Provisional Application No. 60/197,932, filed on April 13,2000.

Background This application relates to optical amplifiers, and more specifically, to fiber optical amplifiers for wavelength-division multiplexed (WDM) applications.

Optical fibers may be doped with active ions such as rare earth ions to operate as optical gain media. Under proper optical pumping conditions, such a doped fiber may be optically pumped at a pump wavelength within the absorption spectral band of the doped fiber and emit light at an emission wavelength that is longer than the pump wavelength and within the emission spectral band of the doped fiber. When an input optical signal at the emission wavelength is injected into the optically-pumped doped fiber, the input optical signal is amplified by the active ions which convert the pump energy into the optical energy at the emission wavelength.

Doped fibers may be designed to produce optical gain over a limited spectral range. In WDM applications, different optical channels at different WDM wavelengths may be amplified by using the same doped fiber amplifier.

One commonly used fiber amplifier for the 1.5-micron band is the Erbium-doped fiber amplifier (EDFA). The typical bandwidth of some conventional EDFAs is about 30 nm and hence is much less than the low-loss bandwidth of commercial fibers. In order to fully use the available bandwidth of the fibers and to meet the demands for broad bandwidth in fiber communications, it is desirable to develop fiber amplifiers with wider bandwidths.

One way to expand the bandwidth of the optical fiber amplifiers is to use combine fiber amplifiers with different operating spectral ranges. For example, a fiber amplifier, e. g., a silica-based EDFA, in the L-C spectral band may use an EDFA in the C band from about 1530 nm to about 1560 nm and another EDFA in the L band from about 1570 nm to about 1600 nm a parallel EDFA configuration. In operation, input WDM channels may be split into C-band channels for amplification in the C- band EDFA and L-band channels for amplification in the L- band EDFA. The amplified C-band channels and L-band channels are then combined or"stitched"together to produce amplified channels in the L-C band from about 1530 nm to about 1600 nm.

Summary This application includes techniques and devices that combine two or more parallel fiber amplifiers of different operating spectral ranges with sampled Bragg gratings such as fiber Bragg gratings (FBGs) to achieve a wide operating spectral range for amplifying optical signals. A sampled fiber Bragg gratings may be designed to produce multiple Bragg reflection bands over a wide spectral range, e. g., over tens of nanometers, while each Bragg reflection band has a narrow bandwidth to carry a single WDM channel with a sharp roll-off at the band edge (e. g., a fraction of one nanometer). At last two sampled FBGs with about the same channel spacing may be used to split the input WDM channels for amplification by different fiber amplifiers and to combine amplified WDM channels from different fiber amplifiers together to produce the final amplified WDM channels.

This use of sampled FBGs may substantially reduce or eliminate the unusable spectral region between or in the overlapping region of two fiber amplifiers with adjacent and different operating spectral ranges. In addition, the sampled FBGs effectively suppress amplified spontaneous emission outside the WDM channel frequencies.

In one implementation, at least two sampled FBGs with about the same channel spacing but different operating spectral ranges may be used, where the Bragg reflection band at the highest channel frequency in the sampled FBG with a lower spectral range may be one channel spacing less than the Bragg reflection band at the lowest channel frequency in another sampled FBG with a higher spectral range. In another implementation, at last two sampled FBGs with about the same channel spacing and operating spectral ranges may be used.

Brief Description of the Drawings FIGS. 1 and 2 show fiber amplifiers that use two parallel fiber amplifiers in different operating spectral ranges and two different sampled FBGs as the channel splitter and combiner.

FIG. 3 shows the Bragg reflection bands of the two sampled FBGs and the gain profiles of the different fiber amplifiers used in FIGS. 1 and 2 according to one implementation.

FIG. 4 shows a first example of a fiber amplifier that uses two parallel fiber amplifiers in different operating spectral ranges and two identical sampled FBGs as the channel splitter and combiner.

FIG. 5 illustrates certain properties and operations of the fiber amplifier in FIG. 4.

FIG. 6 shows one implementation of the fiber amplifier in FIG. 4.

FIG. 7 shows a second example of a fiber amplifier that uses two parallel fiber amplifiers in different operating spectral ranges and two identical sampled FBGs as the channel splitter and combiner.

FIGS. 8,9,10A, 10B, 10C, and 11 show measurements of the fiber amplifier in FIG. 6.

Detailed Description A L-C band silica-based EDFA with a C-band EDFA and a L-band EDFA in parallel operates by first splitting input WDM channels in a wide frequency span over both the L and C bands in the frequency domain into channels in the C band and channels in the L band. Upon amplification in two separate EDFAs, the L-band channels and the C-band channels are then combined in an output fiber to produce the amplified output channels. The above channel splitting in the frequency domain may be implemented by using a wideband wavelength-selective optical element to separate optical energy in a selective, continuous operating spectral range from optical energy outside that continuous operating spectral range. The channel combining may be done in a similar manner. Examples of such wideband wavelength-selective elements include a wavelength selective coupler and a wideband fiber Bragg grating.

