Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-01-24
2002-07-09
Tarcza, Thomas H. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06417958
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the utilization of a co-propagating Raman amplifier in an optical communication system and, more particularly, to the utilization of a co-propagating Raman amplifier in a wavelength-division-multiplexed (WDM) optical communication system.
BACKGROUND OF THE INVENTION
Raman amplifiers have played an important role in advancing optical communication systems, primarily as a result of being capable of both increasing the capacity of such systems (in terms of higher data rate and more channels) and the transmission distance of the systems. Currently, most Raman amplifiers use a counter-pumped configuration in which the pump and message signals propagate in opposite directions through the fiber amplifier. To date, there have been limited applications of a “co-pumped” Raman amplifier where the pump and message signals propagate in the same direction through the fiber amplifier. The co-pumped architecture has been avoided due to the (presumed) increased presence of noise in the co-pumped amplifier, where noise is defined as “pump-signal crosstalk” (i.e., the noise originating from the pump being coupled to the message signals through Raman gain) and “signal-pump-signal crosstalk” (i.e., the encoded signal(s) impressing information to the same or different signal wavelength via the Raman process). The issue of pump-signal crosstalk has been addressed by the applicants in their co-pending application Ser. No. 60/186,797, filed Mar. 3, 2000. The ability to reduce pump-signal crosstalk using the methods disclosed in the co-pending application has lead to the ability to analyze and overcome the problems associated with signal-pump-signal crosstalk.
In general, Raman amplification is an extremely fast process, where the amplitude modulation of the encoded signal channels over a limited bandwidth is impressed upon the Raman pump. Thus, in this environment, even a perfectly “quiet” pump (i.e., a pump without noise) will become noisy during Raman amplification. This noise on the pump may then be impressed upon other message signals through the process of Raman amplification. For the purposes of understanding the teaching of the present invention, this effect will be defined as “signal-pump-signal” crosstalk (hereinafter referred to as “SPS crosstalk), since the crosstalk between the signal channels is mediated by the pump. See, for example, the article entitled “Cross talk in Fiber Raman Amplification for WDM Systems”, by W. Jiang et al., appearing in the
Journal of Lightwave Technology, Vol
. 7, No. 9, 1989 at app. 1407-1411. In this theoretical paper, the crosstalk between two channels in a Raman amplifier was calculated. The crosstalk, even in the linear amplifier range (i.e., pump non-depletion), was shown to be severe for the co-propagating configuration. In its conclusion, the Jiang et al. reference stated that a counter-propagating arrangement would be preferred, since as the pump travels against the message signal, a stronger averaging effect exists, reducing the crosstalk.
It is also known in the art that SPS crosstalk depends on the modulation frequency of the channels, due to the relative propagation speed difference (i.e., group velocities) between the pump and message signals, as well as between the signals themselves. Such relative propagation speed difference introduces the walk-off of information in time, therefore effectively averaging the SPS crosstalk for higher frequencies. Such an effect results in a limited crosstalk bandwidth over which the SPS crosstalk may occur, which is much smaller as compared to the electrical bandwidth of the data. The crosstalk bandwidth in the counter-propagating configuration is known to be relatively small when compared to that encountered in the co-propagating configuration. See, for example, an article entitled “Bandwidth of cross talk in Raman amplifiers” by F. Forghieri et al. appearing in the OFC '94 Technical Digest at page 294. In this paper, the crosstalk bandwidth was defined as the frequency bandwidth in which the modulation depth onto the second continuous-wave signal channel is more than −20 dB. In this definition, the crosstalk bandwidth is determined by the fiber dispersion parameters at the wavelengths of the pump and signals, as well as the amount of Raman gain and pump depletion. In their study, the crosstalk bandwidth was determined to be approximately 100 MHz for a co-pumped Raman amplifier, as compared to only a 10 kHz bandwidth for a counter-pumped arrangement. Therefore, the Forghieri et al. paper concludes that the performance of an intensity modulated WDM system using a co-pumped Raman amplifier was severely limited by SPS crosstalk, noting as preferable, then, the counter-propagating configuration. A similar conclusion was made in the article “Crosstalk due to stimulated Raman scattering in single-mode fibers for optical communication in wavelength division multiplex systems” by H. F. Mahlein appearing in
Optical and Quantum Electronics
16, (1984), p. 409 et seq.
It is well-known by those skilled in the art that the term “relative intensity noise” (RIN) is often used to characterize fluctuations in photocurrents. RIN is defined in terms of detected electrical power as the power-spectral density of the photocurrent in a 1—Hz bandwidth at a specified frequency divided by the average power of the photocurrent. Although the term “RIN” indicates that this quantity is usually used to characterize fluctuations arising from noise, the same quantity can be used to characterize fluctuations due to signal modulation, as is the case here.
As mentioned above, SPS crosstalk depends on both pump depletion and Raman gain. It is known by those skilled in the art that the amount of Raman gain is known as the “on/off” gain, and defined as ratio of output signal power in the presence of the Raman pump to the output signal power in the absence of the Raman pump. If a Raman pump amplifies an optical signal to an optical power comparable to the power of the Raman pump, the Raman pump will experience “pump depletion”. This means that the power of the pump, at some position within the amplifier, will become significantly less than it would be in the absence of the signal, and that the “net” Raman amplification will be reduced. In practice, pump depletion is measured at the output of the amplifier and is defined as the intensity difference of the pump output “with” and “without” signal channels. In general, the smaller the pump depletion and Raman gain, the smaller the SPS crosstalk. Therefore, it is expected from the prior art studies that co-pumped Raman amplifiers are limited to applications with small levels of Raman gain and pump depletion. See, for example, the article entitled “Wide-Bandwidth and Long-Distance WDM Transmission using Highly Gain-Flattened Hybrid Amplifier” by S. Kawai et al., appearing in
IEEE Photonics Technology Letters
, Vol. 11, No. 7, Jul. 1999. In this article, 4 dB of Raman gain in a co-propagating-pump geometry was used in a discrete Raman amplifier as a part of the gain-flattened hybrid amplifier.
The prior art is replete with references describing the SPS crosstalk problem present in fiber Raman amplifiers and the utilization of a counter-propagating amplifier configuration to overcome this problem. The Jiang et al. article referenced above describes a solution involving limiting the signal gain and the injected pump power to values well below the threshold for Raman amplification. The Forghieri et al. and Mahlein articles suggest a solution in terms of eliminating all components within the crosstalk bandwidth (for example, 100 MHz), which is not practical in WDM applications. Several papers have stated that a counter-propagating configuration eliminates SPS crosstalk through averaging. For example, see an article by S. A. E. Lewis et al. appearing in
Electronic Letters
, Vol. 35, No. 11, 1999, at page 923. To date, therefore, most Raman amplifiers have been limited to the counter-propagating configuration.
However, as WDM optical communication sys
Du Mei
Nielsen Torben N.
Rottwitt Karsten
Stentz Andrew John
Hughes Deandra M.
Koba Esq. Wendy W.
Tarcza Thomas H.
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