1. Field of the Invention
The present invention relates to an optical amplifier for use in optical fiber communication systems and optical signal processing systems.
2. Description of the Related Art
The basic configuration of a conventional optical amplifier is shown in FIG. 22. This optical amplifier is comprised by two amplifying sections having different gain band regions (L-amplifying section and S-amplifying section), a divider, and a combiner, and attempts to broaden the operational bandwidths by coupling the two gain band regions in the wavelength domain. (refer to xe2x80x9cBroadband and gain-flattened amplifier composed of a 1.55 xcexcm-band and a 1.58 xcexcm-band Er3+-doped fiber amplifier in a parallel configurationxe2x80x9d, M. Yamada et. al., IEE Electronics Letters, Vol. 33, No. 8, 1977, pp710-711) (Reference 1).
Various types of dividers and combiners are utilized in constructing such optical amplifiers, and they can be classified as a dielectric multi-layer filter or a combination of fiber grating in association with circulator. Those based on dielectric multi-layer filter are reported in the following references.
xe2x80x9cBroadband and gain-flattened amplifier composed of a 1.55 xcexcM-band and a 1.58 xcexcm-band Er3+-doped fiber amplifier in a parallel configurationxe2x80x9d, M. Yamada et. al., IEE Electronics Letters, vol. 33, no. 8, 1977, pp710-711. (Reference 1)
Japanese Unexamined Patent Application, First Publication, No. Hei 10-229238, Publication Date, Aug. 25, 1998. (Reference 2)
Japanese Unexamined Patent Application, First Publication, No. Hei 11-204859, Publication Date, Jul. 30, 1999. (Reference 3)
International Application Published under the Patent Cooperation Treaty, PCT/US98/16558, xe2x80x9cOptical amplifier apparatusxe2x80x9d, International publication date, Apr. 8, 1999, International Publication Number WO 99/17410. (Reference 4)
Those based on a combination of circulator and fiber grating are reported in the following references.
European Patent Application, EP 0 883 218 A1, xe2x80x9cWide band optical amplifierxe2x80x9d, Publication date Sep. 12, 1998. (Reference 5)
xe2x80x9cA gain-flattened ultra wide band EDFA for high capacity WDM optical communication systemsxe2x80x9d, Y. Sun et. al., Technical Digest of ECOC""98, pp.53-54, 1998. (Reference 6)
The following reference does not specify what type of device is used.
Japanese Unexamined Patent Application, First Publication, No. Hei 4-101124, Publication Date, Apr. 2, 1992. (Reference 7)
All of the above references assume that the wavelength division properties are either perfect or present no particular problems.
However, as will be shown with specific examples in the following, wavelength division properties of these devices are not perfect, and problems are encountered depending on the manner and conditions of applying the amplifiers.
First, the most common problem of such amplifiers is encountered when the dividers and combiners are made of dielectric multi-layer filters. FIGS. 1A and 1B show the configuration of a conventional optical amplifier, where FIG. 1A shows a design based on dielectric multi-layer filters of the long wavelength transmission type (L-type) for the divider and combiner, represented by L-divider (3) and L-combiner (4), and FIG. 1B shows a design based on dielectric multi-layer filters of the short wavelength transmission type (S-type) for the divider and combiner, represented by S-divider (5) and S-combiner (6).
The amplifying section has a gain medium and a pumping section for excitation, and examples of such optical amplifiers are rare-earth doped fiber amplifier, fiber Raman amplifier and semiconductor laser amplifier. The rare-earth doped fiber amplifiers include erbium-doped fiber amplifier and the like, and according to xe2x80x9cWideband erbium-doped fibre amplifiers with three-stage amplificationxe2x80x9d, H. Masuda et. al., IEE Electronics Letters, vol. 34, no. 6, 1998, pp567-568 (Reference 8), it is advantageous when such an amplifier has a gain equalizer to broaden the region of flat gain because such an amplifier can produce a large total gain bandwidth.
