Optical: systems and elements – Optical amplifier – Beam combination or separation
Reexamination Certificate
2002-08-30
2004-07-20
Hellner, Mark (Department: 2872)
Optical: systems and elements
Optical amplifier
Beam combination or separation
C359S337000
Reexamination Certificate
active
06765716
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to Low noise optical amplifier (LNOP) and Optical communication system using LNOP, and particularly to Low noise optical amplifier and Optical communication system, which are designed to prevent degradation of the signal-to-noise ratio (SNR) at the receiving station by separating the internally generated amplified spontaneous emission (ASE) from the amplified optical signals.
2. Description of the Related Art
Optical amplifiers are widely used in various fields of optical transmission systems such as CATV distribution network, long-haul transmission between central telephone offices, and undersea inter-terrestrial transmission systems between nations. Also, Semiconductor Optical Amplifiers (SOA), which are superior to the fiber amplifiers in terms of integration and low-price, are key elements in various optical switches and wavelength converters.
In an optical amplifier, the optical signal is amplified without optoelectronic(OE) conversion. Thus, a structure of the optical amplifier is much simpler than that of a conventional regenerator and enables high speed transmission.
Particularly, the development of Erbium Doped Fiber Amplifier (EDFA), which enables direct optical amplification over broad wavelength in the 1550 nm region, lead to great innovation in optical communication technique. In addition, Wavelength Division Multiplexing (WDM) technique using the fact that optical signals with different wavelengths have negligible interaction to each other, enabled enormous expansion in transmission capacity.
FIG. 1
is a simplified diagram showing an EDFA (Erbium Doped Fiber Amplifier).
A conventional Erbium Doped Fiber Amplifier consists of a Pump LD (
11
), a Wavelength Division Multiplexer (WDM) (
13
), and a spool of Erbium Doped Fiber (EDF) (
14
).
The Erbium Doped Fiber (
14
) is usually made by doping Erbium ions into the core of Single Mode Optical Fiber (SMF) of silica. Erbium ion in silica-host has a meta-stable energy level which is capable of emitting 1.55 &mgr;m wavelength photons by stimulated emission. That is to say, electrons in the said meta-stable energy level have a relatively long excited-state lifetime (~11 msec). When an optical signal in the 1.55 &mgr;m band is simultaneously present during the lifetime of excited electrons, the optical signal causes a stimulated emission of photon and therefore the optical signal is amplified.
As explained above, it is necessary that Erbium electrons stay in meta-stable state for a signal to be amplified by stimulated emission. In order to excite Erbium electron into meta-stable state, WDM (
13
) and Pump LD (
11
) are used. Pump LD (
11
) is designed to produce high optical power in wavelength of 980 nm or 1480 nm. The wavelength of 980 nm or 1480 nm is preferred due to its high efficiency in transferring Erbium electrons into the excited state.
WDM (
13
) is an element for simultaneously delivering pump light emited from the pump LD (
11
) and optical signal introduced through the input port (
12
) to the EDF (
14
).
Irrespective of signal amplification by stimulated emission, Erbium in the excited state always produces spontaneous emission, and this spontaneously emitted photon itself also produces stimulated emission as it propagates along EDF (
14
). Therefore, in the output port (
15
) of an amplifier, there always exist amplified spontaneous emission (ASE) along with amplified signals. Due to the existence of amplified spontaneous emission (ASE), the signal-to-noise ratio (SNR) is degraded at the output port of the amplifier.
FIG. 2
is a schematic diagram illustrating the beat noise between amplified optical signals and ASE spectral components and
FIG. 3
is the one illustrating the beat noise between ASE spectral components.
Particularly, the optical spectrum vs. wavelength graph in
FIG. 2
shows the formation of beat noise (
18
) as a result of photoelectric mixing between amplified signals (
16
) and amplified ASE (
17
).
Likewise,
FIG. 3
shows the formation of beat noise (
19
) as a result of photoelectric mixing between different ASE spectral components (
17
), irrespective of the existence of optical signals.
The two kinds of beat noises are similar to the well-known beat phenomena in acoustics. Since the photodetector output current is proportional to the intensity of incident optical signals and not the amplitude thereof (Square-law detector), when two light waves having neighboring frequencies f
1
and f
2
arrive at a photodetector, the photoelectric current has two signals: one having the sum frequencies of input light (f
1
+f
2
); the other having differential frequencies (f
1
−f
2
). Generally, frequency of a light source used for optical communication is on the order of 2×10
14
, which is much greater than the typical electrical bandwidth (B
e
) of a photodetector. For this reason, the photoelectric signals having the same frequency as the optical signal or the sum of two frequencies (f
1
+f
2
) are detected only in the average sense at the receiving end. However, since the difference (f
1
−f
2
) of two frequencies (f
1
, f
2
) could be less than the bandwidth of a photodetector, there exist signals having the beat frequency (|f
1
−f
2
|) at the receiving port. Thus, even though the transmitting end intends to send DC signals, there exist oscillating signal components at the receiving end. Furthermore, in case where the light source has a number of frequency components, differential frequency components are produced due to differential combinations of frequencies. When these frequency components exist all together, randomly fluctuating beat noises are generated. If amplified optical signals and ASE exist together as they do at the output port of the amplifier, two types of beat noises are produced at the same time; signal-spontaneous beat noise which occurs between optical signals and ASE having frequency close to that of the optical signals, and spontaneous—spontaneous beat noise which occurs between ASEs.
These beat noises are the dominant noise sources at the receiving end and cause distortion of signals.
Conventionally, a single channel optical amplifier can be evaluated by gain (G), output saturation power and Noise Figure (NF).
Noise Factor (F) is the input signal-to-noise ratio (SNR
in
) divided by output signal-to-noise ratio (SNR
out
), while Noise Figure (NF) is given as 10 times the common logarithm value of Noise Factor (i.e. NF=10log
10
F). Thus, the fact that noise figure of an optical amplifier can not be lower than 3 dB means that all optical amplifiers degrades the input SNR down to at least half of the original value.
In semi-classical viewpoint, noise figure theories of an optical amplifier can be explained as follows.
First of all, in measuring SNR (signal-to-noise ratio) of optical signals entering through the input port of the optical amplifier, it is assumed that an ideal laser source with a certain wavelength, intensity and bandwidth is directly connected to an ideal detector without any functional or systematical loss.
In this ideal set-up, still there exist two types of noises; thermal noise and laser shot noise. Under the assumption that detector load impedance and temperature are constant, thermal noise is given by a constant irrespective of the intensity of input optical signals. For optical outputs exceeding a certain value, thermal noise becomes negligible compared to laser short noise, and thus can be ignored.
Accordingly, SNR
in
(input signal-to-noise ratio) is given by the following equation.
SNR
in
=
P
s
2
hv
B
e
(
1
)
(hv: photon energy, P
s
: optical power of input signals, B
e
: bandwidth of detector)
By the equation, it could be found that signal-to-noise ratio of an input signal entering through the input port of the optical amplifier (SNR
in
) is proportional to the optical power (P
s
) of the input signal, or more specifically to the input number of photons per unit time (P
s
/hv).
Choi Jung-Ho
Kweon Gyeong-Il
Lee Chang-Ho
Hellner Mark
LG Cable Ltd.
Staas & Halsey , LLP
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