Optical sample and hold architecture

The optical hold unit (100) of the present invention includes an optical modulator (108) that has an electrical input, an optical input, and an optical output. A 1.times.N optical splitter (106) is also provided that has an optical input and N optical outputs. In addition, N optical paths (112) are individually coupled to the N optical outputs and carry one of the N output signals. Each optical path has an associated propagation delay. Optical delay elements may be located in any of the N optical paths that carry the output signals. The optical delay elements serve to lengthen the propagation delay (114a-e) of the optical path (112a-e) in which the optical delay element is located. In an alternative embodiment, the optical hold unit (200) includes an optical modulator (108) that has an electrical input, an optical input, and an optical output. An optical resonator (202) is also provided and connected to the optical output of the modulator (108). The optical resonator (202) also includes a partially transmissive output (222) to which an optical path is connected. The optical resonator (202) may also include a gain medium (208) or an optical switch (210).

DETAILED DESCRIPTION OF THE INVENTION 
Turning now to FIG. 1, that figure illustrates one embodiment of an optical 
hold unit 100. The optical hold unit 100 generally includes an input 
section 102 and an output section 104. The input section 102 includes a 
1.times.5 amplifier splitter 106, an electroabsorption modulator (EAM) 
108, and a colliding pulse modelocked laser (CPM) 110. The output section 
104 includes five optical paths 112a-e (collectively, "optical paths 112") 
and additional propagation delay delays 114a-e (collectively "delays 114") 
associated with the optical paths 112. 
FIG. 1 also shows the numerous signals involved in the operation of the 
optical hold unit 100. The radio frequency (RF) input signal 116 is 
applied to an electrical input of the EAM 108, the pulsed signal 118 is 
applied to an optical input of the EAM 108, and the EAM itself produces 
the modulated output 120. In particular, one input pulse 122 is labeled 
and is shown modulated by an RF input signal value 124 to produce a 
modulated output pulse 126. A zero delay output signal 128, first delay 
output signal 130, second delay output signal 132, third delay output 
signal 134, and fourth delay output signal 136 are also illustrated. 
Delayed versions of the modulated output pulse 126 are illustrated as 
delayed modulated output pulses 138-146. 
In operation, the RF input signal 116 and the pulsed signal 118 are applied 
as inputs to the EAM 108. Any signal source (for example, a satellite 
receiver) may generate the RF input signal 116. It is further noted that 
the RF input signal 116 need not consist only of RF frequency components. 
Rather, the RF input signal 116 may include higher frequency components or 
lower frequency components down to even DC. 
As with the RF input signal 116, the pulsed signal 118 may be generated 
using a variety of signal sources. FIG. 1 illustrates a particular example 
in which a CPM laser 110 generates the pulsed signal 118. The CPM laser 
110, may, for example, generate pulses 0.5 ps in width at a pulse rate of 
10 GHz. Preferably, the pulse rate is at least twice the highest frequency 
component of the RF input signal 116 in order to satisfy the sampling 
theorem. The pulse width is preferably as short as possible to mimic an 
infinitely narrow impulse function. 
The EAM 108 (which is commercially available for example, from TRW, Redondo 
Beach, Calif. 90278) modulates the pulsed signal 118 with the RF input 
signal 116 to produce the modulated output 120. As each individual input 
pulse (for example the input pulse 122) enters the EAM 108 at a particular 
instant in time, the EAM 108 produces an output pulse (for example, the 
modulated output pulse 126) that is proportional to the RF input signal 
116 at that instant in time (for example, the RF input signal value 124). 
The modulated output 120 thereby represents samples of the RF input signal 
116. 
The modulated output 120 next enters the 1.times.5 amplifier splitter 106. 
The 1.times.5 amplifier splitter 106 is an optical splitter that provides 
five optical outputs each carrying an optical signal that is a copy of an 
input pulse provided to the 1.times.5 amplifier splitter 106. As shown in 
FIG. 1, the input pulses are provided by the modulated output 120. Each 
optical signal is amplified by the amplifier splitter 106 so that its 
power is approximately equal to the power of the input pulse. 
Many variations of the amplifier splitter 106 are possible. For example, a 
non-amplifying splitter may be substituted in applications where the 
splitting losses do not hamper the ability of subsequent processing 
elements to work with attenuated optical outputs. In addition, if 
amplification is in fact needed, it need not take place inside the 
splitter itself. In other words, a non-amplifying splitter may be used, 
followed by optical amplifiers, for example, along one or more of the 
optical paths 112. An example of one suitable optical amplifier that may 
be used is a semiconductor optical amplifier. 
Furthermore, the optical hold unit architecture disclosed in FIG. 1 is 
easily extendable to a 1.times.N splitter. In some applications, for 
instance, it may be necessary to split an input pulse into ten identical 
copies. A 1.times.10 splitter may then be substituted for the 1.times.5 
amplifier splitter 106. In summary, the 1.times.5 amplified splitter 106 
is only one of many types and configurations of splitters suitable for use 
with the optical hold unit 100. 