In optical communication systems, optical amplifiers are generally designed to receive wavelength-multiplexed light signals, and in optical signal processing systems for instruments and the like, optical amplifiers are generally designed to receive wavelength- multiplexed light signals or single wavelength light signals.
FIGS. 2A and 2B show configurations of the divider, where FIG. 2A represents an L-type dielectric multi-layer filter (L-divider), and FIG. 2B represents an S-type dielectric multi-layer filter (S-divider). The L-divider receives input light containing a short-wavelength xcexs and a long wavelength xcexl in the common port (c), and transmits a long wavelength xcexl from the transmission port (l) and reflects a short wavelength xcexs from the reflection port (s). On the other hand, the S-divider receives input light containing a short-wavelength xcexs and a long wavelength xcexl in the common port (c), and reflects a long wavelength light xcexl from the reflection port (l) and transmits a short wavelength light xcexs from the transmission port (s).
FIGS. 3A and 3B show configurations of the combiner, where FIG. 3A represents an L-type dielectric multi-layer filter (L-combiner), and FIG. 3B represents an S-type dielectric multi-layer filter (S-combiner). The L-combiner receives input light containing a short-wavelength xcexs from the reflection port (s) and a long wavelength xcexl from the transmission port (l), and outputs light containing a long wavelength xcexl and a short wavelength xcexs from the common port (c). On the other hand, the S-combiner receives input light containing a short-wavelength xcexs from the transmission port (s) and a long wavelength xcexl from the reflection port (l), and outputs light containing a long wavelength xcexl and a short wavelength xcexs from the common port (c).
Referring to FIGS. 1A and 1B, the dividers (L- and S-dividers) 3 and 5, perform the steps described above, and divide the multiplexed signal light containing a long wavelength xcexl and a short wavelength xcexs into a long wavelength signal light xcexl and a short wavelength signal light xcexs, which are input into the respective amplifying sections (L- and S-amplifying sections) 1 and 2, and are combined in the combiners 4 and 6 and multiplexed light signals are thus output.
However, there are problems in the performance of the light amplifiers described above, which will be explained in the following.
FIGS. 4A and 4B show gain spectra obtained in the amplifying sections (L- and S-amplifying sections) 1 and 2, where FIG. 4A shows an overall view of the gain region while FIG. 4B shows details of gains in the vicinities of the wave boundaries (wavelengths xcextr-s to xcextr-l) of the L- and S-amplifying sections. In FIG. 4B, wavelengths in the L-amplifying section are denoted by 1 l* and those in the S-amplifying section are denoted by xcexs*, and the peak gains in the L-, S-amplifying sections are denoted by G while the wavelength-specific gains for xcex l*, xcexs* are denoted by G*.
FIGS. 5A and 5B show loss spectra in the L-divider, and show the losses relating to a transmission loss between ports c and s, and the same between ports c and l (refer to FIGS. 2A and 2B), which are denoted respectively by Lcs1 and Lcl1 where 1 indicates that the losses are related to long wavelengths. FIG. 5A shows the loss in the overall view of the gain region and FIG. 5B shows the details of the loss in the wave boundary. For both Lcs1 and Lcl1, the loss becomes larger as the wavelength of the signal waves moves away from the respective boundary wavelengths (wavelengths xcextr-s to xcextr-l) into fringes of the respective gain regions.
However, in the long wavelength side of the loss spectrum (i.e., at the xcextr-l end), the loss Lcs1 between the port s (for reflected light) and the common port c is limited to a certain constant value, because of the contribution from residual reflection components in the dielectric multi-layer filter.
The difference in the wavelengths xcextr-s to xcextr-l (referred to as the boundary bandwidth) is typically 5xcx9c10 nm at a 1.5 xcexcm wavelength, and the limiting value and the loss at the wavelength xcexs* are typically about 10 dB and 20 dB, respectively. The boundary bandwidth and the slope of the curve of loss spectrum in the vicinity of the wave boundary are dependent on the parameters (composition and the number of layers of lamination) of the dielectric multi-layer filter.
