequency lock loop which has a frequency discriminator 17. 
The optical hybrid circuit 11 has two output ports from which mixed lights 
acquired by mixing the signal light with two optical local oscillation 
signals having a mutual phase difference of 90.degree.. Of the two outputs 
from the optical hybrid circuit 11, output 111 has a phase delay of 
90.degree. as compared with that of the output 112. These outputs are 
supplied to associated mixers 141 and 142 respectively through a circuit 
of a photodiode 121 and an amplifier 131 and a circuit of a photodiode 122 
and an amplifier 132. 
The mixer 141 receives an electrical local oscillation signal from an 
electrical local oscillator 18, which has its phase delayed by 90.degree. 
via a phase shifter 19. This electrical local oscillation signal is 
multiplied by the output of the amplifier 131. The same type mixer 142 
receives an electrical local oscillation signal directly from this 
electrical local oscillator 18. This electrical local oscillation signal 
is multiplied by the output of the amplifier 132. 
The outputs of the mixers 141 and 142 are supplied to an adder 20 
respectively through amplifiers 151 and 152, and are added there. The 
result of the addition is given to an equalizer 21 which serves as a 
compensating circuit. This equalizer 21, having a predetermined transfer 
function already set therein, compensates for a delay of the optical fiber 
10 by compensating the output signal from the adder 20. 
The output of the equalizer 21 is supplied to a heterodyne demodulator 22 
for demodulation, and a base-band signal is output from this demodulator 
22. With no consideration being given to the group delay of the optical 
fiber 10, however, the equalizer 21 can be omitted. 
FIG. 3 presents an equivalent circuit diagram for explaining the operation 
of equalization delay distortion compensation in this embodiment. 
Signal light f.sub.in (t) input to the optical fiber 10 is expressed by: 
##EQU3## 
In this equation, the third term represents a optical carrier, and the 
first and second terms an upper side-band and a lower sideband, 
respectively. Variables A(p), .theta.(p), B(p), and .psi.(p) give the 
sideband waveforms and phases, A(p) and B(p) are illustrated in FIG. 3. 
With the transfer function of the optical fiber 10 being H(.omega.) which 
is expressed by 
EQU .vertline.H(.PSI.).vertline..ident.G(.omega.), arg 
H(.omega.).ident..PSI.(.omega.) (8) 
then signal light f.sub.out given by the following equation is at the 
output end of the optical fiber 10. 
##EQU4## 
This signal light is supplied to the optical hybrid circuit 11 and is 
divided into two components, which result output currents i.sub.1 (t) and 
i.sub.2 (t) at photodiodes 121, 122. These currents are given by the 
following equations: 
##EQU5## 
where W is the band width of the detectors. 
In the equation (10) the three terms have the same sign, while in the 
equation (11) only the second term has a different sign from the first and 
the third. This means that an in-phase sideband, even when folded over, 
has the sign unchanged whereas a quadrature-phase side wave band has the 
sign inverted. 
##EQU6## 
then the outputs i.sub.1 '(t) and i.sub.2 '(t) of the mixers 141 and 142 
can be expressed by the following equations: 
##EQU7## 
These outputs i.sub.1 '(t) and i.sub.2 '(t) are supplied to the adder 20 
for addition. The output i.sub.0 of this adder 20 is expressed as follows: 
EQU i.sub.0 (t)/S=a(t)+b'(t)+c(t) (20) 
Accordingly, with the transfer function of the equalizer 21, H.sub.o 
(.omega.), being given by 
EQU H.sub.o (.omega.).alpha.H.sup.-1 (.omega.+.omega..sub.s -.omega..sub.IF 
-.omega..sub.OFF) (21) 
then, the delay of the optical fiber can be compensated for, as should be 
obvious from the equations (12), (15) and (16). Since .vertline.H.sub.o 
(.omega.).vertline. can be assumed to be constant, the following should 
only be satisfied: 
EQU .vertline.H.sub.o (.omega.).vertline.=const (22) 
EQU argH.sub.o (.omega.)=-argH(.omega.) (23) 
In other words, according to the homodyne receivers and conventional 
phase-diversity type receivers, as is illustrated in FIG. 4, the upper and 
lower side bands of a signal are folded in the baseband, which makes it 
impossible to compensate the delay of the optical fiber, whereas according 
to the double phase diversity receiver, the upper and lower side bands can 
be separated again when the base band signals are up-converted to the 
intermediate frequency band, and added, thus ensuring compensation for the 
delay distortion of the optical fiber as per the heterodyne scheme. 