Such a wideband device generally does not have a sharp transition at the edge of its continuous operating spectral range but has a gradual roll-off on the order of many nanometers or tens of nanometers. One result of this frequency roll-off is a"dead"frequency zone between the C-band channels and the L-band channels in

which no signal channels can be properly transmitted. In some cases, this wasted bandwidth region may be in the range of 8 to 16 nm and hence results in a significant or even unacceptable amplifier overhead. It would be highly advantageous for system designers to be able to utilize this unusable inter-band wavelength region.

In recognition of the above, the techniques and devices of the present application use a distinctly different approach to the above channel splitting and combining in the frequency domain. Instead of using a wavelength-selective optical element to produce a continuous wideband in a selected spectral range (e. g., the C or L band), a sampled FBG is used to produce discrete, narrow spectral bands only at desired WDM channel frequencies in the selected spectral range (e. g., C or L band).

Such a sample FBG may be fabricated in a fiber with two overlapping spatial patterns: a spatial grating pattern and a spatial sampling pattern. The grating pattern is generally a spatial periodic grating pattern, e. g., a spatial modulation of the refractive index of the fiber core, and is formed in the fiber core along the longitudinal direction of the fiber. This grating pattern effectuates a. Bragg grating in the fiber core.

Such a Bragg grating can interact with a guided optical signal to selectively reflect each spectral component XB (Z) within a Bragg reflection band that satisfies the Bragg condition, XB (z) = 2neff (z) A (z), and transmits other spectral components that fail the Bragg condition, where z represents the position along the fiber, neff (z) the effective index of refraction of the fiber, and A (z) the period of the grating. The grating parameter, neff (z) A (z), may be a constant along the fiber although it may also be a linearly or nonlinearly chirped function of the position z. In the present application, such a grating may be designed to produce a narrow Bragg reflection band to carry a single WDM channel. Since each Bragg reflection band is narrow, the edge at each side of the Bragg reflection band can be sharp, e. g., with a roll-off at a fraction of one nanometer. The spatial sampling pattern is also a periodic pattern either a constant sampling period or a chirped sampling period and may be formed in the fiber core. The sampling period is greater than the grating period. This sampling pattern is superimposed over and modulates the grating pattern to duplicate the Bragg reflection band produced by the grating pattern at different, discrete WDM channel frequencies. This feature of the sampling essentially

uses discrete Bragg reflection bands to replace the. continuous operating spectral range of aforementioned wideband devices.

A number of features may be achieved in a wideband fiber amplifier with parallel fiber amplifiers of different operating spectral ranges by using sampled FBGs. For example, the above dead spectral zone may be substantially reduced or eliminated because the sampling patterns of sampled FBGs may be designed to produce Bragg reflection bands at desired WDM channels so that the two different channels amplified by different fiber amplifiers but next to each other in frequency may be designed to separate by a few channel spacings or just one channel spacing. Since each of the Bragg reflection bands at different WDM frequencies is a narrow band to carry only one channel, its frequency roll-off at the band edge is sharp and may be only a fraction of one nanometer. Hence, the problem of the gradual roll-off at the band edge of many wideband devices with a continuous operating spectral range has been addressed. In addition, since optical energy at discrete frequencies of the Bragg reflection bands of the FBGs is selected for output and optical energy at other frequencies in the broad operating spectral ranges of the fiber amplifiers

is rejected from the output, the sampled FBGs effectively suppress amplified spontaneous emission outside the WDM channel frequencies.

The following describes several embodiments of optical fiber amplifiers with two or more parallel fiber amplifiers of different operating spectral ranges by using sampled fiber Bragg gratings (FBGs) for splitting input WDM channels to be amplified and combining amplified WDM channels. It is understood that the specific reference to L and C bands is only exemplary and should not be construed as limitations.

FIG. 1 shows a L-C band fiber amplifier 100 with two parallel optical amplifying fiber branches, a first fiber branch 110 with a C-band EDFA 114 for amplifying WDM channels in the C band and a second fiber branch 120 with a different, L-band EDFA 124 for amplifying WDM channels in the L band. A first 3-port optical circulator 103 is used to couple an input fiber 101 which carries input L-C band WDM channels 104 and input fiber terminals of the two fiber branches 110 and 120 to direct an optical signal from the input fiber 101 connected to port 1, to the input fiber terminal of the second branch 120 connected to port 2, and to the input fiber terminal of the first fiber branch 110 connected to port 3. A second

3-port optical circulator 130 is used to couple an input fiber 102 and output fiber terminals of the two fiber branches 110 and 120 to direct an optical signal from the second fiber branch 120 connected to port 1, to the output fiber terminal of the first branch 110 connected to port 2, and to the output fiber 102 connected to port 3.

