Nonlinear fiber amplifiers used for a 1430-1530nm low-loss window in optical fibers

[0002] This application is a cont of Ser. No. 09/046,900 filed Mar. 24, 1998, U.S. Pat. No. 6,101,024.

[0003] The present invention relates generally to optical amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optics applications. More particularly, the invention relates to an optical fiber amplifier and method for producing an amplified broadband output from an optical signal having a wavelength in the range of 1430-1530 nm.

[0004] Because of the increase in data intensive applications, the demand for bandwidth in communications has been growing tremendously. In response, the installed capacity of telecommunication systems has been increasing by an order of magnitude every three to four years since the mid 1970s. Much of this capacity increase has been supplied by optical fibers that provide a four-order-of-magnitude bandwidth enhancement over twisted-pair copper wires.

[0005] To exploit the bandwidth of optical fibers, two key technologies have been developed and used in the telecommunication industry: optical amplifiers and wavelength-division multiplexing (WDM). Optical amplifiers boost the signal strength and compensate for inherent fiber loss and other splitting and insertion losses. WDM enables different wavelengths of light to carry different signals parallel over the same optical fiber. Although WDM is critical in that it allows utilization of a major fraction of the fiber bandwidth, it would not be cost-effective without optical amplifiers. In particular, a broadband optical amplifier that permits simultaneous amplification of many WDM channels is a key enabler for utilizing the full fiber bandwidth.

[0006] Silica-based optical fiber has its lowest loss window around 1550 nm with approximately 25 THz of bandwidth between 1430 and 1620 nm. For example, FIG. 1 illustrates the loss profile of a 50 km optical fiber. In this wavelength region, erbium-doped fiber amplifiers (EDFAS) are widely used. However, as indicated in FIG. 2, the absorption band of a EDFA nearly overlaps its the emission band. For wavelengths shorter than about 1525 nm, erbium-atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopings have been added. For example, as shown in FIG. 3a, codoping of the silica core with aluminum or phosphorus broadens the emission spectrum considerably. Nevertheless, as depicted in FIG. 3b, the absorption peak for the various glasses is still around 1530 nm.

[0007] Hence, broadening the bandwidth of EDFAs to accommodate a larger number of WDM channels has become a subject of intense research. As an example of the state-of-the-art, Y. Sun et al. demonstrated in Electronics Letters, Vol. 33, No. 23, pp. 1965-67 (1997), a two-band architecture for an ultra-wideband EDFA with a record optical bandwidth of 80 nm. To obtain a low noise figure and high output power, the two bands share a common first gain section and have distinct second gain sections. The 80 nm bandwidth comes from one amplifier (so-called conventional band or C-band) from 1525.6 to 1562.5 nm and another amplifier (so-called long band or L-band) from 1569.4 to 1612.8 nm. As another example, M. Yamada et al. reported in Electronics Letters, Vol. 33, No. 8, pp. 710-711 (1997), a 54 nm gain bandwidth achieved with two EDFAs in a parallel configuration, i.e., one optimized for 1530-1560 nm and the other optimized for 1576-1600 nm. As yet another example, H. Masuda et al. reported in Electronics Letters, Vol. 33, No. 12, pp. 1070-72 (1997), a 52 nm EDFA that used two-stage EDFAs with an intermediate equalizer.

[0008] These recent developments illustrate several points in the search for broader bandwidth amplifiers for the low-loss window in optical fibers. First, bandwidth in excess of 40-50 nm require the use of parallel combination of amplifiers even with EDFAS. Second, the 80 nm bandwidth achieved by Y. Sun et al., may be very close to the theoretical maximum. The short wavelength side at about 1525 nm is limited by the inherent absorption in erbium, and long wavelength side is limited by bend-induced losses in standard fibers at above 1620 nm. Therefore, even with these recent advances, half of the bandwidth of the low-loss window, i.e., 1430-1530 nm, remains without an optical amplifier.

[0009] It is an object of the present invention to provide an optical amplifier for a range of wavelengths between about 1430 nm and 1530 nm.

