Apparatus and method for Raman gain control
View Patent ↗The invention pertains to optical fiber transmission systems, and is particularly relevant to optical transport systems employing Raman optical amplifiers. In particular the invention teaches an apparatus and method to control the Raman gain based upon power measurements at one end of the transmission fiber.
1. A system for controlling Raman gain of an optical signal on a fiber span, the system comprising:
an optical tap connected to an end fiber span;
an optical coupler connected to the end of the fiber span;
an optical power meter connected to the optical tap, wherein the optical power meter is configured to measure an optical power of the optical signal;
a Raman gain control unit connected to the optical power meter, and
a Raman pump laser connected to the optical coupler and the Raman gain control unit, wherein teh Raman pump laser is configured to produce a Raman pump power,
wherein the Raman gain control unit is configured to read teh measured optical power of the optical signal from the optical power meter, and
wherein the Raman gain control unit is further configured to adjust the Raman pump laser according to a factor multiplied by the measured optical power of the optical signal.
2. The system of claim 1 wherein the factor is determined by the equation:
k
=
λ
S
λ
P
L
N
(
G
)
α
S
α
P
L
N
(
G
)
-
1
where
λ S =wavelength of the optical signal;
λ p =wavelength of the Raman pump laser;
G=net Raman gain;
α P =attenuation of the fiber at λ P ;
α S =attenuation of the fiber at λ S ; and
k=the factor.
3. The system of claim 2 wherein the Raman gain control unit is configured to adjust the Raman pump laser according to the formula:
P
Pump
=
P
Pump
,
0
+
k
(
P
S
,
Out
L
S
L
P
)
where
P Pump,0 =Raman pump power when the optical signal is not present;
P S,Out =optical signal output power;
L S =signal loss between an output of the fiber span and the optical power meter; and
L P =pump loss.
4. The system of claim 1 wherein the factor is equal to about 1.4.
5. The system of claim 1 wherein the Raman pump power counter-propagates relative to the optical signal.
6. The system of claim 1 wherein the optical power meter is configured to measures an output optical power of the optical signal.
7. The system of claim 1 wherein the factor is determined by the equation:
k
=
λ
S
λ
P
(
G
O
-
1
L
N
(
G
O
)
-
1
)
where
λ S =wavelength of the optical signal;
λ S =wavelength of the Raman pump laser; and
G O =unsaturated Raman gain; and
k=the factor.
8. The system of claim 7 wherein the Raman gain control unit is configured to adjust the Raman pump laser according to the formula:
P
pump
=
P
pump
,
0
+
k
(
P
s
,
i
n
L
S
L
P
)
where
P pump,0 =Raman pump power when the optical signal is not present;
P s,in =optical signal input power;
L S =power ratio between and input of the fiber span adn the optical power meter; and
L p =pump loss between the Raman pump laser and an input of the fiber span.
9. The system of claim 1 wherein the Raman pump power co-propagates relative to the optical signal.
10. The system of claim 1 wherein the optical power meter is configured to measure an input optical power of the optical signal.
11. The system of claim 1 wherein the Raman gain control unit is configured to continually adjust the Raman pump laser.
12. The system of claim 1 wherein adjustment of the Raman pump laser does not require utilization of a dedicated probe wavelength.
13. A method of controlling Raman gain in a fiber span, the method comprising:
transmitting, via an end of the fiber span, a Raman pump power on the fiber span;
transmitting an optical signal on the fiber span;
measuring, via the end of the fiber span, an optical power of the optical signal on the fiber span; and
adjusting the Raman pump power according to a factor mulitiplied by the measured optical power of the optical signal.
14. The method of claim 13 wherein the optical signal counter-propagates relative to the Raman pump signal.
15. The method of claim 13 wherein the measured optical power signal is the optical signal power at the output of the fiber span.
16. The method of claim 13 further comprising adjusting the Raman pump power so that so that P pump =P pump,o +k*P s,out /L s L p ,
where
P pump,0 =Raman pump power when the optical signal is not present;
P S,Out =optical signal output power;
L S =power ratio between an output of the fiber span and an optical power meter;
L P =pump loss between a pump and an input of the fiber span; and
k=the factor.
17. The method of claim 13 further comprising adjusting the Raman pump power continually.
18. The method of claim 13 wherein the optical signal co-propagates relative to the Raman pump power.
19. The method of claim 13 further comprising measuring the optical power of the optical signal at an input of the fiber span.
20. The method of claim 13 further comprising adjusting the Raman pump power so that P pimp =P pump,o +k*P s,out /L s L p
where
P pump,0 =Raman pump power when the optical signal is not present;
P S,In =input optical power of the optical signal;
L S =power ratio between an input of the fiber span and an optical power meter;
L P =pump loss between a pump and an input of the fiber span; and
k=the factor.
21. The method of claim 13 wherein the factor is determined by the equation:
k
=
λ
S
λ
P
L
N
(
G
)
α
S
α
P
L
N
(
G
)
-
1
where
λ S =wavelength of the optical signal;
λ p =wavelength of the Raman pump laser;
G=net Raman gain;
α P =attenuation of the fiber at λ P ;
α S =attenuation of the fiber at λ S ; and
k=the factor.
22. The method of claim 13 further comprising adjusting the Raman pump power without utilization of a dedicated probe wavelength.
23. The method of claim 13 wherein the factor is equal to about 1.4.