Fiber optic mode scrambler

The present invention is an optical transmission system comprising an optical source, a first lens and a second lens, a multimode optical waveguide, and a phase-only filter. The optical source generates an optical signal having a predetermined wavelength which is received by the first lens. The signal is then sent from the first lens through the phase-only filter and then to the second lens. The second lens then focuses the signal into the multimode optical waveguide. The multimode optical waveguide has predetermined dimensions and has multiple modes. The phase-only filter excites a plurality of modes in the optical waveguide to approximate equilibrium modal power distribution as the optical signal is received into the optical waveguide.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical transmission system. More 
particularly, the invention relates to a high reliability fiber optic mode 
scrambler utilizing a phase-only filter. 
Guided optical transmission systems are well known systems for rapid and 
highly reliable transmission of information. The basic guided optical 
transmission system consists of a transmitter, a receiver, and a guided 
channel connecting the two. Typically, an optical signal is introduced to 
the guided channel by the transmitter, travels through the guided channel 
and is then detected by the receiver. 
Furthermore, a typical guided optical transmission system will include 
multiple guided channels, such as optical fibers, which are connected with 
connector elements. Such fiber-to-fiber connections are needed for a 
variety of reasons. Several fibers must be spliced together for lengths of 
more than a few hundred kilometers because only limited continuous lengths 
of fiber are normally available from manufacturers. Also, moderate lengths 
of fiber are easier to install in most applications than are very long 
cables. 
Transmitters in typical guided optical transmission systems are either 
light emitting diodes (LEDs) or laser diodes (lasers). An LED is a pn 
junction semiconductor that emits light when forward biased. LEDs are 
incoherent light sources. Typical LEDs do not emit great amounts of power 
and are relatively slow. On the other hand, lasers have significantly more 
power and can be operated much faster. Lasers are coherent light sources. 
The guided channel in a typical guided optical transmission system is a 
step-index fiber consisting of a central core with a designated refractive 
index surrounded by a cladding with a lower designated refractive index. 
Step-index fibers have three common forms: a glass core, cladded with a 
glass having a slightly lower refractive index; a silica glass core, 
cladded with plastic; and a plastic core, cladded with another plastic. 
The core/cladding interface allows a properly oriented optical signal to 
propagate within the core of the fiber with nearly total internal 
reflection, that is, none of the signal leaks into the cladding. As long 
as the signal enters the core of the fiber at an angle less than the 
critical angle, nearly all of the signal will remain in the core of the 
fiber so that there is little loss in the optical signal. 
The fiber in a guided optical transmission system can be a single mode 
fiber or a multimode fiber. The multimode fiber has a multitude of 
transmission modes in which an optical signal can travel while the single 
mode fiber has only one transmission mode for an optical signal. The 
advantage of a multimode fiber is that significantly more information can 
be transmitted through the multimode fiber. A single mode fiber, limited 
to a single mode of transmission, is able to transmit much less 
information than a multimode fiber. 
Most applications in which optical transmission systems are used require 
the extremely rapid transmission of data. Often, speed considerations will 
dictate that a laser source is necessary, as is the use of a multimode 
fiber so that more information can be transmitted at the same time. In 
addition, as stated above, most practical applications will require 
multiple fiber lengths utilizing connectors. The combination of highly 
coherent laser sources with a multimode fiber utilizing connector elements 
presents several problems. 
Modal noise is one difficulty in such guided optical systems. When highly 
coherent light sources such as laser diodes are used with multimode 
fibers, the coherent source excites very few modes in the fiber. However, 
the fiber modes then interfere with one another causing random variations 
in optic power. This random power variation is known as modal noise. With 
typical LED sources, mode interference is not a problem because the light 
is so incoherent that interference will not greatly affect the overall 
power detection. However, with highly coherent light sources such as 
lasers, the modal interference can be both additive and subtractive such 
that any one of the few modes transmitting the optical signal can have a 
very significant portion of the optical power concentrated in that one 
mode. 
This concentration in a single mode becomes extremely significant in 
systems which utilize mode-selective loss elements such as connectors. 
These connectors between the fibers cause losses in the optical signal. It 
is very difficult to perfectly align the fibers, even with the use of 
connector elements. Even slight misalignment of the cores of the fibers 
will cause mode-selective loss. Elements such as connectors are therefore 
sometimes called mode-selective loss elements. Certain transmission modes 
are cut off or terminated by these elements. Thus, such connectors may 
cause a signal to lose its higher order modes. When the terminated modes 
are carrying a significant amount of the optical signal power, there will 
be signal error at the detector. Consequently, multimode fibers cannot be 
used in many high speed applications. 
One approach to overcoming these problems has been to use mode scramblers 
with corrugated surfaces to microbend the fibers. Such corrugated mode 
scramblers physically bend the fiber such that the angle of reflection 
between the signal and the core/cladding interface will be altered as the 
signal passes through the portion of the fiber which is bent by the 
corrugated mode scrambler. In this way, the optical signal will be 
reflected into many more modes than the few in which the coherent laser 
source originally transmits the signal. Thus, the corrugated scrambler can 
approximate equilibrium power distribution in the fiber. 