One particularly suitable amplifier splitter that may be used with the 
present invention is disclosed in a co-pending application entitled 
"Active Multimode Optical Signal Splitter", Ser. No. 08/866,656 filed May 
30, 1997 and assigned to the assignee of the present application. The 
above referenced co-pending application is expressly incorporated in its 
entirety by reference. 
Still with reference to FIG. 1, the output section 104 of the optical hold 
unit 100 is shown including five optical paths 112 and associated optical 
delay elements 114. The optical paths 112 may be constructed, for example, 
with optical fibers, although other materials that provide a path for 
optical energy are also suitable (for example, semiconductor waveguides 
and crystalline structures). 
Each of the optical paths 112 may optionally include an optical delay 
element. In FIG. 1, the optical paths 112b-e include optical delay 
elements that introduce the propagation delays into the optical paths 
112b-e, respectively. In one embodiment of the present invention, the 
optical paths 112 are optical fibers and the optical delay elements 114b-e 
are additional lengths or sections of optical fiber. The additional 
sections of optical fiber increase the overall path length associated with 
the optical paths 112b-e and therefore the propagation delay associated 
with the optical paths 112b-e. 
The optical delay elements need not, for example, be implemented as 
completely separate potions of optical fiber spliced together with an 
optical path. Rather, the optical paths 112 may be manufactured at the 
outset to different lengths. In such a case, the optical delay element 
(the additional length of fiber) is built into the optical path as 
manufactured. 
Note that the optical path 112a has no additional section of optical fiber 
and therefore no additional propagation delay associated with it. In other 
words, the length of optical fiber associated with the optical path 112a 
is considered the zero delay reference, even though it, of course, has 
some finite propagation delay. Each of the optical paths 112b-e are 
extended in length beyond that of the zero delay reference optical path 
112a and therefore are considered to include the propagation delays 
114b-e. 
The propagation delays 114b-e thus represent the extra time required for 
optical energy to propagate through the optical paths 112b-e over the time 
required for light to propagate through the optical path 112a. In FIG. 1, 
the optical path 112b has a propagation delay 114b of 20 ps, the optical 
path 112c has a propagation delay 114c of 40 ps, the optical path 112d has 
a propagation delay 114d of 60 ps, and the optical path 112e has a 
propagation delay 114e of 80 ps. The propagation delays 114b-e are not 
restricted to 20 ps, but may be freely chosen independently of one another 
to match any delay (or no delay) required. 
Optical delay elements that introduce the propagation delays 114b-e may be 
implemented in a variety of ways (or combination of ways) other than 
lengthening the optical paths 112. As one example, semiconductor 
waveguides may be inserted into the optical paths 112. Semiconductor 
waveguides are particularly useful when the required propagation delays 
114b-e are very small. Because the processes that build semiconductor 
waveguides do so very precisely, even very small delays may be implemented 
in a semiconductor waveguide with very exacting tolerances. As another 
example, the optical paths 112 may be coupled to a section of material 
that has a refractive index different than that of the optical paths 112. 
The difference in propagation speed in the material (as indicated by its 
index of refraction) may then create the necessary propagation delays for 
each optical path 112. 
FIG. 1 also shows the effect of each propagation delay 114a-e on the copy 
of the modulated output 120 provided by the 1.times.5 amplifier splitter 
106. The zero delay output signal 128 shows that the copy of the modulated 
output 120 on the optical path 112a is delayed very slightly according to 
the propagation delay inherent in the optical path 112a. The first delay 
output signal 130, second delay output signal 132, third delay output 
signal 134, and fourth delay output signal 136 are delayed 20 ps, 40 ps, 
60 ps, and 80 ps respectively from the zero delay output signal 128. 
As a result, each modulated output pulse, for example the modulated output 
pulse 126, repeats five times (as the delayed modulated output pulses 
138-146) at 20 ps intervals across the optical paths 112. The modulated 
output pulse 126, in effect, has been extended from a single pulse at 
approximately time 0.2 ns to a "broadened" pulse existing from time 0.2 ns 
to 0.28 ns. In other words, the optical hold unit 100 has held the 
modulated output pulse 126 for 80 ps. Thus, additional processing elements 
(for example, electronic A to D converters) have additional time to 
operate on the modulated output pulse 126 by working with the held version 
represented by the delayed modulated output pulses 138-146. 
Note that the delayed modulated output pulses 138-146 are, in reality, 
pulses and not continuous waveforms. However, the delay elements 114b-e 
and the number of outputs provided by the splitter 106 may be adjusted to 
make the pulses as close together over an arbitrary period of time as is 
necessary to prevent slower electronic processing elements from 
recognizing the pulse nature of the modulated output pulses. Additionally, 
each of the delayed modulated output pulses 138-146 may be combined into a 
single output using an N.times.1 optical combiner, for example. 
