Photonic mixer for photonically multiplying two electrical signals in two optically interconnected interferometric modulators operated at modulation outside the linear range

A Mach-Zehnder modulator (MZM), which divides a CW laser beam into two optical portions, is biased at a 180.degree. phase difference between the two optical portions. An RF signal and an LO signal are simultaneously applied to one of the optical portions of the laser beam to produce phase changes between the two optical portions. The two optical portions of the laser beam are then recombined into an optical output beam, which is detected by a photodetector. The photodetector generates a photocurrent, which contains a component at a beat frequency--i.e., the frequency difference between the RF and LO frequencies. The waveform of the photocurrent component at the beat frequency is substantially the same (except for amplitude and a fixed phase shift) as the waveform of the RF signal.

TECHNICAL FIELD 
This invention relates generally to electrical signal mixing, and more 
particularly provides a technique for photonically mixing electrical 
signals by means of a Mach-Zehnder modulator. 
BACKGROUND ART 
A Mach-Zehnder modulator (MZM) conventionally functions as a 
"high-fidelity" electrical-to-optical modulator--i.e., as a device for 
converting electrical signals to optical signals. Ordinarily, an MZM is 
operated at a bias so that an input voltage signal applied to the MZM 
modulates the amplitude of a continuous wave (CW) laser beam passing 
through the MZM. The amplitude-modulated laser beam is then detected by a 
photodetector, which produces a "high-fidelity" photocurrent signal (i.e., 
an electrical current output signal) that is substantially distortionless 
with respect to the input voltage signal. 
The input voltage signal applied to the MZM to modulate the amplitude of 
the CW laser beam is typically a radiofrequency (RF) signal. The output 
signal from the photodetector (i.e., the photocurrent signal) is said to 
be distortionless with respect to the RF input signal--i.e., the MZM 
functions as a "high-fidelity" electrical-to-optical modulator--when the 
waveform of the photocurrent signal is the same as the waveform of the RF 
input signal. High-fidelity operation of the MZM is achieved by biasing 
the MZM to operate in a linear range in which the photocurrent signal 
varies substantially linearly with respect to the RF input signal. 
It had not been recognized in the prior art that an MZM (or any other type 
of interferometric modulator) operating outside the linear range would 
have practical utility. Specifically, it was not realized in the prior art 
that an interferometric modulator such as an MZM operating outside the 
linear range could be used to advantage for photonically mixing electrical 
signals. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide a technique for 
photonically mixing electrical signals. 
It is a particular object of the present invention to provide a technique 
for photonically mixing an information-bearing RF signal with a local 
oscillator (LO) signal of specified frequency by means of an 
interferometric modulator such as a Mach-Zehnder modulator (MZM) operating 
outside the linear range. 
In accordance with the present invention, a CW laser beam is guided into an 
input section of a channel waveguide in an MZM. The channel waveguide 
branches into a first arm and a second arm, which rejoin inside the MZM to 
form an output section of the channel waveguide. The CW laser beam 
propagating through the waveguide splits so that corresponding portions of 
the laser beam are propagated through the first and second arms, and then 
recombine in the output section of the waveguide where the first and 
second arms rejoin. The RF and LO signals are applied simultaneously to 
the first arm of the channel waveguide, while the second arm is maintained 
at ground potential. The optical phase of the portion of the laser beam 
propagating through the first arm is changed by the RF and LO signals 
relative to the optical phase of the portion of the laser beam propagating 
through the second arm--thereby causing amplitude modulation of the 
recombined laser beam in the output section of the channel waveguide. 
The amplitude-modulated laser beam passes from the output section of the 
channel waveguide of the MZM to a photodetector, which generates a 
corresponding photocurrent signal. The photocurrent signal contains 
frequency components, which include: (1) the RF frequency; (2) the LO 
frequency; (3) a beat frequency (i.e., the frequency difference of the RF 
and LO frequencies); and (4) a sum frequency (i.e., the sum of the RF and 
LO frequencies). Under typical operating conditions, the LO signal 
contains only a single frequency. The waveforms of the photocurrent signal 
at the RF frequency, the beat frequency and the sum frequency correspond 
to the waveform of the RF input signal. Typically, only the component of 
the photocurrent signal at the beat frequency is retained. (The components 
at the other frequencies are highly attenuated using bandpass filters.) 
