Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-01-12
2003-10-07
Moskowitz, Nelson (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341320, C372S003000, C372S006000, C372S020000
Reexamination Certificate
active
06631025
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to low noise optical amplifiers for fiber optic transmission systems, and more particularly to low noise discrete, distributed and hybrid Raman amplifiers for broadband communication systems.
2. Description of Related Art
Stimulated Raman scattering is an important nonlinear process that turns optical fibers into amplifiers and tunable lasers. Raman gain results from the interaction of intense light with optical phonons in silica fibers, and Raman effect leads to a transfer of energy from one optical beam (the pump) to another optical beam (the signal). The signal is downshifted in frequency (or upshifted in wavelength) by an amount determined by vibrational modes of silica fibers. The Raman gain coefficient g
r
for the silica fibers is shown in FIG.
1
. Notably, the Raman gain g
r
extends over a large frequency range (up to 40 THz) with a broad peak centered at 13.2 THz (corresponding to a wavelength of 440 cm
−1
). This behavior over the large frequency range is due to the amorphous nature of the silica glass and enables the Raman effect to be used in broadband amplifiers. The Raman gain also depends on the composition of the fiber core and can vary with different dopant concentrations.
Raman amplification has some attractive features. First, Raman gain is a good candidate for upgrading existing fiber optic links because it is based on the interaction of pump light with optical phonons in the existing fibers. Second, there is no excessive loss in the absence of pump power, other than the loss of the fiber inserted—an important consideration for system reliability.
Cascading is the mechanism by which optical energy at the pump wavelength is transferred, through a series of nonlinear polarizations, to an optical signal at a longer wavelength. Each nonlinear polarization of the dielectric produces a molecular vibrational state corresponding to a wavelength that is offset from the wavelength of the light that produced the stimulation. The nonlinear polarization effect is distributed throughout the dielectric, resulting in a cascading series of wavelength shifts as energy at one wavelength excites a vibrational mode that produces light at a longer wavelength. This process can cascade through numerous orders. As an example, cascade Raman orders for different pump wavelengths are illustrated in FIG.
2
. Because the Raman gain profile has a peak centered at 13.2 THz in silica fibers, one Raman order can be arranged to be separated from the previous order by 13.2 THz.
Cascading makes stimulated Raman scattering amplifiers very desirable. Raman amplification itself can be used to amplify multiple wavelengths (as in wavelength division multiplexing) or short optical pulses because the gain spectrum is very broad (a bandwidth of greater than 5 THz around the peak at 13.2 THz). Moreover, cascading enables Raman amplification over a wide range of different wavelengths. By varying the pump wavelength or by using cascaded orders of Raman gain, the gain can be provided over the entire telecommunications window between 1300 nm and 1600 nm.
Raman gain can be used in both discrete and distributed amplifiers. The main advantages of distributed Raman amplification are that the effective noise figure (NF) is improved and existing systems can be upgraded. Intuitively, the NF improves because the signal is continuously amplified and never gets too weak. The additional system margin allowed by distributed amplification can be used to upgrade the system speeds, increase the spacing between amplifiers or repeaters, or to handle the variability in fibers for installed systems. When using distributed amplification, the pump light can be counter-propagating to the signal direction. Simulations and experiments have shown the improvement in noise figure achieved using distributed amplification. For example, a calculation from first principles for a chain of optical amplifiers shows the improvement in signal-to-noise ratio (SNR) for more closely spaced amplifiers. The case of purely uniform amplification gives an improvement of about NF=2 dB compared with amplifiers spaced evenly every 21.7 km and an improvement of about NF=4 dB compared with amplifiers spaced evenly every 43.4 km (where NF (dB)=SNR
IN
(dB)−SNR
OUT
(dB)).
Experiments have also verified the improvement in NF performance for distributed amplification. For instance, experiments in a 514 km Raman amplifier chain have shown an improvement in noise performance of 2 dB compared with a similar amplifier chain using lumped EDFA's spaced roughly every 45 km. This is less than the ideal case because the pump light attenuates along the length of the fiber, leading to periodic but non-uniform amplification. In addition, a combination of distributed Raman amplification and EDFA's has been used to extend the repeater spacing to 240 km for a 5280 km WDM 8-channel system. The performance demonstrated in this experiment was comparable to that of a system of similar length and capacity using conventional EDFA's spaced by 80 km. Therefore, the additional NF margin from distributed amplification can be used to significantly increase the repeater spacing of long-haul transmission systems. Furthermore, a distributed Raman amplifier is tested in a 45 km length of transmission fiber that is pumped by two pumps at 1453 nm and 1495 nm. The resulting transparency gain bandwidth is 92 nm, and the Raman amplifier is shown to perform better than a lumped EDFA with a NF equal or higher than 5 dB.
Another use of hybrid or distributed amplifiers is to reduce nonlinearity impairments from four-wave mixing (4WM) and Raman gain tilt that become increasingly important as new bands are added and the channel count increases. One way of minimizing these nonlinearity impairments is to reduce the power per wavelength channel. This can be achieved without degradation of the signal-to-noise ratio at the receiver by using hybrid or distributed Raman amplification. In particular, distributed Raman amplification can be achieved by pumping the fiber composing the transmission line with a Raman oscillator or laser diodes directly. The pump light produces Raman gain for the signal using the inherent Raman gain in the transmission fiber. Since the gain is inherent to the transmission line, this provides a graceful means of upgrading even existing fiber-optic systems.
The power per channel can be reduced because distributed Raman amplification cancels or compensates for the loss in the fiber. Said another way, the distributed Raman gain has an effectively better noise figure than its discrete amplifier counterparts. The channel power can be lowered to the point that nonlinearities become insignificant. For example, in a typical transmission system at power of 0 dBm (1 mW) is used at OC-48 or 2.5 Gb/s and 6 dBm (4 mW) at OC-192 or 10 Gb/s per channel. With the addition of distributed amplification, OC-192 systems have been demonstrated in the laboratory with power per channel as low as −13 dBm (0.05 mW).
Distributed Raman amplification can also help in gain control or gain clamping, i.e., It is undesirable to have the gain level change when channels are added or dropped, such as when optical add/drop multiplexers are used. This gain clamping problem can be solved to a large extent by using distributed Raman amplification because the power per channel is significantly reduced. The lower power insures that there will be negligible gain or pump depletion. Therefore, the combination of lower power per channel and negligible gain depletion provides an effective gain clamping.
That nonlinear effects in fiber transmission systems can be avoided by use of distributed or hybrid Raman amplification has been illustrated in a number of recent experiments. Transmission in DSF around the zero-dispersion region in a single wavelength band has been demonstrated. Dense-WDM (DWDM) transmission of 32 channels with 50 GHz spacing and bit-rate of 10 Gb/s over 8×80 km has been demonstrated.
Freeman Michael
Islam Mohammed N.
Moskowitz Nelson
Xtera Communications Inc.
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