[0010] It is another object of the present invention to provide a broadband nonlinear polarization amplifier (NLPA) for generating a broadband output from an optical signal having a wavelength between about 1430 nm and 1530 nm.

[0011] It is yet another object of the present invention to provide a parallel optical amplification apparatus having a combination of the NLPA and EDFA for the almost full 25 THz bandwidth between 1430 nm and 1620 nm in the low-loss window of optical fibers.

[0012] In accordance with the invention, a broadband NLPA is implemented by using a combination of cascaded Raman amplification and either parametric amplification (PA) or four-wave mixing (4WM) in optical fibers. To achieve the broad bandwidth, one intermediate order of the Raman cascade is arranged to be at a close proximity to the zero-dispersion wavelength of an amplifying fiber. This intermediate order phase matches PA (if its wavelength is greater than the zero-dispersion wavelength) or 4WM (if its wavelength is less than the zero-dispersion wavelength). PA/4WM generates sidebands and broaden the pump band. In subsequent Raman orders, the gain bandwidth is further broadened due to the convolution of the Raman gain band with the pump band. To produce an amplified broadband signal out of the NLPA of the invention, the optical signal to be amplified must have a wavelength greater than the zero-dispersion wavelength, which in turn must be greater than the pumping wavelength from a pumping means of the NLPA.

[0013] In one embodiment, a broadband NLPA employs a 1240 nm pump and an open-loop fiber with a zero-dispersion wavelength corresponding to one of the Raman orders (e.g., either 1310 nm or 1390 nm or dispersion-flattened in between). Another embodiment uses a Sagnac Raman cavity that is pumped at either 1117 nm or 1240 nm. Feedback by the Sagnac Raman cavity reduces the required pump power, and the broadband cavity design supports much of the generated bandwidth.

[0014] The present invention also relates to a parallel optical amplification apparatus having a combination of optical amplifiers. In one embodiment, the parallel optical amplification apparatus comprises two parallel stages of NLPAs with one NLPA optimized for 1430-1480 nm and the other for 1480-1530 nm. In another embodiment, the full 25THz of the low-loss window of approximately 1430 nm to 1620 nm in optical fibers is exploited by using a parallel combination of a NLPA of the invention and a EDFA.

[0015] NLPAs have the advantage that the gain band is set by the pumping wavelengths, and gain can be provided over virtually the entire transparency region in optical fibers (i.e., between 300 nm and 2000 nm). Moreover, because NLPAs utilize inherent properties of glass fibers, NLPAs can be used even in existing fibers by modifying the terminal ends. Hence, NLPAs are fully compatible with fiber-optic systems and can take advantage of the mature fiber-optic technologies.

[0016] These and other objects, features and elements of the present invention will be better understood from the following detailed description of preferred embodiments of the invention in which:

[0017] FIG. 1 depicts the loss profile of a 50 km fiber and the gain band of a typical EDFA.

[0018] FIG. 2 depicts absorption and gain spectra of an EDFA.

[0019] FIG. 3adepicts emission spectra of four EDFAs with different core compositions.

[0020] FIG. 3bdepicts absorption cross-section of erbium-doped glass of different compositions.

[0021] FIG. 4 depicts a measured Raman-gain spectrum for fused silica at a pump wavelength of 1000 nm.

[0022] FIG. 5 plots power gain coefficient 2 g versus phase vector mismatch Δk for parametric amplification.

[0023] FIG. 6 demonstrates basic concepts of the NLPA of the invention.

[0024] FIG. 7 illustrates the spectral broadening and gain expected from PA for a pump power of 1W and different separations between the pump and zero-dispersion wavelength.

[0025] FIG. 8 illustrates the spectral broadening and gain expected from PA for a pump and zero-dispersion wavelength separation of 1 nm and for varying pump powers.

[0026] FIG. 9 is a schematic illustration of a first embodiment of an NLPA using an open-loop configuration, and FIGS. 9a-cshow the three choices of fibers.

