Wavelength selective coupler for high power optical communications

A wavelength selective optical fiber coupler having various applications in the field of optical communications is disclosed. The coupler includes a first substrate that has an optical input end for receiving a first optical signal. A first grating is formed in the first substrate. A second substrate has an optical input end for receiving a second optical signal. A second grating is formed in the second substrate. The first and second gratings are joined to transfer energy from the second optical signal to the first substrate for combination with the first optical signal. The combined signals are output from an optical output end of the first substrate. The gratings can comprise, for example, in-fiber gratings. Alternatively, at least one of the gratings can be provided in a polished optical block. The coupler can be used to combine a plurality of pump lasers operating at slightly different wavelengths, for input to an optical fiber amplifier having a broad pump band. A specific embodiment of a high power optical fiber amplifier using a neodymium fiber pump laser is also disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to the communication of signals via optical 
fibers, and more particularly to a wavelength selective optical coupler 
that is useful in combining a plurality of optical signals for 
communication via a common transmission path and in providing a high power 
optical fiber amplifier. 
Cable television systems currently distribute television program signals 
via coaxial cable, typically arranged in tree and branch networks. Coaxial 
cable distribution systems require a large number of high bandwidth 
electrical amplifiers. For example, forty or so amplifiers may be required 
between the cable system headend and an individual subscriber's home. 
The use of a television signal comprising amplitude modulated vestigial 
sideband video subcarriers (AM-VSB) is preferred in the distribution of 
cable television signals due to the compatibility of that format with the 
standards of the National Television Systems Committee (NTSC) and the 
ability to provide an increased number of channels within a given 
bandwidth. An undesirable characteristic of AM-VSB transmission, however, 
is that it requires a much higher carrier-to-noise ratio (CNR) than other 
techniques, such as frequency modulation or digital transmission of video 
signals. Generally, a CNR of at least 40 dB is necessary to provide clear 
reception of AM-VSB television signals. 
The replacement of coaxial cable with optical fiber transmission lines in 
television distribution systems has become a high priority. Production 
single mode fiber can support virtually unlimited bandwidth and has low 
attenuation. Accordingly, a fiber optic distribution system or a 
fiber-coax cable hybrid would provide substantially increased performance 
at a competitive cost as compared to prior art coaxial cable systems. In 
order to implement such systems, optical couplers are necessary to couple 
different optical signals to the distribution network or to components of 
the distribution network, such as optical amplifiers. 
Amplification of optical signals within an optical fiber network has been a 
problem in the attempt to distribute AM-VSB television signals. As noted 
above, amplifiers are required between a cable system headend and a 
subscriber's home in order to provide signals to the subscriber at an 
acceptable power level. Semiconductor optical amplifiers of the type 
typically used in fiber optic systems produce high levels of distortion 
products that are not compatible with multi-channel AM-VSB video signals. 
This is due to the short lifetime of the carrier excited state within the 
semiconductor optical amplifier. The recombination time of such an 
amplifier operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, 
which is short compared to the period of a typical AM-VSB subcarrier 
operating in the television band of about 55.25 MHz to 1 GHz. 
Optical fiber amplifiers, such as erbium-doped fiber amplifiers, have been 
proposed for applications in long distance transmission and subscriber 
loop distribution systems. See, e.g., W. I. Way, et al, "Noise Figure of a 
Gain-Saturated Erbium-Doped Fiber Amplifier Pumped at 980 nm", Optical 
Amplifiers and Their Applications, 1990 Technical Digest Series, Vol. 13, 
Conference Edition, Optical Society of America, August 6-8, 1990, Paper 
TuB3, pp. 134-137, and C. R. Giles, "Propagation of Signal and Noise in 
Concatenated Erbium-Doped Fiber Optical Amplifiers", Journal of Lightwave 
Technology, Vol. 9, No. Feb. 2, 1991, pp. 147-154. 