For a wavelength band (1.55 .mu.m) of the abnormal dispersion region of 
silica optical fibers, the delay compensation can be effected by using as 
the equalizer 21 a medium having a flat amplitude characteristic and 
having a positive dispersion, such as a strip line. On the contrary for 
longer wavelength band (equal to or less than 1.3 .mu.m) having the normal 
dispersion, the output i.sub.O of the adder 20 in the equation (20) should 
consist of a'(t), b(t) and c'(t). This may be done by changing the 
connection of the local oscillator 18 or changing the polarity of one of 
the inputs to the adder 20. 
In addition, arg H(.omega.) is inserted into the equations (9) to (17), the 
first term of arg H(.omega.) presents a uniform time delay and the second 
term represents the dispersion. 
The reception scheme of such a double phase diversity receiver produces the 
effects as shown in the following table given in comparison with the 
results of other receiving schemes. 
__________________________________________________________________________ 
Double-Stage 
Scheme 
Direct Phase Phase- 
Item Detection 
Heterodyne 
Homodyne 
Diversity 
Diversity 
__________________________________________________________________________ 
Receiver 
10-25 dB 
3 dB lower 
Best Same as 
Same as 
Sensitiv- 
lower than 
than homo- hetero- 
heterodyne 
ity heterodyne 
dyne system dyne or 
system slightly 
lower 
Required 
Half of 
2 to 3 times 
Half of 
Half of 
Half of 
Detector 
the bit 
of the bit 
the bit 
the bit 
the bit 
Band rate rate rate rate rate 
Require- 
Very loose 
(1) Sync de 
Very Same as 
Same as 
ment for modulation: 
severe 
(2) of 
hetero- 
Width of Very severe hetero- 
dyne 
Laser but looser dyne system 
Spectral than homo- system 
Line dyne system 
(2) Other 
cases: Very 
loose 
Possible 
Only All possible 
All All All possible 
Modula- 
intensity possible 
possible 
tion modula- except 
except 
Scheme 
tion FSK FSK 
Possible 
Direct 
Sync and 
Baseband 
Self mul- 
Sync and 
Demodu- 
baseband 
async de- 
signal 
tiplier 
async de- 
lation 
signal 
modulation 
directly 
(square 
modulation 
Scheme 
available 
possible for 
obtained 
detector) 
possible 
any modula- for ASK 
for any 
tion system Delay modulation 
multiplier 
system 
for PSK 
Delay 
Impossible 
Possible 
Impossible 
Impossible 
Possible 
Equali- 
zation 
Other Same num- 
Same num- 
ber of 
ber of 
detectors 
only 
and demo- 
detectors 
dulators 
as ports 
as ports 
required 
required 
__________________________________________________________________________ 
Let us now check the effects of double-stage phase-diviersity scheme item 
by item. 
(1) Reception Sensitivity 
Since the double-stage phase-diversity receiving scheme divide signal light 
into more than two parts prior to detection, its reception sensitivity is 
lower than that of the idealistic homodyne scheme, but can be kept at 
substantially the same receiving sensitivity of the heterodyne scheme. 
(2) Required Detector Band 
Similar to the homodyne scheme and phase diversity scheme, the optical 
current after detection is in the baseband, and the required detector band 
can be half the bit rate from the Shannon's theorem. 