[0027] FIG. 10 is a schematic illustration of a second embodiment of an NLPA using a Sagnac Raman cavity that is pumped at 1240 nm.

[0028] FIG. 11 is a schematic illustration of a third embodiment of an NLPA using a Sagnac Raman cavity that is pumped at 1117 nm.

[0029] FIG. 12 is a schematic illustration of a first embodiment of a parallel optical amplification apparatus having two stages of NLPAs.

[0030] FIG. 13 is a schematic illustration of a second embodiment of a parallel optical amplification apparatus that is a combination of an EDFA and an NLPA.

[0031] The present invention provides a structure for exploiting almost the full 25 THz of bandwidth available in the low-loss window of optical fibers from 1430 nm to 1620 nm. The broadband NLPA amplifier of the invention combines Raman amplification with either PA or 4WM to achieve bandwidth performance improvements that neither technology by itself has heretofore been able to deliver.

[0032] More specifically, the broadband NLPA of the invention comprises an input port for inputting an optical signal having a wavelength λ, a distributed gain medium for receiving the optical signal and amplifying and spectrally broadening the same therein through nonlinear polarization, a pumping means operated at wavelength λ_pfor generating a pumping light to pump the distributed gain medium, and an output port for outputting the amplified and spectrally broadened optical signal. The distributed gain medium has zero-dispersion at wavelength λ₀such that λ≧λ₀≧λ_p. The pumping light cascades through the distributed gain medium a plurality of Raman orders including an intermediate order having a wavelength λ_rat a close proximity to the zero-dispersion wavelength λ₀to phase match four-wave mixing (if λ_r<λ₀) or parametric amplification (if λ_r>λ₀).

[0033] A first embodiment of the NLPA uses open-loop amplification with an optical fiber gain medium. A pump source operated at 1240 nm is used. The pump may be retro-reflected to increase the conversion efficiency. A second embodiment of the NLPA uses a Sagnac Raman cavity that is pumped at 1240 nm. Feedback in the Sagnac Raman cavity reduces the required pump power, and the broadband cavity design supports much of the generated bandwidth. A third embodiment of the NLPA uses a Sagnac Raman cavity pumped at 1117 nm for a very broadband operation.

[0034] The present invention also relates to a parallel optical amplification apparatus having a combination of optical amplifiers. In one embodiment, the parallel optical amplification apparatus comprises two parallel stages of NLPAs with one NLPA optimized for 1430 to 1480 nm and the other for 1480 to 1530 nm. In another embodiment, the full 25 THz of the low-loss window in optical fibers is exploited by using a parallel combination of a Raman amplifier and a rare earth doped amplifier. Preferably, an NLPA of the invention is used cover the low-loss window of approximately 1430 nm to 1530 nm, and an EDFA is used to cover the low-loss window of approximately 1530 nm to 1620 nm.

[0035] To provide a better understanding of the amplification mechanisms at work in the present invention, we first describe stimulated Raman scattering, Raman cascading, PA and 4WM. Stimulated Raman scattering effect, PA and 4WM are the result of third-order nonlinearities that occur when a dielectric material such as an optical fiber is exposed to intense light. The third-order nonlinear effect is proportional to the instantaneous light intensity.

[0036] 1. Stimulated Raman Scattering

[0037] 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_rfor the silica fibers is shown in FIG. 4. Notably, the Raman gain g_rextends 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.

[0038] 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 - an important consideration for system reliability.

[0039] 2. Raman Cascading

[0040] 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. 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.

[0041] 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.

[0042] 3. Parametric Amplification and Four-Wave Mixing

[0043] PA/4WM involve two pump (P) photons that create Stokes (S) and anti-Stokes (A) photons. Both PA/4WM and Raman amplification arise from the third order susceptibility X⁽³⁾in optical fibers. More specifically, the real part of X⁽³⁾, the so-called nonlinear index of refraction n₂, is responsible for PA/4WM, while the imaginary part of χ⁽³⁾associated with molecular vibrations corresponds to the Raman gain effect. In silica fibers, about ⅘ths of the n₂is an electronic, instantaneous nonlinearity caused by ultraviolet resonances, while about ⅕th of n₂arises from Raman-active vibrations, e.g., optical phonons (see further description in M. N. Islam, Ultrafast Fiber Switching Devices and Systems, Cambridge University Press, 1992). The imaginary part of this latter contribution corresponds to the Raman gain spectrum of FIG. 4.