The first arm of the channel waveguide is d.c.-biased so that, when no RF 
and LO signals are applied thereto, the phase of the portion of the laser 
beam propagating through the first arm is shifted by substantially 
180.degree. with respect to the phase of the portion of the laser beam 
propagating through the second arm. The d.c.-bias voltage that produces 
this 180.degree. phase shift is designated V.sub..pi.. When the RF signal 
and the LO signal are applied to the first arm, the recombined laser beam 
in the output section of the channel waveguide has an amplitude 
modulation, which contains a multiplication of the RF and LO signals. The 
corresponding photocurrent signal at the beat frequency of the RF and LO 
signals has a maximum value when the MZM is operated at the V.sub..pi. 
bias. When, the MZM is operated at a bias voltage away from V.sub..pi., 
the photocurrent signal at the beat frequency of the RF and LO signals 
becomes weaker. 
It is another object of the present invention to provide a technique for 
photonically mixing an information-bearing RF signal with an LO signal of 
specified frequency by means of two MZMs, which operate outside the linear 
range with matched photodetectors in order to increase sensitivity. 
When two MZMs are used, each MZM functions independently of the other as a 
photonic mixer operating at the V.sub..pi. bias. The V.sub..pi. bias 
changes the phase of the portion of the CW laser beam in the first arm 
relative to the portion of the CW laser beam in the second arm in each of 
the first and second MZMs by 180.degree.. The first MZM has a first output 
port that is connected to a first photodetector, and a second output port 
that is connected to the second MZM. The output from the first output port 
(i.e., the "first optical output") of the first MZM serves as the optical 
input to the first photodetector. The RF and LO signals can be applied 
simultaneously to the first arm of each of the two MZMs. However, when no 
RF and LO signals are applied to the first arms of the first and second 
MZMs, the V.sub..pi. bias causes all the energy of the CW laser beam 
propagating through the first MZM to exit therefrom via the second output. 
Thus, the optical output from the second output port (i.e., the "second 
optical output") of the first MZM serves as the CW laser beam that is 
guided to the second MZM. 
When the RF signal and the LO signal are applied to the first arms of the 
first and second MZMs, the V.sub..pi. bias causes a portion (actually a 
very small fraction) of the energy of the CW laser beam propagating 
through the first MZM to exit therefrom via the first output port as the 
"first optical output". The output from the second output port of the 
first MZM (i.e., the "second optical output") is only slightly diminished 
when the RF and LO signals are applied, so that for practical purposes the 
energy of the CW laser beam guided into the second MZM can be considered 
as being substantially the same as the energy of the CW laser beam guided 
into the first MZM. The "first optical output" of the first MZM (i.e., the 
optical input to the first photodetector) exhibits an amplitude 
modulation, which contains a multiplication of the RF and LO signals. 
In response to the amplitude-modulated "first optical output" of the first 
MZM, the first photodetector generates a corresponding first photocurrent 
signal containing a beat-frequency component, which indicates the 
frequency difference of the RF and LO signals. Since the second MZM is 
operated at the same V.sub..pi. bias as the first MZM, and since the RF 
and LO signals are applied to the first arms of the first and second MZMs 
simultaneously, a portion of the energy of the CW laser beam propagating 
through the second MZM exits therefrom via a first output port as the 
"first optical output" thereof. The "first optical output" of the second 
MZM exhibits the same amplitude modulation as is exhibited by the "first 
optical output" of the first MZM. 
The amplitude-modulated output from the first output port of the second MZM 
is detected by a second photodetector, which generates a corresponding 
second photocurrent signal containing a beat-frequency component likewise 
indicating the frequency difference of the RF and LO signals. The first 
and second photocurrent signals are 180.degree. out-of-phase with each 
other, because the amplitude modulation of the optical inputs to the first 
and second photodetectors are 180.degree. out-of-phase with each other. 