By approximating equilibrium power distribution the effect of 
mode-selective loss element such as connecters are greatly lessened. When 
there are only a few modes in which the optical signal is traveling, any 
modes cut off by the mode selective loss elements will greatly diminish 
the overall signal power and cause error in detection. When more modes are 
utilized, however, each mode terminated by the mode-selective loss 
elements has much less impact. 
Despite the advantages of such corrugated mode scramblers, there are many 
limitations to these devices, and their usefulness is very limited. First, 
corrugated mode scramblers impose intolerable strain on the fiber. In 
order for the corrugated scrambler to be effective, significant strain 
must be put on the fiber. The corrugated scrambler alters the angle of 
reflection between the core/cladding interface by physically bending the 
fiber. This bending stretches one side of the fiber and places tension on 
the other. In most applications this strain is intolerable. Most fibers 
are comprised of glass or plastic. Any strain on these fibers increases 
the risk that they will break. Even slight bends in fiber can cause 
cracks. These cracks can affect the optical signal traveling through the 
fiber and will eventually lead to breakage of the fiber. A broken or 
cracked fiber will not properly transmit an optical signal. 
Second, in order for the corrugated scrambler to effectively approximate 
equilibrium power distribution in the fiber, multiple bends in the fiber 
are necessary. The fiber must be subjected to a series of alternating 
bends. The package size needed to contain these plurality of bends is too 
large for many practical implementations. Consequently, the corrugated 
scrambler has limited practical usefulness. 
The present invention is an optical transmission system which solves these 
and related problems in the prior art. 
SUMMARY OF THE INVENTION 
The present invention is an optical transmission system comprising an 
optical source, which generates an optical signal, a first lens, a second 
lens, a multimode optical waveguide, a phase-only filter, and a detector. 
The phase-only filter is utilized in the transmission system to 
approximate equilibrium power distribution in the multimode optical 
waveguide. Approximation of equilibrium power distribution decreases the 
affect of mode selective loss elements, thereby avoiding errors in 
detecting the optical signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is an overview of prior art fiber optic transmission system 10. 
Prior art fiber optic transmission system 10 includes laser diode 12, 
optical signal 14, lens 18, first fiber 20 with launch end 21, connecter 
22, second fiber 24 with detection end 25, and detector 26. 
Laser diode 12 of prior art fiber optic transmission system 10 generates 
optical signal 14 which is transmitted through lens 18. Lens 18 directs 
optical signal 14 into first fiber 20 at launch end 21, through connecter 
22, and into second fiber 24. Optical signal 14 eventually reaches 
detection end 25 of fiber 24 and is detected by detector 26. Fibers 20 and 
24 are multimode fibers, capable of supporting multiple modes of 
transmission. 
Laser diode 12 is a highly coherent light source. Thus, optical signal 14 
which is transmitted through lens 18 into first fiber 20 is transmitted 
into very few modes in fiber 20 at launch end 21. These modes tend to 
interfere with one another as optical signal 14 propagates from launch end 
21 to detection end 25. This modal interference, also known as modal 
noise, can potentially cause errors in optical signal 14 at detector 26. 
This is particularly true if detection of optical signal 14 occurs before 
equilibrium power distribution is reached in fiber 20. Since detector 26 
typically is located a relatively short distance along fiber 20, a few 
hundred meters or less, equilibrium power distribution will not be reached 
and errors in optical signal 14 will occur at detector 26. 
Errors in optical signal 14 will occur at detector 26 due to mode-selective 
loss elements like connector 22. Connecter 22 will cause slight core 
misalignment between fibers 20 and 24. Such misalignment will cause mode 
selective loss. Certain modes, usually higher order modes, will be cut off 
and lost by connecter 22. This mode selective loss will eliminate a 
significant portion of optical signal 14 at connector 22. Thus, the lost 
portion of signal 14 will never be transmitted into fiber 24. This will 
cause errors to be detected by detector 26. 
FIG. 2A shows a fiber speckle pattern, and FIG. 2B shows misaligned fiber 
cores. FIG. 2A illustrates a speckle pattern caused by the fiber modes 
interfering with one another. The spots are bright where the net 
interference is additive (in-phase modal fields) and dark where the net 
interference is subtractive (out-of-phase modal fields). As is evident 
from FIG. 2B, misalignment of the core causes mode selective loss, that 
is, certain modes from first fiber 20 will be terminated at connecter 22 
and not transmitted to second fiber 24. This mode selective loss will 
likely cause errors in signal detection by detector 22 since significant 
optical signal power is lost at connecter 22. 
FIG. 3 shows fiber optic transmission system 30 in accordance with the 
present invention. Fiber optic transmission system 30 includes laser diode 
32, optical signal 34, first lens 36, phase-only filter 37, second lens 
38, first fiber 40 with launch end 41, connecter 42, second fiber 44 with 
detection end 45, and detector 46. 