[0044] Whereas Raman amplification is attractive for providing optical gain, PA/4WM offers an efficient method to broaden the bandwidth of the optical gain. PA/4WM has a much smaller frequency separation between pump and signal than Raman amplification, and the frequency difference may depend on the pump intensity. Just as in Raman amplification, the main advantage of PA/4WM gain is that it is present in every fiber. However, unlike the Raman effect, both PA and 4WM require phase-matching. 4WM is usually inefficient in long fibers due to the requirement for phase-matching. However, PA can act as self-phase-matched because the nonlinear index of refraction is used to phase match the pump and sidebands. This is particularly true when operating near the zero-dispersion wavelength in fibers. When 4WM and PA occur near the zero-dispersion wavelength of a single-mode fiber, phase-matching becomes automatic in the fiber. In 4WM, sidebands are generated without gain when the pump wavelength falls in the normal dispersion regime (where the pumping wavelength is shorter than the zero-dispersion wavelength). PA is 4-photon amplification in which the nonlinear index of refraction is used to phase match the pump and sidebands. For PA the pump wavelength must lie in the anomalous group velocity regime (i.e., where the pumping wavelength is longer than the zero-dispersion wavelength) and proper phase matching requires that pump and signal be co-propagating.

[0045] To illustrate the PA/4WM gain, consider the gain coefficient as derived in R. H. Stolen and J. E. Bjorkholm, IEEE J. Quantum Elect., QE-18, 1062 (1982): $\begin{array}{cc}g=\sqrt{{\left(\gamma P\right)}^2-{\left[\left(\frac{\Delta \kappa }{2}\right)+\gamma P\right]}^2}& \left(1\right)\end{array}$

[0046] The first term under the square root sign corresponds to the third order nonlinearity that couples the pump photons to the sidebands. The second term corresponds to the phase mismatch between the waves and it consists of two parts: one due to the wave-vector mismatch at the different wavelengths and the other due to the increase in nonlinear index induced by the pump. The nonlinearity parameter is defined as $\begin{array}{cc}\gamma =\frac{\omega }{c}\frac{n_2}{A_{\mathrm{eff}}}=\frac{2\pi }{\lambda }\frac{n_2}{A_{\mathrm{eff}}}& \left(2\right)\end{array}$

[0047] Also, assuming that we are operating near the zero-dispersion wavelength λ₀, the propagation constant can be expanded as $\begin{array}{cc}\mathrm{\Delta \kappa }=-{\frac{{\lambda }^2}{2\pi c}\left[\frac{dD}{d\lambda }\right|}_{{\lambda }_s}\left({\lambda }_p-{\lambda }_{\sigma }\right)]{\Omega }^2& \left(3\right)\end{array}$

[0048] where

[0049] Ω=ω_p−ω_s=ω_a−ω_p.  (4)

[0050] When the pump wavelength falls in the normal dispersion regime, then D <0, ∂D/∂λ>0, (λ_p−λ₀)<0, so that Δk >0. In this case, g is always imaginary, and there is no gain during the sideband generation process. This corresponds to the case of 4WM. If operation is in the anomalous group velocity dispersion regime, then D>0, ∂D/∂λ>0, (λ_p−λ₀)>0, so that Δk <0. This is the regime of PA, and the nonlinearity helps to reduce the phase mismatch (i.e., the two parts in the second term in Equation (1) are of opposite sign). There is gain for PA, and the gain is tunable with the pump power. As an example, the power gain coefficient 2 g is plotted schematically in FIG. 5 for operation in the anomalous group velocity regime. The peak gain (g_peak=γP) occurs at Δk_peak=−2 γP. The range over which the gain exists is given by 0>Δk>−4γP. Thus, the peak gain is proportional to the pump power, and the Δk range is determined by the pump power. Consequently, from Equation (2) we see that the bandwidth can be increased by increasing the pump power, increasing the nonlinear coefficient n₂or decreasing the effective area A_eff. Alternately, for a given required frequency range over which gain is required, the pump requirements can be reduced by increasing the effective nonlinearity (n₂/A_eff).