The beat frequency components of the first and second photocurrent signals 
are combined electronically to produce a combined photocurrent signal, 
which is used to regenerate the information content of the RF signal. 
Photocurrent noise due to intensity fluctuations in the CW laser beam 
propagating through the second MZM is in phase with photocurrent noise due 
to intensity fluctuations in the CW laser beam propagating through the 
first MZM. Thus, subtractively combining the beat frequency components of 
the first and second photocurrent signals causes cancellation of the 
photocurrent noise, while doubling the amplitude of the beat frequency 
component of the photocurrent signal that would be obtained by using just 
one photodetector. Consequently, a photonic mixer using two MZMs in 
accordance with the present invention is significantly more sensitive than 
a photonic mixer using just one MZM.

BEST MODE OF CARRYING OUT THE INVENTION 
In FIG. 1, functional features of a typical Mach-Zehnder modulator (MZM) 10 
as known in the prior art are illustrated. MZMs are commercially marketed 
by a number of suppliers, including, e.g., Crystal Technology in Palo 
Alto, Calif.; United Technologies Photonics in Hartford, Conn.; and GCE 
Advanced Optical Products Ltd. in Chelmsford, Essex, England. 
The MZM 10 shown in FIG. 1 comprises an elongate substrate 11 in which a 
channel waveguide is formed. The substrate 11 could be made of, e.g., 
gallium arsenide or lithium niobate. An input section 12 of the channel 
waveguide receives a continuous wave (CW) laser beam for propagation 
through the MZM 10. At a first Y-junction on one side of a mid-way 
position along the substrate 11, the channel waveguide branches into a 
first arm 13 and a second arm 14. The arms 13 and 14 extend parallel to 
each other, and rejoin at a second Y-junction on the other side of the 
mid-way position along the substrate 11 to form an output section 15 of 
the channel waveguide. The laser beam, which is of constant power, is 
generated by a conventional laser device 20, and is guided into the input 
section 12 of the channel waveguide by conventional means (e.g., an 
optical fiber). The laser beam splits at the first Y-junction into a first 
portion that propagates through the first arm 13, and a second portion 
that propagates through the second arm 14. 
A first metallic electrode 21 is formed (as by vapor deposition) on a 
surface portion of the substrate 11 over the first arm 13; and a second 
metallic electrode 22 is formed in like manner on a surface portion of the 
substrate 11 over the second arm 14. The first electrode 21 is connected 
by a conductor 23 to an electrical signal input pin 24. The second 
electrode 22 is connected by a conductor 25 to a metallic casing 26, which 
serves as an electrical ground or is itself electrically grounded. In the 
particular embodiment illustrated in FIG. 1, the casing 26 encloses (or 
forms part of an enclosure around) the MZM 10. The input pin 24 extends 
out through the casing 26 to receive an information-bearing electrical 
signal applied thereto. Opposite ends of the elongate substrate 11 extend 
out through the casing 26 to function as input and output ports for the 
laser beam that is propagated through the channel waveguide. 
An information-bearing electrical signal--which is typically a 
radiofrequency (RF) signal--is applied to the input pin 24 (and thus to 
the first electrode 21). The RF signal causes a change in the index of 
refraction of the material comprising the first arm 13 of the channel 
waveguide, and thereby produces a change-of-phase of the first portion of 
the laser beam that is being propagated through the first arm 13 relative 
to the second portion of the laser beam that is being propagated through 
the second arm 14. The first and second portions of the laser beam are 
then combined in the optical output section 15 of the channel waveguide to 
form an optical output signal, which is conveyed (typically by means of an 
optical fiber) as an input optical signal to a photodetector 30. 
A block diagram illustrating the functional components of the MZM 10 is 
shown in FIG. 2. The optical output signal from the MZM 10 is an 
amplitude-modulated signal with carriers at optical frequencies. The 
photodetector 30 detects this amplitude-modulated optical signal, and 
generates a corresponding electrical output signal (i.e., a photocurrent 
signal)--which has a waveform determined by the RF signal applied to the 
pin 24. In conventional operation (i.e., where the MZM 10 functions as an 
electrical-to-optical modulator), the first arm 13 of the channel 
waveguide is d.c.-biased so that the resulting photocurrent signal 
generated by the photodetector 30 has a waveform that is substantially the 
same (except for amplitude and a fixed phase shift) as the waveform of the 
RF signal--i.e., so that the photodetector 30 produces a "high fidelity" 
electrical output signal that retains all the significant information 
contained in the RF signal. 
