Photonic encoding sampler

Details Photonic encoding sampler Photonic encoding sampler

2001-05-03

2003-09-30

Bruce, David V. (Department: 2882)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S045000, C385S027000

Reexamination Certificate

active

06628849

ABSTRACT:

FIELD OF THE INVENTION
The present application relates to photonic sampling, and more specifically to an optical sampler that provides encoding capability.
BACKGROUND OF THE INVENTION
Analog-to-digital conversion is well known as a process in which a continuous analog signal, which theoretically has an infinite number of values or states, is converted to a digital signal, which has a finite number of values or states. Typically, in analog-to-digital conversion, the analog signal is first sampled. The sampled analog signal is represented as a series of pulses. Each pulse has a magnitude equal to the magnitude of the analog signal at a discrete moment in time. After sampling, the discrete time signal is then quantized by rounding the value of each pulse to the closest one of a finite number of values. The resulting signal is a digital version of the analog signal.
Sampling may be accomplished by electronic sampling techniques that are well known in the art. However, electronic sampling is limited by the speed at which the electronic circuitry can be clocked to sample the analog signal. Additionally, temporal jitter in the occurrence of the sampling clock may also limit the analog-to-digital conversion performance by causing non-uniform sampling, which will increase the quantization error.
Fortunately, sampling jitter limitations can be overcome by using photonic sampling. Photonic sampling makes use of ultra-short laser pulses with high temporal stability to sample an analog electrical input. Compared to electronic samplers, the photonic approach is capable of shorter sampling windows (sub-picosecond) and higher sampling rates, approaching 100 gigasamples per second (GSPS), and thus can sample wideband analog inputs.
One type of photonic sampler may be provided by a Mach-Zehnder interferometer. Mach-Zehnder interferometers are typically constructed within a slab of transparent material using processes similar to those used for constructing semiconductor devices. The waveguides used within a Mach-Zehnder interferometer are typically constructed from lithium niobate, due to its inherent electro-optically active characteristics.
A Mach-Zehnder interferometer
100
constructed within a slab
101
of electro-optically active material is shown in FIG.
1
. As shown in
FIG. 1
, optical pulses
10
entering the device
100
are split through a “Y” splitter
111
into two directions and are directed into the two arms
121
,
123
of the device
100
. The optical pulses in each arm
121
,
123
have one-half the power of the original pulses
10
. An analog signal
20
is applied to a modulation input
170
. The modulation input
170
is coupled to electrodes
131
that are placed adjacent to the upper arm
121
of the device
100
. Application of the analog signal
20
to the modulation input
170
causes an electric field to be induced across the upper arm
121
. The arms
121
,
123
of the device
100
comprise material, such as lithium niobate, that changes its refractive index under the influence of an electric field. So when the analog signal
20
is applied, the refractive index of the material within the upper arm
121
will change and thus change the speed of propagation of the optical pulses in that arm
121
. Hence, the phase of the optical pulses in the upper arm
121
will change in relation to the optical pulses in the lower arm
123
.
If no electric signal is applied to the device
100
, the optical pulses are recombined in phase at the “Y” junction
113
. Since the signals in each arm
121
,
123
are coherent with each other, they reinforce during the recombination, and optical pulses
30
at the original strength are output. If an electric signal is applied, there will be a phase difference between the pulses routed through the upper arm
121
and the pulses routed through the lower arm
123
. When the optical pulses recombine, some or all of the optical power will be lost because the signals will interfere with each other. If the phase difference is a full 180 degrees, then the output will be zero. Hence, the pulse output
30
by the device
100
will have an optical power proportional to the analog signal
20
applied to the device
100
. Thus, the optical output
30
will represent a sampled optical version of the analog input signal
20
.
Another type of photonic sampler may be provided by a dual output Mach-Zehnder interferometer. As shown in
FIG. 2
, the dual output Mach-Zehnder interferometer
200
has a 3 dB directional coupler
213
at the point where the two interferometer arms
121
,
123
recombine. If an input optical pulse undergoes the same phase shift in the two arms
121
,
123
, then an optical pulse is produced at both of the outputs
207
,
209
of the device
200
. This is because the optical pulse excites only the in-phase, that is, even, mode of the two waveguide guide coupler structure
213
. If the optical pulse undergoes relative phase shifts of 180°, or &pgr;, again an optical pulse is produced at both of the outputs
207
,
209
of the device
200
. This is because only the odd mode of the two waveguide coupler structure
213
is excited. Although the amplitudes of the optical fields exiting the two coupler outputs are complementary, they have the same intensities. If the optical pulses undergo relative phase shifts of ±90°, or ±&pgr;/2, in the interferometer arms
121
,
123
, an optical pulse is produced at only one of the outputs
207
,
209
of the device. This is because both the even and odd modes of the two-guide coupler
213
are now excited, with equal amplitudes. Whether the upper output
207
or lower output
209
of the device
200
transmits a pulse depends on whether the phase shift is +90° or −90°. If the optical pulses in the interferometer arms
121
,
123
undergo intermediate amounts of relative phase shift, then some, unequal, amount of optical power is transmitted from both outputs of the coupler
207
,
209
.
A conventional dual output Mach-Zehnder interferometer
200
is usually operated such that an analog input voltage of 0 volts applied at the modulation input
170
results in the in-phase condition. An analog input voltage corresponding to ±V
&pgr;
/2, which is somewhat higher than the full-scale ADC voltage, results in the ±90° phase shift condition. The length of the coupler is typically designed to achieve a net phase shift of &pgr;/2 or &pgr; between the two coupler modes, depending on the construction of the coupler and how much mode conversion occurs at the input and output ends of the coupler. Thus, the dual output Mach-Zehnder interferometer provides a pair of optical signals which represent a sampled differential optical version of the analog input signal. The optical signal pair may be converted back to a single sampled signal by using a pair of photo detectors (not shown) coupled with a comparator (not shown).
A problem with the dual output Mach-Zehnder interferometer
200
depicted in
FIG. 2
is fabricating the directional coupler section
213
such that the signals output from the two interferometer arms
121
,
123
are properly coupled to produce a differential signal output. Careful manufacture of the lengths of the two waveguides in the directional coupler section will produce the desired coupling. Another method of obtaining the desired coupling is described by Nazarathy et al. in U.S. Pat. No. 5,253,309, issued Oct. 12, 1993, and shown in the Mach-Zehnder interferometer
700
FIG.
7
. Nazarathy et al. control electrodes
220
added to the Mach-Zehnder interferometer structure depicted in
FIG. 2
to arrive at the structure depicted in FIG.
7
. The control electrodes
220
adjust the splitting ratio between the signal output by the interferometer arms
121
,
123
. According to Nazarathy et al., an exemplary splitting ratio would be 50/50. If no phase shift is induced in the interferometer arms
121
,
123
and the directional coupler section has a signal ratio of 50/50, equal intensity pulses will be produced at the outputs
207
,
209
, as described abo

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