Efficient bi-directional optical fiber amplifier for missile guidance data link repeater

A bi-directional erbium doped fiber (EDF) optical fiber amplifier (50) employing two separate EDF amplifier channels (52, 62) for two counter-propagating signals allows the optimization of each individual signal channel for efficient bi-directional optical signal repeater performance. The pump sources (53, 63) for each EDF channel (52, 62) can be switched on and off independently, to serve as an in-line signal switch, without affecting another signal channel.

BACKGROUND OF THE INVENTION 
The present invention relates to optical fiber amplifiers, and more 
particularly to an efficient bi-directional optical fiber amplifier. 
The realization of an optimized high efficiency bi-directional optical 
fiber amplifier is essential for the development of long range fiber optic 
data links ("FODL") for powered weapon system guidance. Applicable systems 
include air-to-ground or surface-to-surface missiles of extended range as 
well as long range land combat missiles. One exemplary such application is 
described in U.S. Pat. No. 5,005,930, for "Multi-Directional Payout Fiber 
Optic Canister," D. K. Schotter, and assigned to a common assignee with 
the present application. The entire contents of that patent are 
incorporated herein by this reference. Other potential applications for 
military systems include unmanned ground vehicle (UGV) and unmanned 
undersea vehicle (UUV). Potential commercial applications include long 
range radar and bi-directional satellite ground station relay links, CATV 
head-end link and fiber to the home (FTTH) fiber optics systems. 
Erbium doped fiber (EDF) can be used to amplify optical signals in the 
1.53-1.58 .mu.m band by converting pump lasers operating at a wavelength 
of 1.48 .mu.m or 0.98 .mu.m into signal power. For a long haul 
bi-directional fiber optics link, typically over 100 kilometers in length, 
a prior design of an optical signal amplifier operates by a single strand 
of EDF and suffers possible input signal level dependence, near-end 
reflection and in-line optical feedback induced noise, as well as 
cross-channel cross talk. 
FIG. 1 illustrates a design for an EDF bi-directional optical fiber 
amplifier 20 employing a single strand of EDF, of the type described in 
commonly assigned pending application Ser. No. 07/655,615, filed Feb. 15, 
1991, entitled "Amplifier for Optical Fiber Communication Link," by 
Hui-Pin Hsu, Ronald B. Chesler and Gregory L. Tangonan, now abandoned, the 
entire contents of which are incorporated herein by this reference. This 
design employs two signals and two diode laser pumps (22 and 24) coupled 
together by couplers 28 and 30, propagating in the same EDF gain medium 
(EDF 26). In the system of FIG. 1, an input signal at a first wavelength 
.lambda..sub.1 from a first transmitter enters a wavelength division 
multiplexing ("WDM") coupler 10, and is sent via a long length of optical 
fiber on a fiber bobbin 12 to the amplifier 20. The amplifier 20 amplifies 
the signal at the first wavelength, and sends it through another long 
length of optical fiber on a second fiber bobbin 16 to a second WDM 
coupler 18. The coupler 18 sends the signal at the first wavelength 
through a narrow band optical filter 19 centered at .lambda..sub.1, and on 
to a first receiver. A second transmitter sends a second input signal at a 
second wavelength .lambda..sub.2 into the second WDM coupler 18 and 
through the second long length of optical fiber on bobbin 16 to the 
bi-directional amplifier 20. The amplifier 20 amplifies the second input 
signal and sends it via the optical fiber on the first bobbin 12 to the 
first WDM coupler 10, which separates this signal from the first input 
signal and sends it via a narrow band optical filter 14 centered at 
.lambda..sub.2 to the second receiver. The filter 14 rejects light at the 
first wavelength .lambda..sub.1 ; the filter 19 rejects light at the 
second wavelength .lambda..sub.2. 
The first transmitter, second receiver, optical filter 14 and WDM coupler 
10 may be carried by the master vehicle, e.g., a manned airborne vehicle, 
and the second transmitter, first receiver, the WDM coupler 18, and the 
filter 19 can be carried on a slave vehicle such as a missile. 