The noise figure of the fiber amplifier is a parameter that must be 
considered in such systems to optimize overall system performance. Noise 
figures of an erbium-doped fiber amplifier pumped at 980 nm have been 
found to be near 3 dB, which is a desirable performance figure. However, 
an erbium-doped fiber amplifier pumped at 980 nm does not exhibit an 
optimal power efficiency for a communication signal distributed at a 
typical wavelength of about 1550 nm. 
In order to provide a higher power efficiency for a 1550 nm communication 
signal, erbium-doped fiber amplifiers can be pumped at about 1480 nm. 
However, pumping at this wavelength results in a noise figure of about 5 
dB, which is less than optimal. 
One way to increase the power efficiency of a rare earth fiber amplifier, 
such as an erbium fiber amplifier, is to increase the pump power to the 
doped fiber. High power pump lasers suitable for use with rare earth fiber 
amplifiers, and particularly erbium fiber amplifiers, have not been 
readily available at a low enough cost for wide scale use in cable 
television distribution systems. It would therefore be advantageous to 
provide a scheme for providing high power pumping energy at relatively low 
cost. It would be further advantageous to provide an improved technique 
for coupling pump energy to a fiber amplifier using a simple and 
inexpensive optical component, such as a grating. 
Recent progress has been made in placing gratings in optical fibers by 
modifying the fiber index of refraction. Examples of processes for forming 
such gratings can be found in G. Meltz, W. W. Morey and W. H. Glenn, 
Optical Letters, Vol. 14, p. 823, 1989 and R. Kashgap, J. R. Armitage, R. 
Wyatt, S. T. Davey, and D. L. Williams, Electronics Letters. Vol. 26, p. 
730, 1990. These articles describe the formation of gratings by 
photorefractive techniques. Fiber gratings can also be fabricated 
according to the teachings of C. M. Ragdale, et al, "Bragg Grating 
Add-Drop Optical Multiplexers for InP Based Optoelectronic Integrated 
Circuits," Integrated Photonics Research Conference, IEEE OSA Meeting, 
Apr. 9, 1991, Monterey, California, Paper TuD12. 
The gratings disclosed in the articles cited above are perpendicular to the 
direction in which the optical signal propagates through the substrate 
containing the grating. When the grating is placed perpendicular to the 
direction of the lightwave propagation, the light is reflected back upon 
its original path. Thus, such gratings are used as reflectors. 
In the field of optical communications, it is desirable to multiplex 
different optical signals onto a single optical fiber. The optical fiber 
can then distribute the various signals for selective retrieval at a 
receiver. It is also desirable to provide low cost, high power lasers for 
use in communicating signals via optical fibers. As an alternative or 
complement to a high power source laser, the provision of a high power 
optical fiber amplifier is desirable. 
In order to achieve the above, it would be advantageous to provide a 
wavelength selective optical fiber coupler. It would be further 
advantageous to provide a high power optical fiber amplifier that can make 
effective use of a wavelength selective optical fiber coupler to provide a 
low cost solution for the amplification of optical signals in a 
communication network, such as a cable television distribution network. It 
would be still further advantageous to provide a wavelength selective 
optical coupler that can be used to multiplex a plurality of different 
optical information signals onto a common transmission path. 
The present invention provides a wavelength selective optical coupler that 
enjoys the aforementioned advantages. A high power optical amplifier and 
an optical multiplexing scheme embodying the coupler are also provided. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a wavelength selective optical 
fiber coupler is provided. A first substrate has an optical input end for 
receiving a first optical signal. A first grating is formed in the first 
substrate. A second substrate has an optical input end for receiving a 
second optical signal. A second grating is formed in the second substrate. 
Means are provided for joining the first and second gratings to transfer 
energy from the second optical signal to the first optical substrate for 
combination with the first optical signal. The combined signals are output 
from an optical output end of the first substrate. In a preferred 
embodiment, the gratings are optimized to pass a specific wavelength of 
the second optical signal. In order to effect the energy transfer from the 
second optical signal to the first substrate, the gratings are formed from 
lines that are oriented at a nonperpendicular angle with respect to the 
direction of lightwave propagation through their respective substrates. 