[0051] 4. Broadband NLPA by Combining Raman and Either PA or 4WM

[0052] This invention leads to broadband gain for cascaded Raman amplification by arranging at least one intermediate Raman cascade order at a close proximity to the zero-dispersion wavelength λ₀(e.g., within ±5 nm of λ₀, optimally within ±2 nm ). Either 4WM (if λ_r<λ₀) or PA (if λ_r>λ₀) will lead to spectral broadening of that particular Raman order. Then, in subsequent Raman orders the bandwidth will grow even further. It is further advantageous if the cascade Raman wavelength λ_rlies to the long wavelength side of λ₀. (i.e., in the anomalous dispersion regime), so that parametric amplification can occur.

[0053] The basic concept of the broadband NLPA is illustrated in FIG. 6. Starting from the pump wavelength λ_p, cascaded Raman amplification can be used in the first few stages. This is only if the pump is more than one Raman shift or 13.2 THz away from the zero-dispersion wavelength. (If it is desired to keep higher efficiency in these initial steps, a narrow band cavity design can be used, such as designs based on gratings or wavelength selective couplers.)

[0054] The key to the invention of broadening the gain bandwidth is that one of the intermediate Raman cascade orders lies at a close proximity to the zero-dispersion wavelength λ₀. By operating close to λ₀, it almost automatically phase-matches either 4WM or PA. In the subsequent cascaded Raman orders, the gain bandwidth may continue to broaden. This occurs because the effective gain bandwidth of Raman is the convolution of the bandwidth of the pump (in this case, the previous Raman cascade order) with the Raman gain curve. In this regard, see discussion in R. H. Stolen, et al., “Development of the stimulated Raman spectrum in single-mode silica fibers,” Journal of the Optical Society of America B, Vol. 1 (1984), pp. 652-57. Thus, the basic idea is to take advantage of the property of Raman amplification that the gain spectrum follows the pump spectrum. As the pump wavelength changes, the Raman gain changes as well, separated by the distance of optical phonon energy which in silica fibers is an approximately 13.2THz down-shift in frequency.

[0055] If the fiber is conventional so-called standard fiber, then zero-dispersion wavelength λ₀is about 1310 nm. On the other hand, for dispersion-shifted fiber the zero-dispersion wavelength λ₀can shift to longer wavelengths by adding waveguide dispersion. Alternately, a dispersion-flattened fiber could be used for low dispersion values over one or more of the Raman cascade orders. The additional advantage of using dispersion-flattened fiber is that the dispersion slope is small, so the gain bandwidth will be even larger (c.f. Equations (1) and (3)).

[0056] The Raman gain spectrum can follow the pump spectrum only so long as there is nothing in the Raman cavity to restrict the bandwidth of the subsequent orders. Therefore, for these higher cascade orders Raman laser schemes, it is not desirable to use either gratings or wavelength selective couplers. On the other hand, the broadband cavity design of the Sagnac Raman amplifier and laser lends itself naturally to increased bandwidth by tailoring the pump spectrum. A single-pass fiber design constitutes the broadest bandwidth design. However, a broadband cavity such as the Sagnac Raman cavity is advantageous because the feedback is used to lower the threshold and the required pump power. Also, it should be noted that broadening the bandwidth leads to a drop in efficiency, so the pump powers are already higher for the broadband cavity designs.

