Patent Application
20070115536
The invention is based on a priority application EP 05 292 459.4 which is hereby incorporated by reference.
The invention relates to a Raman amplifier comprising a Raman active optical fiber and a Raman pump in operative connection with said optical fiber, the Raman pump comprising: a plurality of pairs of pump light sources emitting polarized pump light, each generating a pair of mutually orthogonal pump fields with respective central frequencies, and a pump field combiner in operative connection with the pump light sources and adapted to couple each of the plurality of pairs of pump fields at mutually orthogonal directions of polarization into said optical fiber.
The invention also relates to an optical transmission system, comprising: a transmitter unit for transmitting an optical signal, a receiver unit for receiving the optical signal from the transmitter unit, and an optical transmission link for propagating the optical signal between the transmitter unit and the receiver unit.
The invention further relates to a method for amplifying an optical signal in a Raman active optical fiber, comprising the steps of: (a) generating at least one pair of pump fields with mutually orthogonal directions of polarization, (b) propagating said at least one pair of pump fields through said Raman active optical fiber in a propagation direction of the optical signal to be amplified.
For amplifying optical signals in optical transmission systems, widespread use has been made of stimulated Raman amplification, which is a non-linear optical process in which an intense pump wave is injected into a Raman active optical fiber propagating one or more optical signals to be amplified. In this context, a particular amplification technique is referred to as Raman co-pumping, wherein the optical signal to be amplified and the Raman pump wave(s) propagate in the same direction on an optical transmission link, e.g. an optical transmission fiber, connecting the transmitter unit and the receiver unit in an optical transmission system.
In order to achieve efficient Raman co-pumping, pump light sources—hereinafter also referred to as “pumps”—with low Relative Intensity Noise (RIN) have to be employed. Pump light sources with acceptable RIN are usually composed of semiconductor diodes. These diodes emit polarized light. In order to obviate the problem of polarization dependent gain (PDG), a widely used method includes depolarizing the Raman pump, which comprises individual pump light sources, by means of using a plurality of diodes, which operate at the same optical wavelength, and coupling their respective emissions with a polarization beam combiner (PBC). This approach, however, suffers from the following disadvantage: When the two orthogonal pump light emissions—hereinafter also referred to as “pump fields”—overlap in the spectral domain, intensity noise coming from the product of the orthogonal pump fields is induced on the Raman amplified optical signal. Experimentally, with Fiber Bragg Grating (FBG) stabilized diodes, a 10 dB degradation due to this overlap has been observed.
Prior art document U.S. Pat. No. 6,384,963 discloses a Raman amplifier for to achieve Raman co-pumping, in which said overlap between the orthogonal pump fields is avoided by separating the spectra of the pump diodes with respect to their emission wavelengths. As can be gathered from
For instance, in an optical transmission system comprising a Raman amplifier with two orthogonal pumps having the same power, being separated by Δν=2.1 THz, and emitting light at 1467.8 and 1483 nm, respectively, an on/off Raman gain over a range of wavelengths from 1510 nm to 1620 nm is shown in
The channel at 1590 nm receives a total gain of 7.5 dB. Among this total gain, 2.3 dB comes from the pump at 1467.8 nm and 5.2 dB gain comes from the pump at 1483 nm. The 1483-nm pump brings 2.9 dB more gain than the 1467.8-nm pump. Consequently, this share of 2.9-dB gain is provided by the polarized pump field at 1483 nm alone. In co-pumped schemes, PDG can be as high as 60%, thus the channel at 1590 nm will suffer 1.8 dB PDG.
It is the object of the present invention to provide a Raman amplifier which can be employed in optical transmission systems using co-pumped Raman amplification, which obviates the above-mentioned disadvantages and which therefore does not induce polarization dependent gain (PDG) on channels of the optical transmission system.
It is further an object of the present invention to provide an optical transmission system, individual channels of which do not suffer polarization dependent gain (PDG).
