Optical power equalizer

Details Optical power equalizer Optical power equalizer

2001-05-14

2003-10-21

Lee, John D. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S122000, C359S566000, C359S569000, C359S573000

Reexamination Certificate

active

06636666

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and apparatus for equalizing the power of at least one frequency in a multi-wavelength optical signal, and limiting the power in an optical system. More particularly, the invention relates to a method and apparatus for equalizing the power of at least one frequency in a multi-wavelength optical signal that equalizes without spectrally dispersing the signal.
2. Background of the Related Art
Extremely low losses in optical fibers have made them the transmission medium of choice for communications networks. These losses, however, are not zero so transmitted optical signals need to be amplified to compensate for the losses. Moreover, signals in all but the simplest networks split among several paths would each require amplification.
In a few short years, the erbium doped fiber amplifier (EDFA) has revolutionized optical networks by providing amplification of optical signals without conversion to and from the electrical domain.
FIG. 1
shows the gain versus input signal wavelength for 3 different EDFAs based on A1/P Silica, Silicate L22, and Fluorozirconate F88, respectively.
FIG. 1
is reproduced from J. Miniscalco, J. Lightwave Tech. 9, 234 (1991), which is hereby incorporated by reference herein. As indicted by
FIG. 1
, each type of EDFA has a gain peak centered at about 1530 nm (nanometers), with substantially reduced gain at wavelengths below about 1490 nm and above about 1580 nm. For an input signal between 1490 nm and 1580 nm, the gain of the EDFA is still strongly dependant on input wavelength. For virtually any given multi-wavelength input signal, the EDFA will amplify each wavelength a different amount.
FIG. 1
shows that even when the multi-wavelength input signal has an even power distribution, the output of the EDFA will not have a uniform distribution of power between the wavelengths.
When amplifying a single wavelength, the pump power and the signal wavelength determine the gain. To maintain a substantially constant optical power level in a conventional substantially single wavelength system throughout the system, each EDFA has a feedback circuit that controls the pump power while monitoring the output power. Thus, even a relatively simple system relying on a single frequency requires a complex feedback mechanism to assure a stable network.
The EDFA has a more complex behavior when it simultaneously amplifies several wavelengths. The gain at a particular wavelength depends, in addition to the pump power and the wavelength, on its input power relative to those at the other wavelengths. Specifically, the gain is greatest for the wavelengths that already have the most power and least for the weakest power wavelengths. The gain differential is modest, so it is generally not a serious problem for small multi-wavelength networks containing just a few EDFA's. But as the size of the network grows, the effects of the gain differential accumulate, and can lead to instability in the network with a few wavelengths dominating.
The output spectrum of the EDFA ideally should be flat, or equalized, to avoid instabilities due to single wavelength domination. It is not sufficient to simply equalize the EDFA gain. The instability still occurs with a flat gain if the input spectrum is not flat. Thus, a general solution to this problem must equalize the EDFA output rather than its gain.
If it is possible to predict with reasonable accuracy the input power distribution to each EDFA, then one can insert compensating filters that attenuate the stronger wavelengths more than the weaker ones so that each EDFA output power distribution is spectrally flat or equalized. For example, see C. R. Giles and D. I. DeGiovanni, “Dynamic Gain Equalization in Two-Stage Fiber Amplifiers,” IEEE Phot. Technol. Letts., 2,866-868, (1990); M. Tachibana, R. I. Laming, P. R. Morkel and D. N. Payne, “Erbium-Doped Fiber: Amplifier with Flattened Gain Spectrum,”
IEEE Phot. Technol. Letts.,
3, 118-120, (1991); and A. E. Willner and S. -M. Hwang, “Transmission of Many WDM Channels Through a Cascade of EDFA's in Long Distance Links and Ring Networks,”
J. Lightwave Technol.,
5, 802-816, (1995), which are hereby incorporated by reference. But such an approach works best if the network is static. A change in the input power distribution of one EDFA will disrupt power equalization throughout the system. In turn, this will upset the input power distribution to other EDFA's, disrupt their equalization and may eventually cause a network-wide instability. Such changes in the power distribution would be common in reconfigurable systems where wavelengths are intentionally switched from one path to another.
It is these reconfigurable optical fiber systems that are most attractive for large multi-wavelength networks. For example, see G. K. Chang, G. Ellinas, J. K. Gamelin, M. z. Iqbal and C. A. Brackett, “Multiwavelength Reconfigurable WDM/ATM/SONET Network Testbed,”
J. Lightwave Technol.,
14, 1320-1340, (1996); and R. E. Wagner, R. C. Alfemess, A. A. M. Saleh and M. S. Goodman, “MONET: Multiwavelength Optical Networking,”
J. Lightwave Technol.,
14, 1349-1355, (1996), which are hereby incorporated by reference. For these networks to function properly, the output of each EDFA must remain equalized even as the input power distribution varies. The gain required to maintain a flat output spectrum will vary as the input varies. A power equalizer must continuously sense the power at each wavelength and alter the power at that wavelength accordingly. The equalizer must treat each wavelength independently so that a strong wavelength will be attenuated without attenuating the weaker ones. Besides these basic requirements, the equalizer should have features that would make it attractive for widespread use such as scalability in the number of wavelengths, low cost, reliability, ease of use, etc.
Several approaches have been demonstrated to try to solve this problem. Experiments showing that the coupling between wavelengths in EDFA's decreases at cryogenic temperatures because the gain becomes inhomogeneously broadened, suggest operating the EDFA at cryogenic temperatures. For example, see L. Eskildsen, E. Goldstein, V. da Silva, M. Andrejco and Y. Silberberg, “Optical Power Equalization for Multiwavelength Fiber-Amplifier Cascades Using Periodic Inhomogeneous Broadening,”
IEEE Phot. Technol. Letts.,
5, 1188-1190, (1993), which is hereby incorporated by reference. However, cryogenic cooling is not economically attractive.
A different approach is to disperse the light spectrally, measure the power at each wavelength, and adjust a tunable filter for each wavelength according to the measured powers. For example, see K. Inoue, T. Kominato and H. Toba, “Tunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers,”
IEEE Phot. Technol. Letts.,
3, 718-720, (1991); F. Su, R. Olshansky, G. Joyce, D. A. Smith and J. E. Baran, “Gain Equalization in Multiwavelength Lightwave Systems Using Acoustooptic Tunable Filters,”
IEEE Phot. Technol. Letts.,
4,269-271, (1992); and F. Khaleghi, M. Kavehrad and C. Bamard, “Tunable Coherent Optical Transversal EDFA Gain Equalization,”
J. Lightwave Technol.,
13,581-587, (1995), which are hereby incorporated by example. This approach requires a large amount of hardware, including a spectrometer, to be attached to each EDFA.
An example as disclosed in U.S. Pat. No. 5,155,780 to Zirngible, of a method to equalize the optical power in a network is an optical limiting amplifier. In the optical limiting amplifier, the input optical signal is divided into two signals. One signal is the input signal of the optical amplifier. The other signal is passed through a saturable absorber. The signal from the saturable absorber is fed back into the optical amplifier via the output of the optical amplifier. The output signal from the saturable absorber varies at a rate which is greater than the variation between the input signal and the saturable absorber, and thus forms a negati

Affiliated with

Also associated with

Rating

Optical power equalizer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical power equalizer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical power equalizer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3160983