Balanced coupler for radiation sources

Details Balanced coupler for radiation sources Balanced coupler for radiation sources

2001-10-03

2003-12-30

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

With optical coupler

Plural

Reexamination Certificate

active

06671429

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical communications and specifically to a balanced coupler for coupling multiple radiation sources and an optical amplifier and transmission system using the balanced coupler.
BACKGROUND OF THE INVENTION
Wave division multiplexing (WDM) increases bandwidth in optical communications by providing for communication over several wavelengths or channels. For long haul optical communications the optical signal must be periodically amplified. Current amplification schemes include Erbium doped fiber amplifiers (EDFA) and Raman amplifiers.
To maximize WDM capacity, it is desirable that the optical bandwidth of the system be as wide as possible. Thus, a wide range of optical signal wavelengths must be amplified. At the same time, it is desirable that the different optical signal wavelengths be amplified by about the same amount by the amplifiers in the amplification system. Thus, it is desirable that the amplification gain profile of the amplification system should be both broad and relatively flat.
Raman amplification can provide a broad and relatively flat gain profile over the wavelength range used in WDM optical communications by using a plurality of different pump laser wavelengths. (See Y. Emori, “100 nm bandwidth flat-gain Raman Amplifiers pumped and gain-equalized by 12-wavelength channel WDM Diode Unit,” Electronic Lett., Vol. 35, no. 16, p. 1355 (1999). and F. Koch et. al., “Broadband gain flattened Raman Amplifiers to extend to the third telecommunication window,” OFC' 2000, Paper FF3, (2000)). Raman amplifiers may be either distributed or discrete (See High Sensitivity 1.3 (m Optically Pre-Amplified Receiver Using Raman Amplification,” Electronic Letters, vol. 32, no. 23, p. 2164 (1996)). The Raman gain material in distributed Raman amplifiers is the transmission optical fiber, while a special spooled gain fiber is typically used in discrete Raman amplifiers.
FIG. 1
is a schematic of a typical optical communication system using Raman amplifiers for periodic amplification of the optical signal. The system includes transmitter terminal
10
and receiver terminal
12
. The transmitter terminal includes a number of optical communication transmitters
14
a
,
14
b
, . . .
14
z
respectively transmitting signals at optical communications wavelengths &lgr;a, &lgr;b, . . . &lgr;z.
The optical signals are multiplexed by multiplexer
16
and are amplified by a series of amplifiers A
1
, A
2
, . . . An. The signals are transmitted from the transmitter
10
to the amplifiers, between the amplifiers, and from the amplifiers to the receiver
12
via transmission optical fiber
26
. For distributed Raman amplification, the optical amplifier will also include transmission optical fiber. The optical signals are then demultiplexed by demultiplexer
18
of receiver
12
to respective optical communications receivers
20
a
,
20
b
, . . .
20
z
. The demultiplexer
18
sends optical communications wavelengths &lgr;a, &lgr;b, . . . &lgr;z to respective optical communications receivers
20
a
,
20
b
, . . .
20
z.
Although
FIG. 1
shows signals directed from transmitter terminal
10
to receiver terminal
12
for ease of illustration, in general the transmitter terminal
10
and receiver terminal
12
are typically transmitter/receiver terminals for bidirectional communication. In this case each of the transmitter/receiver terminals will have transmitters as well as receivers and both a multiplexer and demultiplexer.
FIG. 2
is a schematic of a typical distributed Raman optical amplifier
50
employed as one of the amplifiers in the series of amplifiers A
1
, A
2
, . . . An in the system of FIG.
1
. The amplifier
50
includes optical pump assembly
51
(shown enclosed by dashed lines) and transmission fiber
64
. In this amplification scheme, the pump assembly
51
includes a pump radiation source
52
that provides, for example, twelve different pump wavelengths &lgr;
1
through &lgr;
12
. Specifically, the pump radiation source
52
comprises a plurality of pump sources, i.e., twelve lasers
56
that each emit radiation at a different wavelength of the wavelengths &lgr;
1
through &lgr;
12
, respectively. The radiation from the individual radiation sources
56
of the pump radiation source
52
are then coupled or combined at a pump radiation coupler
54
, and the coupled radiation is output at pump radiation coupler output
58
.
The coupled radiation has a coupled radiation profile that is a combination of the individual radiation profiles of the radiation input into the pump radiation coupler
54
. The pump radiation profile, that will be coupled with the optical signal to be amplified, is therefore the coupled radiation profile in this case. Thus, the pump radiation profile is output from output
58
. The pump radiation profile from output
58
is then coupled at pump-signal combiner
60
with the optical signal
62
. Optical signal
62
, i.e., the data signal, propagates in the transmission optical fiber
64
in a direction opposite to the radiation, i.e., a counterpropagation direction, of the pump radiation profile. The optical signal is amplified along transmission optical fiber
62
.
SUMMARY OF THE INVENTION
It would be desirable to provide an optical coupler system that could provide substantially the same optical output power at each output of the coupler.
According to one embodiment of the invention there is provided an optical coupler system. The optical coupler system comprises: a first optical coupler having at least a first and a second input and a first and a second output; and a second optical coupler having at least a first and a second input and a first and a second output. The first and second outputs of the first optical coupler are connected to the first and second inputs, respectively, of the second optical coupler via first and second optical links, and the radiation that is input to the first input of the first optical coupler is coupled to both the first and second optical links to travel over first and second paths as first path radiation and second path radiation. At the second coupler the second path radiation is incoherently combined with the first path radiation for output on the first output of the second coupler.
According to another embodiment of the invention there is provided an optical coupler system. The optical coupler system comprises: a first optical coupler having at least a first and a second input and a first and a second output; and a second optical coupler having at least a first and a second input and a first and a second output. The first and second outputs of the first optical coupler are connected to the first and second inputs, respectively, of the second optical coupler via first and second optical links. The first and second links provide different optical paths between said first and second optical couplers such that portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at said first output of said second optical coupler.
According to another embodiment of the invention there is provided an optical coupler. The optical coupler system comprises: a series of N couplers optically connected in series, where N is an integer greater than 1, the couplers in the series numbered i=1 to i=N, each ith coupler having at least first and second inputs and at least first and second ouputs; and a series of N−1 groups of optical links, the series of groups numbered j=1 to j=N−1, wherein each optical,link of the jth group of optical links optically connects a respective output of the ith coupler to a respective input of the (i+1)th coupler when i=j. The optical links provide different optical paths between said first and Nth optical couplers such that portions of radiation energy that is input to said first input of said first optical coupler are combined incoherently at said first output of said Nth optical coupler.
According to another embodiment of the invention there is provide

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