Patent Application
20110318014
The invention generally relates to reception of optically transmitted signals and particularly to apparatuses and methods for suppression of noise in connection with reception of optically transmitted telecommunication signals encoded in BPSK (binary phase-shift keying and QPSK (quaternary phase-shift keying) modulation formats as well as in variations of these formats, such as DPSK (differential phase-shift keying) and DQPSK (differential quaternary phase-shift keying).
In QPSK modulation, which is used as an illustrative but non-restrictive example for describing the invention, signal is transmitted in bit pairs. In other words, one symbol contains two bits of information. The four possible bit pairs are encoded into four different phase values, which can be absolute phase values or relative phase differences between two consecutive symbols. This is illustrated schematically in
Reference numeral 1-6 denotes an idealized constellation diagram, in which the radius of a circle denotes the maximum amplitude of the optical pulse carrying the symbol (normalized as one), while the anti-clockwise angle from the real axis Re denotes phase. In the present modulation scheme, the nominal (ideal) phase values are π/4, 3π/4, −3π/4 and −π/4, which is why the constellation diagram 1-6 comprises only four points, which are the intersections of a unity-radius circle and the four possible phase angles. The relation between symbol pair values and phase angles in the constellation diagram 1-6 corresponds to Gray encoding, in which only one bit changes at a time with increasing phase value, but those skilled in the art will realize that the problem and its inventive solution are not restricted to any particular encoding scheme. Real-world optical transmission systems are not ideal, however, and the signal deviates from the idealized representation given in the diagram 1-6. Because of phase and amplitude noise, real-world optical transmission systems produce signals whose constellation diagrams resemble the one denoted by reference numeral 1-7.
BPSK modulation format is a subset of QPSK modulation. Instead of four different phase values, the BPSK modulation contains just two possible phase values, their relative phase difference being yr radians. Consequently, BPSK modulation carries only one bit of information in each symbol.
As shown in the diagrams 1-1, the waveform's amplitude alternates from zero to unity (or to the noise-affected value 1-5) and back to zero for each symbol. This kind of amplitude variation is called return-to-zero (rz) amplitude modulation, but other modulation schemes are possible, such as non-return-to-zero (nrz) amplitude modulation. In many practical phase modulation formats the rz amplitude modulation is superimposed on top of the phase modulation. In such modulation schemes, it is not necessary for the amplitude modulation to carry net information (user information). Instead the amplitude modulation may carry a timing reference for demarcating the individual optical pulses that carry the symbols. Alternatively, the amplitude modulation may be used to reduce possible signal distortions, which may be caused by abrupt changes of signal phase. Within such modulation schemes, user information is typically carried by phase modulation. Later in this document, a phase-modulated signal means a signal in which user information is entirely or predominantly carried by variations in phase, whereas a phase/amplitude modulated signal means a signal in which user information is carried by variations in phase and amplitude.
The first and second optical paths 3-12A, 3-12B have different optical path lengths, and the difference is inversely proportional to the received symbol rate such that the phase modulated signal is transformed to an amplitude-modulated signal at the output of the delay interferometer. In effect, the optical path length difference equals the distance travelled by the optical signal in an optical medium in a time that corresponds to one symbol period, as known by those skilled in the art.
In the known optical receiver 3-0, the delay interferometer section 3-1 is followed by a detection stage 3-5 which comprises photo detection elements, such as photodiodes 3-51 and 3-52, for converting the amplitude-modulated signal in the optical domain into an amplitude-modulated signal in the electric domain, which follows the detection stage 3-5.