Two different sampled FBGs 122 and 112 are implemented at different positions to split input WDM channels and to combine the amplified WDM channels, respectively. The sampled FBG 122 is a C-band reflector that reflects WDM channels 105 at selected WDM channel wavelengths in the C band within the input L-C band WDM channels 104 and transmits optical signals at other wavelengths including L-band WDM channels 123. It is positioned in the fiber link between the coupler 103 and the L-band EDFA 124 in the second fiber branch 120 and operates as a channel splitter to transmit L-band WDM channels 123 to the L-band EDFA 124 and reflect the C- band WDM channels 105 to the first fiber branch 110 to be amplified by the C-band EDFA 114. The sampled FBG 112 a L-band reflector that reflects WDM channels at selected WDM channel wavelengths in the L band and transmits optical signals at other wavelengths including C-band WDM

channels. It is positioned in the fiber link between the C-band EDFA 112 and the circulator 130 in the first fiber branch 110 to transmit amplified C-band WDM channels to the circulator 130 and reflect the amplified L-band WDM channels from the second fiber branch 120 back to the circulator 130. Both amplified L-band channels 125 and C-band channels 115 are combined by the L-band FBG 112 as amplified output channels 132 and are directed to the output fiber 102 by the circulator 130.

FIG. 2 shows another configuration that uses C-band FBG 122 and L-band FBG 112 in which the L-band FBG 112 is used as the channel splitter and the C-band FBG 122 is used as the channel combiner.

FIG. 3 shows the Bragg reflection bands of the two sampled FBGs 112 and 122 and the gain profiles 301 and 302 of the EDFAs 114 and 124 according to one implementation. Under this implementation, the fiber amplifiers in FIGS. 1 and 2 may operate in the entire wavelength range covering the complete set of L-and C- bands. The sampled FBGs 112 and 122 may be configured to have reflection resonance bands throughout the L and C bands and inter-band region between the C and L bands such that the dead zone would be eliminated. Each of two sampled FBGs 112 and 122 may have multiple identical

Bragg reflection bands that can be placed to within one unit of channel wavelength spacing (e. g., 0.8 or 1.6 nm) with respect to the other band. Under this implementation, sampled FBGs 112 and 122 may be fabricated by imparting a periodic modulation onto the refractive index of a fiber Bragg grating, causing the grating to have a multiplicity of resonances. Eggleton et al.,"Long periodic super structure Bragg gratings in optical fibres", Electron. Lett., vol. 30, pp. 1620-1622, 1994. The spacing between the resonances is uniform and can be tailored by varying the sampling period. The Bragg reflection bands of each sampled FBG may match all the ITU grids for WDM channels only in C or L band. As illustrated, the Bragg reflection band 310 at the highest channel frequency in the sampled FBG 114 for reflecting the C-band channels is one channel spacing less than the Bragg reflection band 320 at the lowest channel frequency in the sampled FBG 112 for reflecting L-band WDM channels. There is no dead gap between the Bragg reflection bands of the C-band FBG 122 and the Bragg reflection bands of the L-band FBG 112. Such FBGs can be fabricated by sampling a uniform FBG with a sinc modulation pattern to provide multiple reflection peaks in a desired bandwidth that have substantially the same

signal amplitudes. Ibsen et al. disclosed sinc-sampled FBGs having up to 16 uniform channels with a channel spacing of 0.8 nm. Ibsen et al."Sinc-sampled fiber Bragg gratings for identical multiple wavelength operation", IEEE Photonic. Tech. Lett., vol. 10, pp. 842- 844,1998. If each sampled FBG does not cover the desired spectral range such as L or C band, two or more sinc-sampled FBGs may be cascaded in series to operate as the FBG 112 or 122 in FIGS. 1 and 2 to over the C-band or L-band.

FIGs. 4 and 7 show alternative embodiments of L-C band fiber amplifiers that use two identical sampled FBGs in one fiber branch for channel splitting and combining.

FIG. 4 shows a fiber amplifier 400 uses C-band FBG 122 in the first second branch 120, where the first C-band FBG 122 is connected between the circulator 103 and the L- band EDFA 124 and the second C-band FBG 122 is connected between the circulator 130 and the L-band EDFA 124. The first fiber branch 110 does not have a sampled FBG.