[0057] 5. Example of NLPA Gain Broadening from PA and Cascaded Raman

[0058] To illustrate the cascaded Raman amplification to reach the 1430-1530 nm range of the low-loss window, consider pumping with a commercially available cladding-pumped fiber laser, which operates around 1060 to 1140 nm. The various Raman orders, each separated by 13.2Thz from the previous order, are set forth in Table 1.

[0059] To obtain gain between 1430 nm and 1520 nm, the pump is operated between 1090 nm and 1140 nm, and five cascaded Raman orders are used to reach the desired wavelength. To make use of the broadening from PA or 4WM, a pumping scheme is selected in the middle of this range, i.e., starting with a pump wavelength of 1117 nm. Then, the various Raman orders land at approximately 1175 nm, 1240 nm, 1310 nm, 1390 nm and finally 1480 nm. In particular, the third Raman frequency (1310 nm) passes through the zero-dispersion point of a standard fiber, and the next order (1390 nm) could be close if the fiber is dispersion shifted. Therefore, a broadband gain is expected for wavelengths in the 1430-1530 nm range centered around 1480 nm by using a fiber with a standard dispersion and a pump wavelength of 1117 nm, 1175 nm or 1240 nm.

[0060] Next, consider the broadening expected from PA. Assume that a standard fiber is used and the pump wavelength starts at 1117 nm. The calculations use Equations (1-4) with the following typical parameters for high-Raman cross-section fiber: λ₀=1310 nm, γ=9.9W⁻¹km⁻¹, and a dispersion slope of 0.05ps/nm−km. In FIG. 7, the gain coefficient for PA is plotted versus wavelength at a pump power of 1W and wavelength separations (λ_r−λ₀) of 0.5, 1, 2 and 5 nm. For a wavelength separation of 2 nm, the PA peak gain occurs at ±10 nm, so the spectral broadening is over 20 nm. The closer the pump wavelength approaches the zero-dispersion wavelength, the wider the gain bandwidth. In addition, FIG. 8 plots the gain versus wavelength for a separation of (λ_r−λ₀)=1 nm and pump powers of 0.7, 1, 2, and 3W. The peak gain increases directly proportional to the pump power, while the bandwidth increases as the square root of pump power.

[0061] 6. Preferred Embodiments of NLPA Amplifiers

[0062] FIG. 9 shows a first embodiment of the invention which uses an open-loop design to produce an amplified broadband signal for a range of wavelengths between 1430 nm and 1530 nm. The open-loop design is the simplest nonlinear polarization amplifier although it may have a high pump power requirement. In the NLPA amplifier 20 as illustrated in FIG. 9, an optical signal having a wavelength between 1430 nm and 1530 nm is input from an input port 25 to an optical fiber 30. The optical fiber 30 is pumped by a pumping light generated by a pumping laser 35 operated at a wavelength of about 1240 nm. The optical signal is amplified and spectrally broadened in the fiber by nonlinear polarization, and output through an output port 40. The configuration is so arranged that the optical signal has a wavelength greater than the zero-dispersion wavelength of the fiber, which in turn is greater than the pumping wavelength of 1240 nm.

[0063] In this open-loop configuration, the fiber must have a cut-off wavelength below 1240 nm to be single-mode (spatial) over all wavelengths of the Raman cascade. Three choices of the fiber are preferred in this configuration. First, a standard dispersion fiber with a zero-dispersion wavelength at about 1310 nm. Second, two fibers spliced together with one fiber having a zero-dispersion wavelength at about 1310 nm (first cascade) and the other at 1390 nm (second cascade). Third, a dispersion-flattened fiber with low-dispersion at least between 1310 nm and 1390 nm. The reduced dispersion slope of such a dispersion-flattened fiber increases significantly the bandwidth for PA or 4WM.

[0064] Exemplary 1240 nm pump lasers include: (a) an 1117 nm cladding-pumped fiber laser followed by a coupler-based or grating-based Raman oscillator cavity (with gratings for 1117 nm, 1175 nm and 1240 nm); (b) an optically-pumped semiconductor laser; or (c) a chromium-doped forsterite laser. At one end of the fiber, a 1240 nm retro-reflector 45 is preferably placed to increase pumping conversion efficiency. The retro-reflector is preferably a dichroic mirror or a 1240 nm grating. The input and output ports are preferably WDM couplers, and isolators should be used at the input and output ports to prevent from lasing due to spurious feedback. It is desirous to use a counter-propagating geometry to average out noise fluctuations in this open-loop configuration. It is also possible to use a co-propagating geometry.