A generic problem in optical telecommunications is noise, as described in connection with
The prior art amplitude regeneration section 2-3 comprises two 3 dB couplers 2-31, 2-32 and two semiconductor optical amplifiers (labelled “SOA”, denoted by reference numerals 2-33 and 2-34). In case of BPSK modulated signals, the two outputs of the coupler 2-13 of the first delay interferometer contain complementary high and low amplitude signals, which are both directed to the couplers 2-31 and 2-32. Both of these couplers divide the signals and direct them to the SOA components. The high amplitude signals are thus propagating through the SOAs in one direction and the low amplitude signals are propagating through the SOAs to the opposite direction. The regeneration effect described in reference document 1 is such that when high and low amplitude signals cross an amplifying medium, and especially when the high amplitude-signal saturates the gain of the amplifying medium, the low amplitude signal may experience a lower gain factor than the high amplitude signal. This means that the high amplitude signal is amplified relatively more than the low amplitude signal. In reference document 1 this process is called discriminative gain. The non-linear amplifying element is thus having a characteristic comparable to saturable absorption, where the low amplitude level signal is suppressed when compared to the high amplitude level signal. After the SOAs the high and low amplitude level signals are recombined in the couplers 2-31 and 2-32. The optical arrangement of two 3 dB couplers and two connecting optical paths are known in the art as the Mach-Zehnder interferometer. In case the optical paths are symmetric or have relatively similar characteristic, the Mach-Zehnder interferometer is known to direct the optical energy diagonally through the arrangement. In other words, the signal to the input 2-35 is directed to the output 2-38, while the signal to the input 2-36 is directed to the output 2-37. Therefore, in case of symmetric arrangement of couplers 2-31, 2-32 and optical paths containing the non-linear elements 2-33, 2-34, the high and low amplitude signals are directed to coupler 2-41 of the second delay interferometer 2-4, and not backwards to coupler 2-13 of the first delay interferometer 2-1.
Another regeneration scheme is disclosed in reference document 2. While the layouts of the amplitude regeneration sections disclosed in reference documents 1 and 2 are different, it can be seen that the two regeneration circuits share a common phase regeneration principle: a phase-modulated input signal, which suffers from phase noise, is applied to a first delay interferometer which converts the phase-modulated signal to an amplitude-modulated signal; after the phase-to-amplitude conversion the amplitude-modulated signal is regenerated. After the amplitude regeneration, the signal is applied to a second delay interferometer which converts the regenerated signal back to a phase-modulated output signal, which exhibits less phase noise than the input signal does.
The amplitude regenerator of reference document 1 is a coupled amplitude regenerator, whereas the one disclosed by reference document 2 is non-coupled. As used herein, a coupled amplitude regenerator means an amplitude regenerator in which there is some coupling between the two signal paths A and B within the amplitude regenerator. In contrast, the two signal paths of a non-coupled amplitude regenerator are not coupled to one another within the amplitude regenerator. A coupled amplitude regenerator provides the benefit over an uncoupled one that it is more readily implemented via non-linear amplification. Limiting amplification tends to be faster than non-linear attenuation in conventional semiconductor optical amplifiers. On the other hand, a non-coupled amplitude regenerator may provide other advantages, such as simpler construction and better yield in manufacturing.
Yet another BPSK regeneration scheme is disclosed in reference document 3, which suggests a phase-sensitive amplifier for phase noise averaging of consecutive optical pulses. The regeneration scheme disclosed in reference document 3 is based on self-phase modulation in highly non-linear fibers. Similar to the regeneration schemes disclosed in references 1 or 2, the technique of reference 3 is restricted to regeneration of BPSK-modulated signals. The scheme benefits of simple construction, but is more susceptible to amplitude noise than the schemes disclosed in references 1 or 2, which may limit its usefulness in real-life transmission systems.
A regeneration scheme of rz amplitude modulated signals is disclosed in references 4 and 5, which suggest the use of bandpass filtering in conjunction of self-phase modulated signal. Conventional techniques to compensate for the signal degradation due to noise typically involve correction of noise-induced bit errors in the electric domain by means of forward error-correction algorithms or RF filters.
A straightforward technique for reducing noise in an optical receiver 3-0 is regenerating the optical signal by a regeneration apparatus, such as one of the regeneration apparatuses disclosed in reference 1, 2, 4 or 5, prior to applying the optical signal to the optical receiver 3-0. It is an open question, however, whether such a straightforward combination of an optical regenerator and optical receiver provides optimal noise suppression performance.
An object of the invention is to develop further improvements to noise suppression in connection with optical reception and/or modulation format conversion circuits. Such improvements may relate to noise suppression performance, circuit complexity, manufacturing economics or any combination thereof. This object is achieved by apparatuses and methods as disclosed in the attached independent claims. The dependent claims and the present description with the attached drawings illustrate specific embodiments of the invention.
In order to keep the complexity of the description of the present invention within reasonable limits, the majority of the present description relates to modulation schemes in which all useful information is carried via phase modulation. It should be understood, however, that the invention is applicable to a variety of encoding schemes in which useful information is encoded by modulating one or more physical parameters such as phase, frequency and polarization state.