FIGS. 6A and 6B show the loss spectra of the S-type divider, where FIGS. 6A, 6B relate, respectively, to the transmission losses between ports s and l, and between ports c and l shown in FIGS. 2A and 2B (referred to as Lcs2, Lcl2, where 2 indicates that the losses are related to short wavelengths). FIG. 6A refers to the overall view of the loss region and FIG. 6B shows details of losses in the wave boundary. Both Lcs2, Lcl2 show a tendency to increase as the wavelengths of the signal waves move away from the respective boundary wavelengths (wavelengths xcextr-s to xcextr-l) into fringes of the gain regions.
However, in the short wavelength side of the loss spectrum (i.e., at the xcextr-s end), the loss Lcs2 between the port l (for reflected light) and the common port c is limited to a certain constant value, which is caused by the contribution from residual reflection components in the dielectric multi-layer filter.
The difference in the wavelengths xcextr-s to xcextr-l (referred to as the boundary bandwidth) is typically 5xcx9c10 nm at a 15 xcexcm wavelength, and the limiting value and the loss at the wavelength xcexs* are typically about 10 dB and 20 dB, respectively. The boundary bandwidth and the slope of the curve of loss spectrum in the vicinity of the wave boundary are dependent on the parameters (composition and the number of layers of lamination) of the dielectric multi-layer filter.
The loss spectra of the L- and S-type combiners are the same as the loss spectra of the L- and S-type dividers shown in FIGS. 5 and 6, because of the reciprocality of light propagation. That is, if the transmission ports are the same, the loss values are the same.
FIGS. 7A and 7B show loss spectra in the optical circuits of a conventional optical amplifier. The losses are incurred in the L- and S-amplifying sections, and the circuit loss is represented by a sum of the losses in the dividers and combiners, and are expressed in the units of dB. FIG. 7A shows the losses in the L-type divider and combiner shown in FIG. 1A, and FIG. 7B shows the losses in the S-type divider and combiner shown in FIG. 1B.
In the case of the L-type amplifier, the loss value (denoted by U) in the S-amplifying section at a wavelength xcexl* is limited to 20 dB, which is twice the limit value (about 10 dB). These values, 20 dB and 10 dB, correspond to non-dimensional numbers, 100 and 10, respectively, so that the former is ten times the latter. On the other hand, the case of the S-type amplifier, the loss value (denoted by Ls) in the L-amplifying section at a wavelength xcexs* is limited to 20 dB, which is twice the limit value (about 10 dB). In other words, the loss of signal light at the wavelength xcexs* is limited to ten times the limit value.
As explained above, in the vicinity of the wave boundaries, because the wave separation properties in the dividers and combiners are not perfect, the output light, for example wavelength xcexl*, from its primary circuit (through the L-amplifying section) is affected by the contribution from the residual light in the reflection circuit (through the S-amplifying section).
For the purpose of providing a quantitative explanation, the optical powers of the primary output light and the residual light are designated, respectively, by P and P*. If the value of P* is not sufficiently small compared with P, coherent interference is generated and interference noise will be superimposed on the signal light to cause operational problems. For example, in an optical communication system, an increase in bit error rate will lead to degradation of the system performance.
In the L-type divider and combiner (refer to FIG. 1A), P and P* are related to the input power Pin (in units of dBm) according to the following expressions.
P=Pin+G and P*=Pin+G*xe2x88x92Llxe2x80x83xe2x80x83(1) 
Pxe2x88x92P*=Gxe2x88x92G*+Llxe2x80x83xe2x80x83(2) 
where G* is the gain in the residual circuit and Ll represents the circuit loss described in FIGS. 7A and 7B, and wavelength-dependent losses in the dividers and combiners are neglected for simplicity. If it is assumed that the difference between G and G* is smaller than Ll and can be neglected, the difference between P and P* is equal to Ll from equation (2). In the example given in FIGS. 7A and 7B, Ll is about 20 dB, but this value is not sufficiently large, so that interference noise is generated.