FIG. 3 includes a plot indicating variation of the photocurrent signal 
generated by the photodetector 30 as a function of voltage of the RF 
signal applied to the input pin 24 of the MZM 10. The variation is 
sinusoidal with a frequency that is determined by the frequency of the 
applied RF signal. To achieve a "high fidelity" photocurrent signal from 
the photodetector 30, the MZM 10 must be biased so that the applied RF 
signal has maximum and minimum amplitude values within a substantially 
linear region on the sinusoidal curve indicating the variation of the 
photocurrent signal with respect to the applied RF signal. As shown in 
FIG. 3, the frequency of the photocurrent signal is equal to the frequency 
of the applied RF signal when the MZM 10 is operated in this linear 
operating region. 
FIG. 4 likewise includes a plot indicating variation of the photocurrent 
signal generated by the photodetector 30 as function of voltage of the RF 
signal applied to the input pin 24 of the MZM 10. As in FIG. 3, the 
variation is sinusoidal with a frequency that is determined by the 
frequency of the applied RF signal. However, FIG. 4 shows the waveform of 
the photocurrent signal generated by the photodetector 30 when the MZM 10 
is biased so that the applied RF signal is centered in a nonlinear 
operating region of the MZM 10--viz., in a region centered on an applied 
voltage value V.sub..pi. for which the optical input signal to the 
photodetector 30 is zero. 
As indicated in FIG. 4, when the MZM 10 is biased to operate at V.sub..pi., 
the photocurrent signal generated by the photodetector 30 has a d.c. 
component (i.e., the average value of the photocurrent signal) and an a.c. 
component. The a.c. component of the photocurrent signal has a frequency 
that is double the frequency of the applied RF signal. The value of the 
d.c. component, and the amplitude of the a.c. component at double the 
frequency of the RF signal, are proportional to the square of the 
amplitude of the applied RF signal. 
The MZM 10 is not conventionally operated outside the linear region 
indicated in FIG. 3, because it is ordinarily desired that the 
photocurrent signal generated by the photodetector 30 be a "high fidelity" 
signal with only negligible low-power harmonics. However, in accordance 
with the present invention, the MZM 10 is operated in a nonlinear region 
(preferably, a region centered on the voltage V.sub..pi.) in order to 
obtain a photocurrent signal consisting predominantly of second-order 
harmonics of the applied RF signal. 
When the electrical signal applied to the input pin 24 of the MZM 10 is a 
combined RF signal and local oscillator (LO) signal, where the LO signal 
has a specified constant frequency and amplitude, the photocurrent signal 
generated by the photodetector 30 contains a component at the beat 
frequency (i.e., the frequency difference of the RF and LO frequencies), 
and a component at the sum frequency of the RF and LO frequencies. 
Typically, the beat frequency is lower (usually very much lower) than the 
frequency of the applied RF signal. Thus, the component of the 
photocurrent signal at the beat frequency is usually much more amenable to 
signal processing than is the RF signal. The waveform of the component of 
the photocurrent signal at the beat frequency is substantially the same as 
the waveform of the RF signal. Hence, the information content of the RF 
signal can be extracted with "high fidelity" from the component of the 
photocurrent signal at the beat frequency. 
FIG. 5 schematically indicates a way of combining the RF and LO signals 
into a combined signal, which can be applied to the input pin 24 of the 
MZM 10. As illustrated in FIG. 5, the RF and LO signals are inputs to a 
conventional RF combiner, which generates an electrical output signal that 
contains the RF and LO signals as components thereof. The electrical 
output signal from the RF combiner is then applied to the input pin 24 
(and thus to the first electrode 21 adjacent the first arm 13) of the MZM 
10. 