At least one of the substrates can comprise an optical fiber. For 
substrates that are optical fibers, the gratings are advantageously 
in-fiber gratings. In an illustrated embodiment, the optical fibers have a 
substantially D-shaped cross section with their respective gratings formed 
in a flat portion thereof. The joining means mate the flat portions to 
couple the evanescent fields of the two fibers. 
In another illustrated embodiment, at least one of the substrates is a 
polished optical block. For example, one of the substrates can comprise a 
polished optical block with its grating situated on a flat surface 
thereof. The other of the substrates can comprise an optical fiber having 
an in-fiber grating situated in a flat portion of the cross section of the 
fiber. The joining means mate the flat portion of the optical fiber with 
the flat surface of the polished optical block such that the gratings 
adjoin each other. 
Apparatus is also provided for combining a plurality of optical signals for 
communication via a common transmission path. Each of the optical signals 
is coupled to a corresponding grating. A collector fiber has a plurality 
of in-fiber gratings corresponding to the plurality of optical signals. 
Means are provided for joining the grating for each optical signal with a 
corresponding in-fiber grating of the collector fiber. An output end of 
the collector fiber is coupled to the common transmission path. 
In a preferred embodiment, the grating for each optical signal and the 
corresponding in-fiber grating of the collector fiber are optimized as a 
pair to pass a specific wavelength of the optical signal. The gratings for 
the optical signals can be in-fiber gratings formed in flat portions of 
corresponding optical signal fibers. The in-fiber gratings of the 
collector fiber are situated in flat portions thereof. The joining means 
mate the flat portions of the optical signal fibers with corresponding 
flat portions of the collector fiber to transfer optical energy from the 
optical signals to the collector fiber. Again, the grating for each 
optical signal and the corresponding in-fiber grating of the collector 
fiber can be optimized as a pair to pass a specific wavelength of the 
optical signal. 
In another embodiment, the gratings for the optical signals are formed in 
flat portions of corresponding polished optical blocks. The in-fiber 
gratings of the collector fiber are situated in flat portions thereof. The 
joining means mate the flat portions of the polished optical blocks with 
corresponding flat portions of the collector fiber to transfer optical 
energy from the optical signals to the collector fiber. Preferably, the 
grating for each optical signal and the corresponding in-fiber grating of 
the collector fiber will be optimized as a pair to pass a specific 
wavelength of the optical signal. The gratings can comprise lines that are 
oriented at a nonperpendicular angle with respect to the direction of 
lightwave propagation into the grating. 
A high power optical fiber amplifier that makes use of pump power received 
from a collector fiber in accordance with the invention is also provided. 
The amplifier comprises an erbium/ytterbium doped optical fiber having an 
input region and an output region. Means are provided for inputting an 
optical signal to the input region of the erbium/ytterbium doped optical 
fiber for amplification. A neodymium fiber laser having an input end and 
an output end is provided to pump the erbium/ytterbium fiber. The output 
end of the neodymium fiber laser is coupled to the input region of the 
erbium/ytterbium doped fiber to effect the pumping. The neodymium fiber 
laser comprises an optical fiber having a first core that provides a 
multi-mode waveguide and an adjacent neodymium doped second core. A source 
of pump energy is coupled to the first core for pumping the neodymium 
fiber laser. In this manner, the first core couples the pumping energy 
from the pump source to the second core. 
In a preferred embodiment, a plurality of neodymium fiber pump lasers are 
provided, operating at slightly different wavelengths within the pump band 
of the erbium/ytterbium fiber. Each of the neodymium fiber pump lasers is 
coupled to pump the erbium/ytterbium fiber. In particular, a collector 
fiber can be provided for coupling the neodymium fiber pump lasers to the 
erbium/ytterbium fiber. The collector fiber has a plurality of in-fiber 
gratings corresponding to the plurality of neodymium fiber pump lasers. An 
in-fiber grating is provided at the output end of each neodymium fiber 
pump laser. The in-fiber grating of each neodymium fiber pump laser is 
joined with a corresponding in-fiber grating of the collector fiber. Means 
are provided for coupling an output end of the collector fiber to the 
erbium/ytterbium fiber. 