An aspect of the invention is an apparatus for processing an optical input signal carrying symbols, the apparatus comprising at least one optical system with the following elements:
The one or more photo detector stages may be implemented as part of the inventive apparatus, in which case the invention is embodied as an optical receiver with improved noise suppression functionality. Alternatively, the invention may be embodied as a signal regenerator which is followed by the one or more photo detector stages. Each photo detector stage typically comprises a pair of photo detectors and a differential combiner operable to create a differential electrical signal from the photo detectors' outputs. Alternatively, the photo detector stages may be implemented by using only one photo detector for each pair of optical partial signals. While the one or more photo detector stages are necessary for converting the optical partial signals to an electrical signal, the invention may be embodied as a regenerator configured for acting as a front end to an optical receiver, which includes the photo detector stage(s). The benefits of the invention, such as improved noise suppression performance and/or reduction of apparatus complexity are equally achieved in embodiments relying on photo detector stages residing in external receivers.
In the above definition of the inventive apparatus, assuming that the parameter group being modulated includes phase, a signal in the first modulation format, which is at least partially phase modulated means that useful information is carried wholly or partially via phase modulation. A signal in the second modulation format, which is at least partially amplitude modulated, means that useful information is carried partially via amplitude modulation and partially via phase modulation. In modulation schemes in which the parameter group being modulated includes frequency or polarization state, these definitions should be adjusted accordingly. For the sake of clarity and brevity, phase will predominately be used as an example of the parameter being modulated in the first modulation format.
As stated in connection with
As used herein, splitting the optical signal into two partial signals means that the signal splitter divides the signal energy into two parts. “Regeneration” or “amplitude regeneration” is a process which involves reduction of amplitude noise. Amplitude noise can be reduced by means of a non-linear amplifier or a combination of a linear amplifier and non-linear attenuator. “Modulation conversion” is a process for changing an optical signal's modulation to a different modulation format. For instance, the optical signal can be converted from a phase-modulation format to a phase/amplitude-modulation format or vice versa. Or, the optical signal can be converted from a phase-modulation format or phase/amplitude-modulation format, respectively, to a different phase-modulation format or phase/amplitude-modulation format. In sections wherein the optical signal is in the phase/amplitude modulation format, the optical signal usually propagates via two optical paths having complementary modulation with respect to one another.
In the modulation format conversion, when one symbol pair of the first phase/amplitude-modulated signal has nominal amplitude values of 0 or 1, this phase/amplitude-modulated signal is transformed to another phase/amplitude-modulated signal with a second symbol pair having nominal amplitude values of 0 and 1, wherein the phase difference of the second symbol pair differs from the phase difference of the first symbol pair.
Some embodiments employ a further noise-reduction technique, which is an enhanced implementation of a regeneration technique referred to as “Mamyshev” regeneration. At the regeneration or reception point, the signal is filtered from the noise with a bandpass filter, which retains the signal and the noise at the transmission band, but removes the noise from the other parts of the spectrum. The noisy amplitude-modulated signal is directed into a nonlinear element or medium, such as a highly nonlinear fiber, which broadens and/or shifts the spectrum of the signal by means of self-phase modulation. After the nonlinear element, the spectrally broadened and/or shifted signal is directed to a second bandpass filter that transmits parts of the broadened and/or shifted signal spectrum. While the input noise affects the spectral broadening and/or shifting, some of the noise can be suppressed by rejecting one or more portions of the spectrally broadened and/or shifted spectrum. The nature of the suppressed noise, and hence also the transmitted noise, can be tailored on the basis of the properties of the second bandpass filter, such as its transmission wavelength and bandwidth. Especially, the noise of a signal with a nominal amplitude value of 0 is suppressed when the second bandpass filter at least partially transmits at a signal band other than the first bandpass filter. On the other hand, the noise of the signal with nominal amplitude value of 1 is suppressed when the second bandpass filter effectively transmits the same signal band as the first bandpass filter, while the second bandpass filter rejects parts of the broadened/shifted signal spectrum.
As used herein, spectrum broadening by the nonlinear element means that any given percentage of signal energy occupies a broader band of the spectrum after the nonlinear element than before it. On the other hand, spectrum shifting means that the wavelength at which signal intensity has its maximum differs between the input and output sides of the nonlinear element. A feature common to both spectrum broadening and shifting is that after the broadening or shifting, a significant portion of signal energy resides in one or more spectrum bands which were substantially devoid of signal energy before the spectrum broadening or shifting.