It is another object of the present invention to provide a method for amplifying an optical signal in a Raman active optical fiber, comprising the steps of: (a) generating at least one pair of pump fields with mutually orthogonal directions of polarization, and (b) propagating said at least one pair of pump fields through said Raman active optical fiber in a propagation direction of the optical signal to be amplified.
According to the first aspect of the present invention the object is achieved by means of a Raman amplifier of the above-mentioned type, wherein the respective central frequencies of the pump fields of at least one, preferably each pair of mutually orthogonal pump fields are different from each other and exhibit a frequency shift below 1.8 THz.
According to the second aspect of the present invention the object is also achieved by providing an optical transmission system of the above-mentioned type, comprising a Raman amplifier according to said first aspect of the present invention, said Raman amplifier being in operative connection with the optical transmission link for amplifying the optical signal.
According to the third aspect of the present invention, the object is achieved by a method of the above-mentioned kind wherein the central frequencies of the pump fields of said at least one pair of pump fields are selected to be different from each other and to exhibit a frequency shift of below 1.8 THz.
As a general idea, the present invention proposes to specify a maximum frequency shift, i.e., Δν≦1.8 THz, between the central optical frequencies of the polarization combined pump fields for to minimize polarization dependent gain (PDG) in optical transmission systems. In a preferred embodiment of the Raman amplifier in accordance with the present invention the frequency shift Δν between the respective central frequencies is lower than 0.8 THz. The “pairs of pump fields” to which the invention relates may be defined as the couples of polarized pump fields that are mutually orthogonal and that are the closest to each other in the spectral domain (compared to all other possible pairs). The polarized pump fields of different pairs are not necessarily aligned along the same state of polarization.
Advantageously, in accordance with a further embodiment of the present invention, a spectral overlap between the orthogonal pump fields in the at least one, preferably each pair of pump fields is limited according to the relation:
(νC2−Δν2′/2)−(νC1+Δν1′/2)>0 (Eq. 1)
wherein Δν1,2′(=Δνi′) is a frequency bandwidth of the pump fields and wherein νC1,C2 is the central frequency of the respective pump field such that:
for a given pump field, wherein P(ν) is the optical power spectrum of the pump field and A is a constant, 0.5<A<0.9, preferably A=0.8. In this way excessive overlap between pump fields is avoided which results in an effective reduction of intensity noise on the Raman amplified signal, as stated above.
In another embodiment in accordance with the present invention, the respective central frequencies of only one pair of pump fields with the lowest central frequencies of pump fields of said plurality of pairs of pump fields exhibit a frequency shift below 1.8 THz. The share of gain provided to the signal by one polarized pump field alone should be limited, for instance to 2 or 3 dB. Due to the specific shape of the Raman gain curve, the pump field (or the pair of orthogonal pump fields) with the highest wavelength (corresponding to the lowest central frequency) almost provide all the gain to the signals of higher wavelengths. It is thus a priority to specify the maximum frequency shift between the orthogonal pump fields having the highest wavelengths.
In a further preferred embodiment, the plurality of pairs of pump light sources are set up for emitting the polarized pump light through said Raman active optical fiber in a propagation direction of an optical signal to be amplified, thus performing Raman co-pumping.
In a preferred further embodiment of the Raman amplifier in accordance with the present invention, the pump field combiner comprises at least one of a polarization beam combiner, a wavelength multiplexer, and an optical coupler. In this way, various ways of technical realization of the Raman amplifier in accordance with the present invention are available for a person skilled in the art. In a general way, for pump depolarization, a plurality, i.e. at least two, polarized pump fields are coupled together, such that they enter the Raman active optical fiber (discrete or distributed Raman amplifier) with orthogonal states of polarization.
In order to efficiently limit a width of the pump fields, which is an important feature with respect to avoiding pump field overlap, in a preferred embodiment of the present invention at least one of the pump light sources is a Fiber Bragg Grating (FBG) diode, which display a spectral width of only 2 to 4 nm (250-500 GHz) and which are wavelength stabilized. Alternatively or additionally, at least one of the pump light sources can be devised as an inner-grating module, which have the additional advantage of presenting only low RIN.