In a preferred embodiment, the in-fiber grating of each neodymium fiber 
pump laser and the corresponding in-fiber grating of the collector fiber 
are optimized to pass a specific wavelength output from the associated 
neodymium fiber pump laser. The in-fiber gratings can be formed in fiber 
portions having a substantially D-shaped cross section.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides apparatus for coupling narrowband light into 
or out of single mode fibers. Multi-mode fiber can also be used in 
accordance with the present invention, although it will not operate as 
efficiently. In accordance with the present invention, light is coupled 
across an adjoining pair of gratings. The gratings are placed at an angle 
to the direction of lightwave propagation, such that light is reflected 
out of the guiding region of the optical fiber. The angle at which the 
grating lines are placed is chosen using Bragg's law to optimize the 
coupling of a specific wavelength of light. See, e.g., Halliday and 
Resnick, Physics, John Wiley & Sons, Inc., New York, New York, Part II, 
pp. 1140-1142 (1962) for an explanation of Bragg's law. 
The gratings are preferably in-fiber gratings, that can be fabricated, for 
example, using a known photorefractive process. A fiber is prepared to 
receive the grating by removing a portion of its cladding material, down 
to the evanescent field of the fiber. This results in a fiber having a 
substantially D-shaped cross section, as illustrated in FIGS. 1-3. In 
particular, a fiber 10 has its cladding 15 removed to provide a flat 
surface 12 into which a grating can be formed. The fiber includes a core 
14 as well known in the art. A second fiber 18 is provided with a flat 
surface 16 that also includes an in-fiber grating. The adjoining in-fiber 
gratings, designated 20 in FIG. 1, overlap to couple light from fiber 18 
into fiber 10. Like fiber 10, fiber 18 includes a conventional core 22. 
The in-fiber grating 20a of fiber 18 is shown in greater detail in FIG. 3. 
A similar in-fiber grating is provided in fiber 10. 
In the two fibers illustrated in FIG. 1, a wavelength selective coupling is 
achieved by matching the interaction length of the gratings 20 to the 
coupling length. In this manner, light is selectively coupled based on 
wavelength, from one fiber to the other. Specifically, the evanescent 
fields of the two fibers are coupled. 
Coupling can be effected without removing very much of the cladding 
material. Since the function of the grating is to strongly enhance 
selective coupling of narrowband light, the coupler of the present 
invention actually deals with more than just the evanescent tail of the 
lightwave. 
In another embodiment, illustrated in FIGS. 4 and 5, a polished block 
coupler 30 is provided from a solid block of optical material. Such 
polished block material is available commercially, e.g., from Canadian 
Instruments of Burlington, Ontario, Canada. A waveguide 32 within the 
polished optical block 30 is provided with a grating 34. The lines of 
grating 34 are provided at an angle that is not perpendicular to the 
direction of wavelength propagation through the waveguide 32. In order to 
transfer energy from waveguide 32 into optical fiber 18, the flat portion 
16 of fiber 18 is placed against the flat top of polished block 30 such 
that in-fiber grating 20a overlaps grating 34 of waveguide 32. Like 
grating 34, the lines of grating 20a are at an angle that is not 
perpendicular to the direction of lightwave propagation within fiber 18. 
There are many applications for an optical coupler in accordance with the 
present invention. For example, wavelength division multiplexers and 
demultiplexers can be provided using a plurality of in-fiber gratings in a 
collector fiber 40 as illustrated in FIG. 6. 
Collector fiber 40 is similar to fibers 10 and 18 illustrated in FIGS. 1-3. 
The cross section of the fiber is substantially D-shaped, and the fiber 
includes a conventional core 42 and cladding 44. In accordance with the 
present invention, a plurality of gratings 48a, 48b, 48c, . . . 48n are 
provided on the flat portion 46 of optical fiber 40. This enables a 
plurality of signal sources to be coupled into collector fiber 40 by 
aligning a corresponding grating from each signal source with one of the 
gratings 48a, 48b, 48c, . . . 48n of collector fiber 40. An example of 
this scheme is illustrated in FIG. 7. 