Drawbacks of the single channel EDF design of FIG. 1 include: 
1) It is difficult to optimize the EDF length to accommodate wide input 
signal variations which dictate the EDF length for maximum signal gain. 
When the EDF 26 is too long, unwanted attenuation reduces the signal gain. 
When the EDF length is too short, both amplified signals will not be able 
to realize the full signal gain allowed. In contrast, the new dual-arm EDF 
design in accordance with this invention allows each EDF to be optimized 
individually for one signal channel for maximum saturable gain. This makes 
the optimum length of EDF less sensitive to the input signal level. 
2) Single strand EDF amplifier designs cannot provide the switch-off option 
for individual signal channels for the bi-directional link. 
3) Single strand EDF amplifiers will incur higher noise figure (NF) due to 
the presence of a co-directional pump. 
4) Single strand EDF amplifiers will also suffer cross-channel cross talk 
when one of the signal channels is operated at a low data rate of less 
than 100 Hz, due to the depletion of excited state population for the 
stimulated emission signal gain. 
It is therefore an object of the present invention to provide a 
bi-directional EDF optical amplifier employing two separate EDF channels 
for counter-propagating signals to allow the optimization of each 
individual signal channel for efficient bi-directional optical signal 
repeater performance. 
SUMMARY OF THE INVENTION 
An optimized high efficiency bi-directional optical fiber amplifier 
employing erbium doped fibers (EDF) is described. In a general sense, the 
amplifier is characterized by first and second separate, unidirectional 
EDF amplifier channels for two respective counter-propagating signals, 
thereby permitting optimization of each individual amplifier channel for 
efficient bi-directional optical signal performance. 
The counter-propagating signals are at respective first and second 
different wavelengths .lambda..sub.1 and .lambda..sub.2. The amplifier 
further comprises wavelength selective means for maintaining separation 
between the signals of different wavelengths so that only signals of the 
first wavelength traverse the first amplifier channel and only signals of 
the second wavelength traverse the second amplifier channel. The 
wavelength selective means includes first and second signal routing 
wavelength division multiplexing (WDM) optical couplers for separating the 
signals into signal components at the first and second wavelengths. The 
first coupler is coupled to the first ends of the EDFs, and the second 
coupler is coupled to the second ends of the EDFs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A high efficiency, switchable, bi-directional fiber amplifier 50 embodying 
the present invention is illustrated in FIG. 2. The bi-directional fiber 
amplifier 50 comprises two separated optical fiber amplifiers 52 and 62 
and a pair of signal routing wavelength division multiplexing (WDM) 
couplers 56 and 66, which route a weak incoming and an amplified output 
signal to or from the two fiber amplifier arms 52 and 62. Each 
uni-directional fiber amplifier arm 52 and 62 has a counter-propagating 
pump (to the signal) source 53 or 63, respectively, a pump/signal WDM 
coupler 55 or 65 for necessary pump and signal routing, and an optical 
fiber amplifier 54 or 64 which can be, for example, a section of erbium 
doped fiber (EDF) for 1.55 micron signal amplification, or other 
rare-earth doped fiber for the amplification of other signal wavelengths. 
By way of example, one of amplifiers 54 and 64 can be an EDF for operation 
at 1.5 micron, and the other amplifier fiber can be doped with neodymium 
(Nd) for operation at 1.3 micron. 
The amplifier 50 is employed in this embodiment in a long-haul optical 
fiber data link between a first airborne vehicle 70 and a second airborne 
vehicle 90. In this exemplary application, the first vehicle 70 is the 
master vehicle, e.g., a manned aircraft, and the second vehicle is a slave 
vehicle, e.g., a guided missile or other un-manned vehicle. Data is 
exchanged between the vehicles 70 and 90 via the optical data link. 
A first transmitter 72 operating at a first optical wavelength 
.lambda..sub.1 is located on the first vehicle 70 with a second receiver 
and a narrow band optical filter 76 centered at the optical wavelength of 
the second transmitter .lambda..sub.2. A first WDM coupler 78 is also 
located on the vehicle 70 with a fiber bobbin 80, around which a long 
length of optical fiber is wound. Typical values for .lambda..sub.1 and 
.lambda..sub.2 are 1500 and 1530 nm, providing a wavelength separation of 
30 nm between the two operating wavelengths. 