(3) Requirement for Width of Laser Spectral Line 
The requirement for the width of the laser spectral line is determined by 
the demodulation scheme, not the detection scheme. The requirement is very 
severe for the homodyne scheme in which detection and demodulation are 
unified. Since the double-stage phase diversity receiving scheme employs 
the heterodyne demodulation, however, the requirement is the exactly the 
same as that of the heterodyne scheme. 
(4) Modulation Scheme 
Due to the use of the heterodyne modulation, the double-stage 
phase-diversity receiving scheme can deal with all of ASK, FSK (Frequency 
Shift Keying) and PSK. 
(5) Demodulation Scheme 
Since the double-stage phase-diversity receiving scheme employs the 
heterodyne demodulation, it can use the same demodulator as the heterodyne 
scheme. In addition, according to the double-stage phase-diversity 
reception scheme, the intermediate frequency is not restricted by the band 
of the detector, so that this scheme can also employ a PSK synchronous 
demodulation whose use is difficult in the heterodyne scheme. 
(6) Delay Equalization 
This can be done in the same manner as done in the heterodyne scheme. 
(7) Other 
The phase diversity scheme requires the same number of detectors and 
demodulators as the number of ports, whereas the double-stage 
phase-diversity receiving scheme requires only one demodulator and the 
same number of detectors as that of the ports. 
FIG. 5 illustrates the second embodiment of the present invention, which is 
a multi-port (K ports) double phase diversity reception scheme. 
An optical hybrid circuit 11 has K output ports to which a circuit of a 
photodiode 121, amplifier 131 and mixer 141, a circuit of a photodiode 
122, amplifier 132 and mixer 142, . . . , and a circuit of a photodiode 
12K, amplifier 13K and mixer 14K are respectively connected. Mixers 141, 
142, . . . and 14K receive a electrical local oscillation signal having a 
phase difference of 2.pi.(K-1)/K via a phase shifter 19 from an electrical 
local oscillator 18. The outputs of these mixers 141, 142, . . . , and 14K 
are supplied respectively through amplifiers 151, 152, . . . and 15K for 
addition. Since the other circuit arrangement is the same as the one shown 
in FIG. 2, the same reference numerals as used to specify the identical or 
corresponding elements in the second embodiment, thus omitting their 
description. 
In this case, with K.gtoreq.3, the first-stage local oscillation signal and 
second-stage local oscillation signal given to the k-th port are cos 
(.omega..sub.S '+2.pi.k/K) and cos (.omega..sub.IF t+2.pi.k/K), the light 
current i.sub.K (t) acquired through the optical hybrid circuit 11 becomes 
EQU i.sub.K (t).alpha.V(t) cos (.omega..sub.OFFt +.psi.-2.pi.k/K) (24) 
and the current i.sub.K '(t) given by the individual mixers 141-14K becomes 
##EQU8## 
This yields the output i.sub.O (t) of the adder 20 expressed as 
EQU i.sub.O (t).alpha.Kv(t) cos {(.omega..sub.IF +.omega..sub.OFF)t+.psi.}(26) 
The results are the same as those obtained by the aforementioned. 
FIG. 6 illustrates the third embodiment of this invention. 
This embodiment is a combination of a polarization diversity and a 
double-stage phase-diversity receiver provided for each of two orthogonal 
polarizations propagated through the optical fiber. In this case, the 
individual double phase diversity outputs after undergoing demodulation in 
demodulators 22 are added together by an adder 23, which in turn outputs 
the added result. Since the other circuit arrangement is the same as the 
one shown in FIG. 2, the same reference numerals as used to specify the 
identical or corresponding elements in the second embodiment, thus 
omitting their description. 
Although the foregoing descriptions of the individual embodiments have been 
given mainly with reference to coherent optical fiber communications, the 
present invention can be widely applied to optical communications 
involving spatial propagation as well as electrical communications, 
radio-wave communications, radars, general instrumentation technology and 
the like which use electrical signals of a long wave, medium wave, short 
wave, ultrashort wave, millimetric wave, submillimetric wave, etc. 