Some embodiments of the inventive apparatus comprise two virtually identical optical systems in parallel. For instance, one of the parallel optical systems may process the I channel of DQPSK signals, while the other parallel system processes the Q channel. Some embodiments provide further savings in cost and complexity by utilizing common elements for both channels.
Other aspects of the invention include a method for processing an optical input signal carrying symbols, the method comprising:
In the following the invention will be described in greater detail by means of specific embodiments with reference to the attached drawings, in which
The regeneration stage 4-3 resembles the amplitude regeneration stage 2-3 of the known regenerator 2-0. As a departure from the prior art, the regeneration stage 4-3 is logically positioned between the optical receiver's modulation conversion stage 4-1, such as a delay interferometer, and the photo-electric conversion stage 4-5. Another departure from the prior art is that the regeneration stage 4-3 is not followed by a 3 dB coupler, such as the coupler 2-41 in
The inventors have discovered that the regeneration stage 4-3 can be implemented with only one of the two SOA components 4-33 and 4-34, whereby the other SOA component can be omitted. This issue will be discussed in more detail in connection with
The circuit 4-0 operates as follows. The delay interferometer 4-1 converts the phase modulated signal (for instance DPSK) to a modulation format which is partially phase modulated and partially amplitude modulated. A pair of consecutive input signals is converted into a pair of high-level and low-level amplitude signals. In case of a noiseless input, the low-level signal intensity is zero. The amplitude and the phase noise are both superimposed on the high and low levels of the optical signal. Depending on the phase difference of the consecutive symbols, the high-level amplitude signal propagates either along the A optical path or the B optical path. When the high-level and low-level signals meet in a nonlinear amplifying element, such as the semiconductor optical amplifier (SOA) 4-33, 4-34, the high-level signal saturates the medium, which results in suppression of the amplitude variation of the high-level signal. The low-level signal experiences the same transmission characteristics as the high-level signal, because both the low-level and the high-level signal are present in the SOA component and propagate through it simultaneously. As a result, both signals experience the same gain, and the amplitude of the noise in the low-level signal is either suppressed or enhanced. The effect of the limiting amplifier is such that the “signal eye” widens because the noise of the high-level signal is suppressed while the statistical distribution, such as the standard deviation, of the low-level signal remains almost constant. As used herein, the signal eye means a gap between low-level noise and high-level noise. It is thus a measure of signal quality. The widening of the signal eye primarily results from the noise of the high-level signal is efficiently suppressed.
In addition to the nonlinear amplification, the one or two nonlinear amplifying elements 4-33, 4-34 may further induce self-phase modulation (SPM) and/or spectral shifting onto the through propagating high- and low-level signals. As discussed in reference document 5, the transmission functions of the SPM broadened high- and low-level signals are different from one another when directed through suitable bandpass filters. This results in further widening of the signal eye. In following text the process of noise suppression due to SPM and/or spectral shifting of the bandpass filtered signals, as discussed in reference documents 4 and 5 and as explained above, is called “Mamyshev regeneration”.
Provided that the optional bandpass filters BPF1, BPF2 are installed, the embodiment shown in
It was stated earlier, in connection with the description of the prior art, that a straightforward technique for reducing noise in an optical receiver 3-0 is regenerating the optical signal by a regeneration apparatus 2-0 as taught by reference document 1, prior to applying the optical signal to the optical receiver 3-0. The inventors of the present invention have found out that this straightforward technique fails to provide the optimal noise suppression characteristics, for the following reason. Assuming that the incoming optical signal power saturates the SOA, its output will indeed suppress the amplitude variation by the effect of limiting amplification. However, the saturated SOA also affects the phase of the optical signal. If two consecutive symbols of the input signal have different amplitudes, the phase difference of these two signals will also be changed. The delay interferometer transforms such phase variation to an amplitude variation, which results in increased amplitude noise at the output of the delay interferometer, thus compromising the performance of the regenerator. This also applies to a single SOA positioned in front of an optical phase-sensitive receiver. Because of pulse-to-pulse amplitude variation, the phase is also varied, which will again be translated into amplitude variation at the output of a delay interferometer.