In general, the single output would carry N identical modulated output 
pulses in series over the total delay time provided by the delay elements 
114a-e. The single output would appear very much like the cavity output 
signal 212 in FIG. 2. The single output may be useful, for example, where 
an electronic processing element is unable to operate on an input waveform 
distributed over multiple input lines. 
Turning now to FIG. 2, that figure illustrates a second embodiment of an 
optical hold unit 200. The electroabsorption modulator (EAM) 108 and 
colliding pulse modelocked laser (CPM) 110 are also shown in FIG. 2. As 
previously described, the RF input signal 116 is applied to an electrical 
input of the EAM 108, the pulsed signal 118 (generated by the CPM 110) is 
applied to an optical input of the EAM 108, and the EAM itself produces 
the modulated output 120. 
An optical cavity 202 is also provided and includes a cavity input 204 and 
a cavity output 206 connected to an optical path (not shown). The optical 
cavity 202 may also include a gain medium 208, an optical switch 210, or 
both. An example cavity output signal 212 and three of the pulses (the 
first pulse 214, second pulse 216, and third pulse 218) that form the 
cavity output signal 212 are also shown. 
One possible construction of the optical cavity 202 includes a first mirror 
220 that uses a transmissive surface towards the cavity input 204 and a 
reflective surface facing the inside of the optical cavity 202. In 
addition, a second mirror 222 is provided that includes a primarily 
reflective and partially transmissive surface facing the inside of the 
optical cavity 202. A Fabry-Perot cavity is one example of such an optical 
cavity and is generally suitable for use in the present invention. 
The Fabry-Perot cavity is generally referred to as an optical resonator. 
Different types of optical resonators may be substituted for the optical 
cavity 202, including ring laser cavities, recirculating delay lines, and 
distributed Bragg reflectors. 
The optical cavity 202 allows optical energy to enter through the first 
mirror 220. The majority of the optical energy is subsequently reflected 
between the first mirror 220 and the second mirror 222 (and additional 
reflective coatings on the inside of the optical cavity 202). Each time 
optical energy in the cavity reaches the second mirror 222, however, a 
portion of the optical energy passes through the partially transmissive 
second mirror 222. Output pulses are thereby generated that form the 
cavity output signal 212. 
Three output pulses 214-218 are shown in FIG. 2. The first output pulse 214 
may, for instance, have been produced at the optical cavity output 206 
after the first pulse of optical energy entered the optical cavity 202 and 
propagated partially through the second mirror 222. The delay before the 
first output pulse 214 is generated is therefore the propagation delay 
across the optical cavity 202. 
Subsequently, the optical energy reflects off of the second mirror 222, 
propagates back to the first mirror 220, reflects off of the first mirror 
220, and propagates back to the second mirror 222. The optical energy that 
passes through the second mirror 222 thus forms the second output pulse 
216. The delay between the first output pulse 214 and second output pulse 
216 is twice the propagation delay across the optical cavity 202. 
Similarly, the third optical pulse occurs after the second output pulse 
216 and twice the propagation delay across the optical cavity 202. 
After a predetermined number of output pulses have been produced (from a 
single input pulse) on the optical cavity output 206, the energy in the 
optical cavity 202 is eliminated. A different input pulse may then be 
introduced into the optical cavity 202 to produce another sequence of 
output pulses. To this end, the optical switch 210 may be opened or closed 
to absorb energy in the optical cavity 202, or to direct energy in the 
optical cavity 202 along a path out of the optical cavity 202. 
For each input pulse entering the optical cavity 202, the subsequent output 
pulses are preferably equal in amplitude. Because the second mirror 222 
only returns a portion of the energy in the optical cavity 202 to the 
first mirror 220, however, a gain medium is placed in the optical cavity 
202 to account for the loss through the second mirror 222. 
Examples of suitable gain mediums include semiconductor optical amplifiers, 
liquid dyes, and Erbium doped fiber amplifiers. It is further noted that 
the energy in the optical cavity 202 may be removed by switching off the 
gain medium 208 instead of, or in addition to operating the optical switch 
210. For example, the pump laser associated with an Erbium fiber amplifier 
may be turned off for an instant to bring its gain to zero. 
Thus, the output signal 212 provides, for each input pulse to the optical 
cavity 202, a series of output pulses. Each of the output pulses for a 
single input pulse is of approximately equal amplitude and appears after a 
predetermined delay from the previous output pulse. In effect, the single 
input pulse has been held for the number of output pulses, minus one, 
times the predetermined delay. Slower electronic processing elements, for 
example, may therefore operate on the held output. 
While particular elements, embodiments and applications of the present 
invention have been shown and described, it is understood that the 
invention is not limited thereto since modifications may be made by those 
skilled in the art, particularly in light of the foregoing teaching. It is 
therefore contemplated by the appended claims to cover such modifications 
and incorporate those features which come within the spirit and scope of 
the invention.