In FIG. 7, collector fiber 50 combines the outputs of a plurality of lasers 
52, 54, 56. In an embodiment illustrated in FIG. 8, collector fiber 50 is 
used to feed a fiber distribution network 62. In such an embodiment, each 
of lasers 52, 54, 56 can provide, for example, a modulated optical signal 
containing a plurality of video subcarriers. The video modulation from 
laser 52 is coupled to the collector fiber 50 via a pair of gratings 60a. 
Similarly, video modulation from laser 54 is coupled to collector fiber 50 
via grating pair 60b. Video modulation from laser 56 is coupled to the 
collector fiber via grating pair 60n. The combined video modulation 
signals are input to a fiber distribution network 62 (FIG. 8) for 
distribution via a plurality of transmission paths 64. 
In another embodiment, collector fiber 50 can be used to supply pump energy 
to an optical fiber amplifier 76 illustrated in FIG. 9. In this 
embodiment, each of lasers 52, 54, 56 (FIG. 7) provides pump energy at a 
slightly different wavelength. Each wavelength is selectively coupled to 
collector fiber 50 via the grating pairs 60a, 60b, and 60n. The combined 
pump signal is input to a wavelength division multiplexer 70, that also 
receives an input signal to be amplified via an optical isolator 72. The 
output of wavelength division multiplexer 70 is coupled to an optical 
fiber amplifier, such as an erbium fiber amplifier 76 via a conventional 
coupler 74. The amplified signal is coupled via coupler 78 to an optical 
isolator 80 and output for transmission. By providing the combined pump 
signals from collector fiber 50, amplification at high powers is achieved. 
Raman fiber amplifiers, Brillioun gain, and four wave mixing applications 
can also be accommodated in accordance with the present invention. 
High power operation of the optical fiber amplifier of FIG. 9 is possible 
because the pump band of the rare earth material (e.g., erbium) is 
generally broad. For example, the 3 dB pump band of the 980 nm and 1480 nm 
wavelengths of erbium is about 20 nm. Thus, a plurality of pump lasers, 
such as pump lasers 52, 54, 56 can provide slightly different wavelengths 
over the 20 nm pump band. For example, eleven separate pump lasers can be 
provided, each operating at wavelengths that are 2 nanometers apart 
starting at 1460 nm and ending at 1480 nm. The grating pair 60a, 60b, . . 
. 60n associated with each laser will be optimized to pass the specific 
wavelength at which the laser operates. 
Such a grating coupler offers a method for low loss coupling. Clearly, the 
pump laser multiplexing system must be designed so that the optical 
bandwidth of the pump laser is efficiently matched to the acceptance 
bandwidth of the grating coupler. The grating can be made short and 
chirped to accommodate pump lasers with a broad spectral pattern. Some 
pump lasers have a broad optical spectrum, while others have a narrow 
optical spectrum. The wavelength selective coupling technique of the 
present invention is particularly advantageous for use with 807 nm pump 
wavelengths where inexpensive low power lasers are available. By using a 
plurality of low cost lasers in conjunction with a collection fiber as 
illustrated in FIG. 7, an overall high pump power can be achieved. 
An example of a low cost 807 nm laser structure is illustrated in FIGS. 10 
and 11. It is known that an erbium fiber which is co-doped with ytterbium 
can be pumped at 1.06 .mu.m, which is the preferred transition in 
neodymium. Although the pumping efficiency of the erbium/ytterbium fiber 
is not as good as pumping straight erbium at either 980 nm or 1480 nm, 
much higher pump powers are available at 1.06 .mu.m, offsetting this 
disadvantage. In the structure illustrated in FIG. 10, a neodymium fiber 
laser operating at 1.06 .mu.m is used to pump an erbium/ytterbium fiber. 