Similarly, in the configuration based on S-type dividers and combiners (refer to FIG. 1B), P and P* are related to the input power Pin (in units of dBm) according to the following expressions.
P=Pin+G and P*=Pin+G*xe2x88x92Lsxe2x80x83xe2x80x83(3) 
Pxe2x88x92P*=Gxe2x88x92G*+Lsxe2x80x83xe2x80x83(4) 
where Ls represents the circuit loss described in FIGS. 7A and 7B. If it is assumed, for simplicity, that the difference between G and G* is smaller than Ls and can be neglected, the difference between P and P* is equal to Ls from equation (4). In the example given in FIGS. 7A and 7B, Ls is about 20 dB, but this value is not sufficiently large, so that interference noise is generated.
If it is assumed that 30 dB is a sufficiently high value of the difference Pxe2x88x92P* to prevent the interference noise from being generated, in the wavelength regions where the gain difference, Gxe2x88x92G*, is less than 10 dB, it is obvious that interference noise cannot be neglected.
In the amplifier based on L-type dividers and combiners, the signal wavelength in the long wavelength region to produce a gain difference Gxe2x88x92G* of less than 10 dB is designated by xcexl** as indicated in FIGS. 4A and 4B, and the gain value at this wavelength is designated by G**. Then, as shown in FIG. 7A, in the region from the vicinity of the signal wavelength xcexs** (where the power difference Pxe2x88x92P* in the short wavelength region becomes less than 30 dB) to the vicinity the signal wavelength xcexl**, the optical power difference Pxe2x88x92P* is less than 30 dB. Therefore, because of such double adverse effects, i.e., insufficient differences in gain as well as output power levels caused by residual reflection components in both L- and S-devices, interference noise generated in the boundary bandwidth causes degradation in the amplifier performance. Similar results occur in the amplifier based on S-type dividers and combiners, and therefore, it creates a difficulty that the useable wavelengths are restricted in the conventional design of amplifiers regardless of the wavelengths of dividers and combiners.
As discussed above, in the conventional technologies based on dielectric multi-layer filters, interference noises are unavoidable in the signal waves in the vicinity of the wave boundaries, and therefore, it creates a problem that the useable wavelengths are restricted. Even in those systems using dividers and combiners not based on dielectric multi-layer filters, the same problems are experienced because the wavelength division properties of the dividers and combiners are not perfect.
It is an object of the present invention to resolve the problems outlined above, and provide an optical amplifier having a broad bandwidth of useable wavelength for processing input signal light.
According to the present invention, the object has been achieved in an optical amplifier comprising: an optical divider for dividing input signal light according to wavelength; two amplifying sections disposed in parallel and having different wavelength amplification regions for amplifying respective light signals emitted from the optical divider; an optical combiner for combining the light signals output from the respective amplifying sections; and an optical filter disposed in series with one of the two amplifying sections for generating a loss for eliminating mixed wave components in a wavelength region corresponding to the wavelength region of the light signal passing through the other amplifying section.
The optical amplifier of such a design enables to narrow the latent noise region by the action of the filter in generating a loss in residual reflection components created by the wave division effect in the optical divider so as to increase the gain performance of the signal processing circuit associated with the optical filter.
Also, the object has also been achieved in another design of the optical amplifier comprising: an optical divider having a dielectric multi-layer filter for dividing input signal light according to wavelength; two amplifying sections disposed in parallel and having different wavelength amplification regions for amplifying respective light signals emitted from the optical divider; and an optical combiner for combining light signals output from respective amplifying sections using a filter having a blocking wavelength region different than that of the dielectric multi-layer filter provided in the optical divider.
The optical amplifier of such a design enables narrowing of the latent noise region as a result of improved gain in the signal processing circuits because the transmission region of the dielectric multi-layer filter used in the optical divider is different than that of the dielectric multi-layer filter used in the optical combiner.
Accordingly, the present optical amplifier provides benefits that the wavelength bandwidth of the latent noise region has been narrowed and, consequently, the bandwidth of useable wavelengths of signal light that can be used for optical processing purposes has been broadened.