Utilizing the setup shown in FIG. 2, an experiment is carried out. An FSK 
modulated optical data signal of 100 Mbit/s one-zero pattern with a 
frequency deviation of about 600 MHz is generated as output optical signal 
in a DFB semiconductor laser with a narrow linewidth of 10 MHz in 1.30 
.mu.m wavelength range by injecting electrical current pattern 
corresponding electrical data signal. The optical signal is fed into a 
90.degree. optical hybird as shown by 101 in FIG. 2, through signal mode 
optical fiber, and local oscillator optical signal is fed into the hybrid 
as shown by 102 in FIG. 2. The local optical oscillator signal is 
generated in the same type of semiconductor laser with almost same 
electrooptical performances as that for the data signal. Both lasers are 
temperature-controlled to .+-.0.01.degree. K. and the frequency of the 
local oscillator is adjusted near the center frequency of the modulated 
signal FSK spectrum. Isolators are inserted in front of the two lasers. An 
ND filter is inserted in the signal path to simulate a fiber. The 
90.degree. degree optical hybrid consists of a .lambda./4-plate and 
polarization beam splitter (PBS). The front-end is a phase-diversity 
receiver using InGaAs PIN photodetectors and high-impedance-type baseband 
amplifiers. The local oscillator power measured at the photodetector 
surface is -3 dBm. In the second-stage phase-diversity frequency 
up-conversion, the baseband signals are up-converted to 650 MHz (central 
frequency) using two double-balanced mixers. The two up-converted 
intermediate frequency signals are added by means of a resistive combiner, 
and fed to a conventional FSK heterodyne signal-filter demodulator 
consisting of a band-pass filter (center: 1 GHz, bandwidth: 400 MHz), an 
envelope demodulator, and a low-pass filter (bandwidth: 50 MHz). FIG. 7 
shows the signal power spectrum of the added upconverted intermediate 
frequency with no modulation. The offset frequency (the difference between 
the optical signal and local oscillator frequencies) is set to be 100 MHz. 
The unwanted signal which would appear at 550 MHz, if the cancellation is 
not complete, is found to be suppressed at least 20 dB below the signal at 
750 MHz, demonstrating that the double-stage phase-diversity (DSPD) scheme 
is functioning as expected. The pure line spectrum at 650 MHz shows the 
spuriously coupled local oscillator signal; this can eventually be 
eliminated afterwards because it is outside the pass band of the band pass 
filter. FIG. 8 shows the signal power spectrum of the added upconverted 
intermediate frequency under FSK modulation. This spectrum is found to be 
identical to that in a conventional signal-port heterodyne receiver using 
the same system, and shown in FIG. 9. The measured bit-error rate (BER) is 
shown in FIG. 10 as a function of the sum of the received optical powers 
detected by two photodetectors in the two branches. The BER of the 
heterodyne receiver is also measured and shown for comparison in FIG. 10. 
Theoretically, the sensitivities of double stage phase-diversity and 
heterodyne schemes are equal, whereas 0.6 dB degradation is observed. This 
degradation is most probably due to imperfect phase and amplitude match 
between the two branches. 
A radio-frequency experiment is also performed. An 1 MHz carrier is 
modulated by a 12 kHz signal having a triangular waveform, and received by 
a double-stage phase-diversity receiver. The receiver has a first-stage 
oscillator frequency of 0.996 MHz and the mixed signals are fed into low 
pass filters with a cutoff frequency of 160 kHz and are up-converted to 
intermediate frequency signals using second-stage local oscillator signals 
of 0.996 MHz. These intermediate frequency signals are added. 
FIG. 11 shows the power spectrum of the added intermediate frequency (IF) 
signals. The upper and lower sidebands are clearly separated, and the 
obtained signal is nothing but what would be obtained as the intermediate 
frequency signal in an ordinary heterodyne receiver.