[0065] To reduce the pump power requirements, a broadband cavity such as the Sagnac Raman cavity is used. FIG. 10 illustrates an embodiment of the NLPA that uses a Sagnac Raman cavity design with a 1240 nm pump. Referring to FIG. 10, the Sagnac Raman cavity of the NLPA 60 is formed by a broadband mirror 70 and a loop mirror comprising a Raman gain fiber 65 and an optical coupler 90 connected thereto. The Sagnac Raman cavity design is described in a U.S. patent application Ser. No. 08/773,482, and this patent application is incorporated herein by reference. An optical signal having a wavelength between 1430 nm to 1530 nm is input through an input port 75 to the Raman gain fiber 65. A pumping laser 80 operated at a wavelength 1240 nm generates a pumping light that pumps the fiber 65 through a coupling means 85. The optical signal is amplified and spectrally broadened in the fiber by nonlinear polarization, and output through an output port 95. The configuration is so arranged that the optical signal has a wavelength greater than the zero-dispersion wavelength of the fiber, which in turn is greater than the pumping wavelength of 1240 nm.

[0066] The Raman gain fiber has the same characteristics as described above for the open-loop design. Similarly, the pumping lasers used in the first embodiment are used in this second embodiment. The broadband NLPA may further include a polarization controller 100 in the Sagnac Raman cavity for controlling polarization state. However, if the fiber is polarization maintained, the polarization controller is not necessary. The optical coupler 90 is nominally 50:50 at least for the optical signal having a wavelength between about 1240 nm and 1430 nm. The coupling means 85 is a WDM coupler which transmits at least at a wavelength between about 1300 nm and 1430 nm. Moreover, the input port and output port each comprises a WDM coupler which transmits at least at a wavelength between about 1240 nm and 1425 nm. A key advantage of the Sagnac Raman cavity is that it has a passive noise dampening property that leads to quieter cascading of various Raman orders.

[0067] As an alternative, the same kind of Sagnac Raman cavity can be used for all five Raman cascade orders between 1117 nm and the low-loss window. FIG. 11 illustrates a third embodiment of a five-order Sagnac Raman amplifier for NLPA operation. A cladding-pumped fiber laser operating around 1117 nm is used as a pumping laser 120. Different fiber combinations described in the first embodiment can be used except that the fibers must have a cut-off wavelength below 1117 nm to accommodate single-mode operation for the pump. An optical coupler 130 is nominally 50:50 at least for the optical signal having the wavelength between about 1117 nm and 1430 nm. A coupling means 125 is a WDM coupler which transmits at least at wavelengths between about 1165 nm and 1430 nm. Moreover, the input and output ports each comprises a WDM coupler which transmits at least at wavelengths between about 1117 nm and 1425 nm. Although the wavelength range of the various components increases, this configuration leads to an even broader gain band since the pump bandwidth is allowed to increase even during the first two cascades between 1117 nm and 1240 nm. Also, the noise dampening property of the Sagnac cavity can be used advantageously over all five Raman orders.

[0068] In general, the NLPA is operated as follows. An optical signal having a wavelength λ is input through an input port into a distributed gain medium having zero-dispersion at a wavelength λ₀, such as an optical fiber, which is pumped by a pumping light from a pumping means operated at a wavelength λ_p, wherein λ≧λ₀≧λ_p. The pumping light cascades through the distributed gain medium a plurality of Raman orders including an intermediate order having a wavelength λ_rat a close proximity to the zero-dispersion wavelength λ₀to phase match four-wave mixing (if λ_r<λ₀) or parametric amplification (if λ_r>λ₀). The amplified and spectrally broadened optical signal is output through an output port.