FIG. 6 shows an alternative way of applying the RF and LO signals to the 
first arm 13 of the channel waveguide. An additional metallic electrode 31 
(i.e., in addition to the electrode 21) is formed on a corresponding 
portion of the substrate 11 over the first arm 13. The electrode 31 is 
connected by a corresponding conductor to a corresponding input pin to 
which the LO signal is applied. The RF signal is applied via a separate 
input pin (i.e., the input pin 24) to the electrode 21. The embodiment of 
the invention as illustrated in FIG. 6 does not require an RF combiner, 
and hence avoids the signal loss inherent in using an RF combiner. Thus, 
the embodiment illustrated in FIG. 6 has a lower noise figure than the 
embodiment illustrated in FIG. 5, and is recommended for use particularly 
in applications involving weak RF signals. 
FIG. 7 provides a schematic illustration of the photonic mixer of FIG. 6. 
FIG. 8 shows an alternative embodiment of the present invention in which 
two MZMs 10 and 10' are used. The first MZM 10 has two optical outputs. 
The second MZM 10' needs only one optical output to function in accordance 
with the invention, but may have two optical outputs. As illustrated in 
FIG. 8, the CW laser beam generated by the laser device 20 is guided (as 
by a conventional optical fiber) into the input section 12 of the channel 
waveguide formed in the substrate 11 of the first MZM 10. The input 
section 12 branches into the two arms 13 and 14, which rejoin to form the 
output section 15. The output section 15 of the channel waveguide then 
branches into two output arms 16 and 17, which lead to two corresponding 
optical output ports. 
A first optical output from the first MZM 10 is guided (as by a 
conventional optical fiber) from the output arm 16 to the first 
photodetector 30. A second optical output from the first MZM 10 is guided 
(as by a conventional optical fiber) from the output arm 17 to an input 
section 12' of a channel waveguide formed in a substrate 11' of the second 
MZM 10'. The input section 12' of the channel waveguide of the second MZM 
10' branches into a first arm 13' and a second arm 14', just as the input 
section 12 of the channel waveguide of the first MZM 10 branches into the 
arms 13 and 14. Electrodes 21' and 22' are formed over the arms 13' and 
14', respectively, of the channel waveguide of the second MZM 10', just as 
the electrodes 21 and 22 are formed over the arms 13 and 14, respectively, 
of the channel waveguide of the first MZM 10. The arms 13' and 14' rejoin 
to form an output section 15', which then branches into two output arms 
16' and 17' leading to two corresponding optical output ports of the 
second MZM 10'. 
An RF signal and an LO signal can be applied simultaneously by means of a 
conventional coupler 35 to the electrode 21 positioned over the first arm 
13 of the waveguide of the first MZM 10, and to the electrode 21' 
positioned over the first arm 13' of the waveguide of the second MZM 10'. 
The electrodes 22 and 22' are both grounded. The first MZM 10 is operated 
at a bias of V.sub..pi. so that the portion of the CW laser beam 
propagating through the first arm 13 undergoes a phase change 180.degree. 
relative to the portion of the CW laser beam propagating through the 
second arm 14. Similarly, the second MZM 10' is operated at the same 
V.sub..pi. bias so that the portion of the CW laser beam propagating 
through the first arm 13' undergoes a phase change 180.degree. relative to 
the portion of the CW laser beam propagating through the second arm 14'. 
As illustrated in FIG. 8, only the optical output exiting through the 
output arm 16' of the second MZM 10' is utilized for purposes of the 
invention. The optical output exiting through the output arm 17' can be 
guided to an energy absorbing device, or can be used to provide an optical 
input to a second photonic mixer. Alternatively, the second MZM 10' with 
two output arms 16' and 17' as illustrated in FIG. 8 could be replaced by 
an MZM with only one output arm as illustrated in FIG. 2. The photonic 
mixer with two MZMs as illustrated in FIG. 8 could be fabricated on a 
single chip. 
When no RF and LO signals are applied to the electrodes 21 and 21', the 
optical output exiting through the output arm 16 of the first MZM 10 is 
zero, and all the energy of the CW laser beam exits from the first MZM 10 
via the output arm 17; and likewise, the optical output exiting through 
the output arm 16' of the second MZM 10' is zero, and all the energy of 
the CW laser beam exits from the second MZM 10' via the output arm 17'. 