A second transmitter 92 operating at a second optical wavelength 
.lambda..sub.2 is located on the second vehicle 90 with a first receiver 
94 which receives optical signals at the first wavelength .lambda..sub.1 
and a second optical narrow band filter 96 centered at the first optical 
wavelength. A second WDM coupler 98 is also located on the second vehicle 
90 with a second fiber bobbin 100, around which a long length of optical 
fiber is wound. 
In this exemplary application, the amplifier 50 is disposed at the center 
of the long-haul optical fiber data link, and serves to amplify the 
optical signals transmitted from either said first or second vehicle 70 or 
90. After the missile 90 is launched from the aircraft 70, optical fiber 
pays out from both fiber bobbins 80 and 100 to maintain the continuity of 
the optical data link between the vehicles. 
Unlike the single arm optical amplifier design shown in FIG. 1, the two-arm 
amplifier 50 of FIG. 2 allows each amplifier 52 and 62 to be optimized for 
a specific signal by selecting the length and dopant of each fiber 
amplifier 54 and 64 and the pump diode wavelength and power of pump 
sources 53 and 63. Also, interference from the counter-propagating signal 
that may deplete the signal gain and/or cause unwanted cross-talk is 
avoided. Furthermore, the two signal channels can be amplified selectively 
by controlling the two pump sources 53 and 63 individually. This allows 
the two-arm amplifier 50 to serve as an optical switch in certain 
bi-directional optical links where there is accessed control to the unit, 
such as in cascaded FTTH applications. In such applications, a controller 
such as controller 110 can be connected to each pump diode source 53 and 
63 to selectively turn on and off the respective pump sources to 
effectively turn on or off the respective amplifier arm. Of course, for 
the exemplary application of FIG. 2, wherein the amplifier 50 provides 
amplification over an unaccessible link, no controller 110 is employed. 
The amplifier 50 will include self-contained power components (such as a 
battery, not shown) to provide power to the amplifier pump sources 53 and 
63. 
The new bi-directional optical fiber amplifier in accordance with this 
invention offers the following advantages: 
1) The two separate amplifier arms allow the optimization of signal 
amplification for the two counter-propagating signals. 
2) The new amplifier scheme also offers a new feature as an in-line signal 
switch for cascaded fan-out distribution systems such as CATV or FTTH. The 
two counter-propagating pump sources can be switched on independently to 
serve as the signal switch for the bi-directional FODL without affecting 
another signal channel. When the pump source is turned off, the EDF 
provides absorptive loss to the signal instead of gain. This feature is 
not available from the single strand EDF bi-directional fiber amplifier of 
FIG. 1. 
3) The two-arm EDF approach in accordance with this invention allows the 
pumped source to be positioned at the far end of the EDF to construct the 
amplifier in a counter-propagating pump scheme which improves the noise 
performance of the optical amplifier by reducing the required pump power. 
The two-arm fiber amplifier of this invention is also applicable to another 
bi-directional optical amplifier design that uses semiconductor optical 
amplifiers in the place of fiber amplifiers. FIG. 3 illustrates an 
exemplary semiconductor optical amplifier 130 connected to optical fibers 
140 and 150. The amplifier 130 comprises a plurality of layers or regions 
132A-N, with the fibers 140 and 150 coupled to the active layer (gain 
region) 132C. The use of semiconductor amplifiers in optical fiber links 
is described, for example, in "Optical Fiber Telecommunications II," 
edited by S. E. Miller and I. P. Kaminow, Academic Press, Boston, Mass., 
1988, at page 820 et seq. 
It is understood that the above-described embodiments are merely 
illustrative of the possible specific embodiments which may represent 
principles of the present invention. Other arrangements may readily be 
devised in accordance with these principles by those skilled in the art 
without departing from the scope and spirit of the invention.