[0069] 7. Broadband Parallel Optical Amplification Apparatus

[0070] The above embodiments demonstrate that a single NLPA can accommodate the full bandwidth of the low-loss window. Moreover, the full bandwidth of the low-loss window may be reached by using a parallel optical amplification apparatus having a combination of two or more Raman amplifiers and rare earth doped amplifiers. Preferably, the NLPAs and EDFAs are used.

[0071] FIG. 12 shows a first embodiment of the parallel optical amplification apparatus using a combination of two NLPAs for a range of wavelengths between 1430 nm and 1530 nm. Referring to FIG. 12, a dividing means 170 divides an optical signal having a wavelength between 1430 nm to 1530 nm at a predetermined wavelength, preferably at 1480 nm, into a first beam having a wavelength less than the predetermined wavelength and a second beam having a wavelength greater than the predetermined wavelength. The first beam is input into a first NLPA 180 for amplification and spectral broadening therein. The second beam is input into a second NLPA 190 for amplification and spectral broadening therein. Outputs from the first and second NLPAs are combined by a combining means 200 to produce an amplified and spectrally broadened optical signal. The input port 170 and output port 200 are preferably WDM couplers.

[0072] It is preferred that the first NLPA 180 is optimized for 1430-1480 nm and centered at 1455 nm, while the second NLPA is optimized for 1480-1530 nm and centered at 1505 nm. From Table 1, these two windows can be achieved in a five-order cascade by starting with a pump wavelength of about 1100 nm for the short-wavelength side and a pump wavelength of about 1130 nm for the long-wavelength side. For the short-wavelength side, the fiber should have a zero-dispersion around 1365 nm, while for the long-wavelength side, the fiber zero-dispersion should be around 1328 nm or 1410 nm.

[0073] The narrower-bandwidth for each NLPA will also lead to an increased efficiency for each amplifier. Furthermore, the components may be more easily manufactured, since the wavelength window is not as large. Finally, the multiple amplifiers may allow for gradual upgrades of systems, adding bandwidth to the EDFA window as needed.

[0074] A spectrum of 1430-1620 nm in the low-loss window is amplified and spectrally broadened by using a parallel optical amplification apparatus comprising Raman amplifiers and rare earth doped amplifiers. FIG. 13 describes a second embodiment of the parallel optical amplification apparatus. The amplification apparatus comprises a broadband NLPA 240 as disclosed in this invention and a EDFA 250. A dividing means 230 of the apparatus divides an optical signal having a wavelength between 1430 nm and 1620 nm at a predetermined wavelength, preferably at 1525 nm, into a first beam having a wavelength less than the predetermined wavelength and a second beam having a wavelength greater than the predetermined wavelength. The broadband NLPA 240 receives the first beam and produces an amplified broadband first beam. The EDFA 250 receives the second beam and produces an amplified broadband second beam. A combining means 260 combines the amplified and spectrally broadened first and second beams to produce an amplified broadband optical signal. It is preferred that both the dividing means 230 and the combining means 260 are WDM couplers.

[0075] To use any of these embodiments with multi-wavelength WDM channels, it will most likely be necessary to include at the output of the amplifier some means for equalizing the gain. This wavelength dependency or nonuniformity of the gain band has little impact on single-channel transmission. However, it renders the amplifier unsuitable for multichannel operation through a cascade of amplifiers. As channels at different wavelengths propagate through a chain of amplifiers, they accumulate increasing discrepancies between them in terms of gain and signal-to-noise ratio. Using gain-flattening elements can significantly increase the usable bandwidth of a long chain of amplifiers. For example, the NLPA can be followed by a gain flattening element to provide gain equalization for different channels. Alternately, the gain flattening element could be introduced directly into the Sagnac interferometer loop of FIGS. 10 or 11.

[0076] It is understood that various other modifications will be readily apparent to those skilled in the art without departing from the scope and spirit of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description set forth herein, but rather that the claims be construed as encompassing all the features of the patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.