Laser diode 32 of fiber optic transmission system 30 generates optical 
signal 34. Optical signal 34 is transmitted through first lens 36. First 
lens 36 directs optical signal 34 through phase-only filter 37 to second 
lens 38. Second lens 38 directs optical signal 34 into first fiber 40 at 
launch end 41, through connecter 42, and into second fiber 44. Optical 
signal 34 is eventually detected at detection end 45 of fiber 44 by 
detector 46. Fibers 40 and 44 are multimode fibers, capable of supporting 
multiple modes of transmission. 
Laser diode 32 is a highly coherent light source. Thus, optical signal 34 
would excite only a few modes in multimode first fiber 40 if signal 34 
were transmitted directly to first fiber 40 at launch end 41. Thus, 
mode-selective loss at connecter 42 would greatly affect optical signal 34 
and cause errors at detection as is the case in system 10. Optical signal 
34 is transmitted through phase-only filter 37, however, before being 
transmitted into first fiber 40 at launch end 41. Phase-only filter 37 
affects and alters optical signal 34 such that large number of modes are 
excited in first fiber 40 at launch end 41. Optic signal 34 is a single 
beam source. Phase-only filter 37 is a phase grating medium which 
diffracts the single beam of optic signal 34 into many modes of first 
fiber 40 (through second lens 38). Phase-only filter 37 causes optical 
signal 34 to approximate equilibrium power distribution at launch end 41. 
Consequently, any mode-selective loss at connector 42 will not cause 
errors in the detection of optical signal 34. 
Consequently, as optical signal 34 propagates from launch end 41 toward 
detection end 45 and passes through connector 42, the mode-selective loss 
characteristics of connector 42 will not cause errors in detection at 
detector 46. Because equilibrium power distribution is achieved at launch 
end 41, optical signal 34 is sufficiently distributed among the multiple 
modes of fibers 40 and 44 so that the termination of some of those modes 
by connecter 42 will not significantly affect the detection of optical 
signal 34. Thus, there will be no errors in detection. 
Phase-only filter 37 is a fully transparent phase grating surface. It may 
be comprised of glass, plastic, silica or similar material. Phase-only 
filter 37 slightly adjusts the wavelength of optical signal 34 in order to 
appropriately diffract optical signal 34 into many modes of first fiber 40 
at launch end 41. Phase-only filter 37 is constructed based on the 
wavelength of optical signal 34 as it is generated by laser diode 32, the 
relative distances between laser diode 34, first lens 36, second lens 38, 
and launch end 41 of first fiber 40, and also on the physical make-up of 
fibers 40 and 44 such as core diameter and refractive index. 
Those skilled in the art will recognize that LaPlace Transforms can be used 
to construct a phase-only filter based on such criteria. A description of 
the design of such a phase-only filter can be found in an article by 
Michael W. Farn entitled "New Iterative Algorithm For The Design Of 
Phase-Only Gratings," SPIE Vol. 1555 Computer and Optically Generated 
Holographic Optics (Fourth in a Series) (1991), which is incorporated 
herein by reference. 
In an alternative embodiment, an amplitude grating can be substituted for 
phase-only filter 37. An amplitude grating can also be constructed to 
diffract optical signal 34 into many modes at launch end 41. However, an 
amplitude grating is usually not as desirable as its construction 
comprises the alternation of transparent and opaque surfaces. Thus, at 
least some portion of signal 34 is lost in the transmission through the 
amplitude grating (due to the opaque surfaces of the grating). This may 
lead to loss in power and error in detection of the signal. 
FIGS. 4 and 5 show an implementation of the present invention including a 
phase-only filter. FIG. 4 shows a cross-sectional view and FIG. 5 shows a 
perspective view. Phase-only filter package 50 is shown including laser 
diode 52, first gradient index lens 54, phase-only filter 56, second 
gradient index lens 58, housing 60, locking nut 62, fiber optic connector 
64, and fiber 66. 
Phase-only filter package 50 is a self-contained laser package that 
utilizes highly coherent laser diode 52 to generate a signal in multimode 
fiber 66 that approximates equilibrium power distribution at transmitting 
end 67 of the fiber. 
Laser diode 52 generates a optical signal (not shown) according to an 
electronic input signal. The optical signal is directed through first lens 
54, phase-only filter 56, and then through second lens 58. Housing 60 is 
cylindrical and substantially contains first lens 54, phase-only filter 
56, and second lens 58. Fiber optic connector 64 grasps fiber 66 and is 
placed in housing 60 at an opposite end of laser diode 52. Locking nut 62 
surrounds connecter 64 and screws onto housing 60. Housing 60 includes 
external threads that interlock with internal threads from locking nut 62. 
Phase-only filter package 50 is especially advantageous as a source that 
can be installed into many applications. It provides an extremely 
high-speed optical source that can be used with multimode fibers. 
FIG. 6 shows an alternative implementation of the present invention. 
Phase-only filter package 70 is installed directly into a computer 
application for use with integrated circuits. Control signals from the 
computer will drive the optical source in phase-only filter package 70. 
Phase-only filter package 70 includes a phase-only filter, as described 
above in reference to package 50, such that the output of phase-only 
filter package 70 can be connected to a multimode fiber without risk of 
signal error from mode selective loss elements. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.