When using FBG stabilized diodes, polarization maintaining fibers should not be used to depolarize the output field of the respective diodes, as the inventors have found out that such a depolarization of the diode field induces additional intensity noise on the Raman amplified signal.
Further advantages and characteristics of the present invention can be gathered from the following description of preferred embodiments with reference to the enclosed drawings. Features mentioned above as well as below can be used in accordance with the invention either individually or in conjunction. The embodiments mentioned are not to be understood as an exhaustive enumeration but rather as examples with regard to the underlying concept of the present invention.
a, b are schematic block diagrams embodiments of the pump field combiner of
a, b are examples of profitable frequency shifts between pump fields in accordance with the present invention;
a, b are on/off Raman gains produced by pump fields shifted in accordance with the present invention, and
c is an example of on/off Raman gains produced by pump fields shifted according to prior art.
The following detailed description of the invention refers to the accompanying drawings. The same reference numerals may be used in different drawings to identify the same or similar elements.
The optical transmission system 1 further comprises a Raman amplifier 5 in operative connection with the optical transmission link 4, 4a. The Raman amplifier 5 comprises an additional optical fiber segment 6 in connection with the optical transmission link fiber 4, 4a of the optical transmission system 1. The Raman amplifier 5 further comprises a Raman pump 7 depicted with dashed lines, which is connected with the optical fiber 6. The Raman pump 7 includes three pairs of polarized pump light sources {8.1a, 8.2a}, {8.1b, 8.2b}, {8.1c, 8.2c} which emit respective polarized pump light at different pumping optical frequencies ν1a-c, ν2a-c. In accordance with the present invention, said pump light sources 8.1a-c, 8.2a-c are preferably devised as semiconductor diodes, e.g. Fiber Bragg Grating (FBG) stabilized diodes or inner-grating modules.
The pump light sources 8.1a-c, 8.2a-c are operatively connected by means of a pump field combiner 9 adapted to couple the generally polarized pump light PL of pump light sources 8.1a-c, 8.2a-c, i.e. of a plurality of pump fields, into the optical fiber 6, 4, 4a in order to achieve Raman amplification of the optical signals OS propagated on the optical transmission link 4, 4a between the transmitter unit 2 and the receiver unit 3. In the following, two possible implementations of the pump field combiner will be described in greater detail with reference to
a shows a pump field combiner 9 which comprises three polarization beam combiners (PBCs) 11.1 to 11.3, each receiving the pump light from a respective pair of pump light sources {8.1a, 8.2a}, {8.1b, 8.2b}, {8.1c, 8.2c} transmitted through polarization maintaining fibers (represented by thick lines). The pump light of the first pump light source 8.1a-c of a respective pair has a direction of polarization which is orthogonal to the direction of polarization of the second respective pump light source 8.2a-c of the same pair. The output light of the PBCs 11.1 to 11.3 is provided as an input to a wavelength division multiplexer 10 thus forming a common pump signal which is provided to the optical fiber segment 6 shown in
In an alternative implementation of the pump field combiner 9 shown in
Since according to
In the embodiment of
In order to achieve efficient Raman co-pumping, a Raman pump 7 with low Relative Intensity Noise (RIN) has to be used. As stated before, pump units with an acceptable RIN generally comprise semiconductor diodes. These diodes emit polarized light. In order to avoid polarization dependent gain (PDG) and its negative effects on system performance, a widely used approach for effectively depolarizing the Raman pump has been to use diodes operating at a common wavelength and to couple the respective emissions with a polarization beam combiner, as shown at reference numerals 7, 9 in the above-described
In this context, the present invention proposes a judicious choice of the overlap and/or the frequency shift between the combined pump fields of the Raman amplifier 5 in accordance with
Alternatively, when using the implementation of the pump field combiner 9 shown in
In addition to their orthogonality, the individual optical frequencies ν1a, νb, ν1c of the first group of pump fields E1-c are shifted in frequency with respect to optical frequencies μ2a, ν2b, ν2c in the other (orthogonal) group of pump fields E2a-c by an amount (frequency shift) Δν, as depicted in
The inventive choice of the respective frequency shifts Δν, Δνa, Δνb in
wherein P(ν) is the optical power spectrum of the pump field and νC is the center frequency of the Raman pump field.