A neodymium fiber laser for use in pumping an erbium/ytterbium fiber can be 
constructed from a neodymium doped fiber having a cross section as 
illustrated in FIG. 11. Fiber 120 includes an inner core 94 and an outer 
core 96. The inner core 94 is doped with Nd.sup.3+. The outer core 96 has 
a substantially rectangular cross section, and provides a multi-mode 
waveguide for propagation of the pump energy, which in the present case is 
807 nm. The fact that Nd.sup.3+ provides a four-level laser system allows 
pump energy input to the outer core 96 (e.g., at 807 nm) to pump the 
neodymium doped inner core 94, thereby providing a 1.06 .mu.m laser 
output. The inner and outer cores are surrounded by cladding 98 in a 
conventional manner. 
Pumping of the laser is provided by a high power laser array 92 illustrated 
in FIG. 10. The broad area of the outer core 96 allows a high power laser 
array, such as a GaAs array, to efficiently couple thereto. Laser arrays 
of the type described are commercially available, for example from Spectra 
Diode Laboratories of San Jose, California. 
Laser array 92 illustrated in FIG. 10 outputs light having an 807 nm 
wavelength. This light pumps the neodymium laser, generally designated 90, 
via a first reflector 112 that passes the 807 nm light into outer core 96 
of fiber 120. For the best laser operation, reflector 112 should exhibit 
high reflectivity at 1.06 .mu.m (the lasing wavelength) and high 
transmission at 807 nm (the pump wavelength). A second reflector 114 is 
optimized based on the neodymium fiber laser design. Generally, reflector 
114 will reflect on the order of ten percent of the 1.06 .mu.m lasing 
energy and pass the balance, to couple the majority of the 1.06 .mu.m 
light into erbium/ytterbium fiber amplifier 106. 
Reflector 112 can be fabricated from a photorefractive grating that is 
designed to have high reflectivity at 1.06 .mu.m and high transmissivity 
at 807 nm. Reflector 114 can be fabricated in the same manner, as a wide 
grating that reflects the pump wavelength of 807 nm to provide more 
efficient operation of the pump. An additional reflector (not shown) can 
be provided adjacent to reflector 114 to reflect any unabsorbed pump 
power, enabling the length of the neodymium fiber to be shortened. 
The 1.06 .mu.m output of laser 90 is multiplexed in a wavelength division 
multiplexer 100 with an input signal to be amplified. The input signal is 
passed through a conventional optical isolator 102 prior to wavelength 
division multiplexer 100. The multiplexed signal is output to 
erbium/ytterbium fiber amplifier 106 via a conventional optical coupler 
104. A coupler 108 couples the amplified signal to an optical isolator 110 
for output. 
Laser 90 can be used to provide multiple pumps for the erbium/ytterbium 
fiber amplifier 106 when used in combination with a collector fiber such 
as fiber 50 discussed above in connection with FIG. 7. If it is desired to 
use multiple fiber laser pumps, reflector 112 in each of a plurality of 
different pump lasers 90 is made to match the wavelength of the particular 
grating pair 60a, 60b, . . . 60n (FIG. 7) used to couple the laser output 
to the collector fiber 50. Since the pump band of the erbium/ytterbium 
fiber amplifier 106 is very broad, starting below 1.0 .mu.m and continuing 
to above 1.2 .mu.m, a substantial number of pump lasers, each operating at 
a slightly different wavelength, can be used to pump the amplifier for 
high power. 
It should now be appreciated that the present invention provides a 
wavelength selective optical fiber coupler having a variety of 
applications. In one application, the coupler is used to combine a 
plurality of optical signals for communication via a common transmission 
path. In another application, the coupler is used to provide a plurality 
of pump wavelengths within the broad pump band of an optical fiber 
amplifier. Such a structure provides a high power optical amplifier. The 
cost of such a high power optical amplifier can be minimized by using a 
plurality of low cost dual core neodymium fiber pump lasers as illustrated 
in FIGS. 10 and 11. 
Although the invention has been described in connection with several 
specific embodiments, those skilled in the art will appreciate that 
various adaptations and modifications may be made thereto without 
departing from the spirit and scope of the invention, as set forth in the 
claims.