FIG. 5 shows the operation of the amplifier 400 in FIG. 4. The C-band FBG is designed to have its Bragg reflection bands located at desired ITU WDM grids where its highest Bragg reflection band is located one grad below the lowest WDM channel in the L band. The C-band

channels are reflected at both FBGs, while the L-band channels pass through the two FBGs before combining with' the C-band channels at the output port of the L-C band EDFA.

FIG. 6 shows one implementation of the EDFAs 114 and 124 in the device 400 in FIG. 4. The L-band EDFA 124 has two amplification stages with 20-m and 80-m Erbium doped fibers, respectively. Both stages are optically pumped by lasers operating near 980 nm. A wavelength-selective fiber coupler, such as a WDM fiber coupler, may be used to couple the pump beam into the 20-m and 80-m fibers.

The C-band EDFA 114 has a 25-m Er-doped fiber.

FIG. 8 shows the gain and noise figure of the L-band and C-band EDFAs measured separately, where solid dots are data for C-band EDFA and hollow squares for L-band EDFA. A small signal gain higher than 22 dB and a noise figure lower than 7 dB is provided for both the bands, as well as the inter-band region. The noise figure of the C-band EDFA is slightly higher than that of the L-band EDFA because of its single stage structure. Although the characteristics for wavelengths longer than 1580 nm. are not shown due to limitations in the wavelength tuning range of a particular signal source used in the measurements, the L-band amplifier can provide a flat

gain for wavelengths up to 1600 nm, which was confirmed by observing the ASE noise spectrum of the saturated amplifier.

The two C-band FBGS used in FIG. 6 were sampled by square-wave periodic sampling patterns due to lack of sinc-sampling fabrication technology. The square-sampled FBGs were designed to have fewer resonances to keep high reflectivity for channels at the edge of the grating bandwidth. The two identical FBGs have multiple reflection peaks in the C-band and a channel spacing of 3.2 nm. FIG. 9 shows the measured reflection spectrum of one of the gratings. Each reflection peak has a 3-dB bandwidth of 0.2 nm. The reflectivity at 1559 nm, adjacent to the L-band signals, is about 92%. The small peak in the L-band due to square-sampling could be a severe problem if there is a channel at this wavelength.

However, it can be completely eliminated if using sinc- sampled gratings or other sampling techniques.

To evaluate the system performance of the L-C band EDFA shown in FIG. 6, the bit-error-rate (BER) characteristics were measured by transmitting 4-channel 2.5 Gb/s, 215-1 PRBS data through the EDFA. A spacing between the highest C-and the lowest L-band channel of 1.6 nm was chosen in the measurement to demonstrate. a

continuous inter-band operation. Two C-band channels (1555.8 nm and 1559.0 nm) and two L-band channels (1560.6 nm and 1570.2 nm) were used in the system measurement.

FIGS. 10A and 10B show the optical spectra measured at the output of the L-band EDFA and C-band EDFA, respectively. The coherent cross-talk induced by the grating splitter is lower than-30 dB for the two channels which are closest to band edge, and could be lower if using sinc-sampled gratings with higher reflectivity for channels in the inter-band region. FIG.

10C shows the output spectrum of the L-C band EDFA. The amplified-spontaneous-emission (ASE) noise level in C- band is suppressed by the C-band sampled FBGs, which is about 15 dB less than the noise level in L-band. Note that the ASE in both L-and C-band could be suppressed if using the schemes in FIGS. 1 and 2 where L-band and C- band FBGs are used. FIG. 11 further shows the BER results measured before and after the L-C band EDFA for all the channels. A power-penalty of less than 0.2 dB was obtained for all four channels after amplification.

For practical application, the impact of the FBG bandwidth should be considered when cascading the L-C band EDFAs. Since the signals are to be reflected by all the FBGs in the EDFA cascades, it is desirable that each

of the FBG reflection-peaks should have a square-filter response to avoid the bandwidth-narrowing effect. An apodisation technique may be applied to fabricate such sampled FBGs for band-splitting. See, e. g., Ibsen et al., "99. 9% reflectivity dispersion-less square-filter fibre Bragg gratings for high-speed DWDM networks", Conference on Optical Fiber Communications, Paper PD21,2000.

Although the present disclosure only includes a few embodiments, it is understood that various modifications and enhancements may be made without departing from the spirit of the techniques and devices as described above.

For example, three or more parallel EDFA at different operating spectral ranges may be used based on the techniques used in the above two-EDFA embodiments. Also, the fiber gratings may be replaced by Bragg gratings formed in waveguides other than fibers such as planar waveguides formed over substrates. Furthermore, the discrete narrow bands for carrying and filtering the WDM channels produced by the sampled FBGs may be generated by wavelength-selective elements other than sampled FBGs.