In the simple case that the frequency spectrum of the pump light source has a symmetrical shape, the central frequency (i.e. pumping frequency) corresponds to the frequency of symmetry. However, in the case of a non-symmetrical shape of the frequency spectrum, the value of the central frequency may not be easy to find. In this case, Eq. 3 may be used as a definition for the central frequency: the frequency “around which” there is the most optical power density, thus solving Eq. 3 for the couple of values {Δν′, νc} simultaneously, as will be described in the following.
Numerically, one may use a band-pass filter of width Δν′ and find the best central frequency νc for this filter to select as much power as possible. For each width Δν′ of this filter, the best central frequency νc is determined. Finally, one chooses the couple {Δν′, νc} that respects Eq. 3. In this sense, Δν′ is the minimum frequency bandwidth such that there exists a central optical frequency νc that satisfies Eq. 3. As a remark, νc may not coincide with the frequency of the power peak, and also, it may depend on the choice of coefficient A.
As stated above, the central optical frequency νc can thus be determined as the center frequency of the frequency bandwidth Δν′ which constitutes the minimum bandwidth which satisfies Eq. 3. With a power spectrum of the pump light source which is almost symmetrical with respect to the pumping frequency as shown in
In a more general way, Eq. 3 can be written as
wherein A is a positive constant with 0.5<A<0.9.
As stated before, when two orthogonal pump fields overlap, intensity noise coming from the product of these orthogonal fields is induced on the Raman amplified signal. Therefore, in accordance with an embodiment of the present invention, the spectral overlap of the pump fields is limited in accordance with the following relation:
(νC2−Δν2′/2)−(νC1+Δν1′/2)>0,
wherein the definitions of Eq. 1 have been used for two pump fields (e.g., pump fields E1a, E2a in
a, b show the overlap of two pump fields, e.g. pump fields E1a, E2a in
In the embodiment of
Furthermore, and in accordance with the general idea of the present invention, a maximum wavelength/frequency shift between pairs of pump fields (E1a,/E2a and E1b/E2b, respectively, in
a, b show the effects of the invention with respect to the Raman gain on the optical signals OS propagated from the transmitting unit 2 to the receiver unit 3 in the optical transmission system 1 of
The six lower curves in
In
As can be gathered from
Again, due to the specific shape of the Raman gain curve again, the pump field (or the pair of orthogonal pump fields) of highest wavelengths almost provide all the gain to the signals of higher wavelengths of the transmission band. (By contrast, the signal channels of lower wavelengths get their gain from low and high-wavelength pump fields together). Consequently, it is a priority to specify the limit of the maximum frequency shift between the orthogonal pump fields of higher wavelengths, leading to a frequency shift of 0.8 THz between the pump fields of higher wavelengths and a frequency shift of 1.8 THz between the pump fields with lower wavelength, as shown in
In this way the proposed solution does not induce polarization dependent gain (PDG) in the co-pumped Raman amplifier. While being non-obvious, the proposed solution is essentially straightforward, as Raman pumps with polarization-combined pump light sources (diodes) already exist as such. On the market, however, a common wavelength is usually specified for the individual pump light sources combined in polarization. In accordance with the present invention, a different approach is proposed in order to devise a Raman amplifier which does neither induce intensity noise nor polarization dependent gain on optical signals to be amplified. Furthermore, the range of possible frequency shifts in accordance with the present invention, i.e., about 0.4 THz to 0.8 THz for FBG diodes, is wide enough to relax commercial constraints. In general, a lower limit of the shift of the center frequencies of the respective pump fields of each pair may be defined by 0.4 THz, thus sufficiently reducing the overlap between the pump fields.
It should be noted that although the proposed solution is described above with respect to a co-propagating geometry (cf.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05 292 459.4 | Nov 2005 | EP | regional |