The CW laser beam exiting from the second MZM 10' can be guided to an 
appropriate energy absorbing device, or can be utilized as an optical 
input to another photonic mixer or other type of optical device depending 
upon the particular application. 
When the RF and LO signals are applied to each of the electrodes 21 and 
21', the portions of the CW laser beams propagating through the first arms 
13 and 13' undergo a.c. phase changes, which produce amplitude-modulated 
output signals at the output arms 16 and 16' of the MZMs 10 and 10', 
respectively. The output signal from the output arm 16 of the first MZM 10 
is detected by the first photodetector 30, and the output signal from the 
output arm 16' of the second MZM 10' is detected by a second photodetector 
40. The first and second photodetectors 30 and 40 are "matched"--i.e., 
they have the same electrical and electro-optical characteristics. 
The matched photodetectors 30 and 40 generate corresponding photocurrent 
signals, each of which contains a beat frequency component indicating the 
frequency difference between the RF signal and the LO signal. Since the LO 
signal has a specified constant frequency and amplitude, the beat 
frequency components of the corresponding photocurrent signals generated 
by the photodetectors 30 and 40 have the same waveform as the RF signal 
(except for amplitude and a fixed phase shift). Since the amplitude 
modulations of the optical output signals from the output arms 16 and 16' 
of the MZMs 10 and 10' are 180.degree. out-of-phase with each other, the 
waveforms of the beat frequency components of the corresponding 
photocurrent signals generated by the photodetectors 30 and 40 are 
identical to each other but are out-of-phase by 180.degree.. 
Photocurrent noise attributable to intensity fluctuations of the optical 
input to the first photodetector 30 is in phase with photocurrent noise 
attributable to intensity fluctuations of the optical input to the second 
photodetector 40. Therefore, by subtractively combining the beat frequency 
components of the photocurrent signals generated by the photodetectors 30 
and 40, a net beat frequency photocurrent signal is obtained in which: (1) 
the amplitude is double the amplitude of the beat frequency component of 
the photocurrent signal generated by a single one of the photodetectors 30 
and 40; and (2) the noise attributable to intensity fluctuations in one 
effectively cancels the noise attributable to intensity fluctuations in 
the other of the matched photodetectors 30 and 40. 
The use of two optically connected MZMs with corresponding matched 
photodetectors (e.g., the MZMs 10 and 10' and the corresponding matched 
photodetectors 30 and 40 as illustrated in FIG. 8) to generate two 
photocurrent signals provides greater sensitivity and has a higher signal 
to noise ratio than the use of only a single MZM and a single 
photodetector, because the noise is significantly lower when two MZMs are 
used. For an application in which the RF signal is so weak that noise 
attributable to intensity fluctuations in the CW laser beam is apt to mask 
the information content of the RF signal, an embodiment using two MZMs 
with matched photodetectors is recommended. 
FIG. 9 provides a schematic illustration of the photonic mixer of FIG. 8. 
The present invention has been described above in terms of particular 
embodiments. However, other embodiments (utilizing, e.g., other types of 
interferometric modulators) would become apparent to practitioners skilled 
in the art upon perusal of the foregoing specification and the 
accompanying drawing. For example, the MZM 10 (and/or the MZM 10') need 
not be of a type that splits the CW laser beam into spatially separated 
portions that propagate through parallel arms as illustrated in the 
drawing. Instead, in another alternative embodiment of the invention, the 
CW laser beam could be mathematically considered as containing two 
superimposed linearly cross-polarized beams. The RF and LO signals would 
be applied to just one of the superimposed beams; and both of the 
superimposed beams would then be guided either to a polarizer (which 
produces a single optical output) or to a polarization beam splitter 
(which can produce two optical outputs). Also, a bias voltage that 
produces a 0.degree. phase shift rather than a 180.degree. phase shift 
could be used. 
The particular embodiments disclosed in the foregoing specification and 
accompanying drawing are to be understood as merely illustrative of the 
invention, which is more generally defined by the following claims and 
their equivalents.