Reference document 1 teaches that the circuit 2-0, which is a sequence of a first delay interferometer 2-1, a limiting semiconductor optical amplifier (SOA) 2-3, and a second delay interferometer 2-4, has an ability to remove amplitude and phase noise. This is true to certain extent, but there are two drawbacks when using the second delay interferometer 2-4. As explained in the preceding paragraph, a saturated SOA alters the phase of the optical signal. Although the limiting amplifier suppresses amplitude noise, it simultaneously generates parasitic (unwanted) symbol-to-symbol variations in the signal phase. After the limiting SOA amplifier, when two consecutive symbols are combined in the second delay interferometer 2-4, this phase variation is translated into amplitude noise, thus degrading performance, which is one of the drawbacks. The second drawback is that the receiver 3-0 will require yet another delay interferometer 3-1 before the amplitude detection stage 3-5, because the second delay interferometer 2-4 transforms the phase/amplitude modulated signal back to a phase-only-modulated signal, and the amplitude detection stage 3-5 cannot detect information in the phase-only-modulated signal.
The second delay interferometer's undesired tendency to generate additional amplitude noise can be circumnavigated by connecting both the A and B arms of the output of the second delay interferometer 2-4 to the respective A and B arms of the input of the third delay interferometer 3-1 (as opposed to the conventional technique of connecting only one output, such as the “OUT” terminal of the regenerator 2-0 to the respective input “IN” of the receiver 3-0). This work-around effectively restores the signal to the modulation format and state it had before the second delay interferometer 2-4. It should be noted, however, that such an arrangement increases expenses and the corresponding teaching is not provided in above-mentioned reference documents.
The photo-electric conversion stage 4-5 typically uses the electrical output signal of the two photodetectors 4-51, 4-52 in a balanced receiver configuration which outputs a single electrical signal as a difference of the two photodetectors' electrical output signals. As is known in the art, in an ideal case the signal from either photodetector alone contains all the information, but a photodetector pair in a balanced receiver configuration is typically used for improved noise tolerance. Some embodiments of the invention only employ a single photodetector, which is installed in either of the optical output arms, and which directly converts the optical signal into an electrical signal. As a result of the noise suppression provided by the regeneration stage 4-3, use of two photodetectors in a balanced receiver configuration may not be required. Accordingly, each pair of photodetectors in a balanced receiver configuration may be replaced by a single photodetector.
In
Diagram 51A, which relates to the A arm input to the circuit 4-0, describes a DPSK modulated input signal having both phase noise and amplitude noise. Since nothing is connected to the B arm input, the diagram 51B is a zero signal. Diagrams 52A and 52B describe the A and B arm signals after the delay interferometer 4-1, both of which exhibit high-level and low-level signals, both containing phase noise and amplitude noise. The high-level signals and low-level signals appear as pairs, such that the delay interferometer's one output produces the high-level signal and the other output produces the low-level signal, and the regeneration stage 4-3, such as the limiting SOA amplifier, always processes the optical signal as symbol pairs, because the regeneration is based on the simultaneous occurrence of the symbols in the SOA.
The ability of the SOA component to suppress input power variations in the saturation region of the gain curve is manifested in the constellation diagrams 53A and 53B of
In experiments and simulations carried out by the inventors, a typical Q-value improvement without Mamyshev regeneration was about 3 dB at a wavelength of 1550.12 nm, input power range of −10 dBm to +5 dBm, and for input Q-value range of 2-15 dB. As used herein, the Q-value is defined as Q=10 log(Δ/(σ1+σ0)), wherein Δ is the measured average power difference of high-level and low-level signals, and σ1 and σ0 are the power standard deviations (noise) of high-level and low-level signals, respectively.
The delay interferometers 7-1A and 7-1B are followed by respective regeneration stages 7-3A and 7-3B. These are followed by respective photoelectric conversion stages 7-5A, 7-5B, which are again implemented as photodiode pairs, or as individual photodiodes, similarly to the corresponding element 4-5 in
The fact that the delay interferometers 7-1A and 7-1B exhibit mutually different phase shifts, such as +π/4 and −π/4 radians as in the present example, can be considered surprising. This is because the outputs of the delay interferometers 7-1A and 7-1B do not exhibit the high-level and low-level signals in the sense of DPSK modulation, where the low-level signal approaches zero in case of noiseless input. Instead, the output of the interferometers 7-1A and 7-1B exhibit normalized amplitude values of 0.92 and 0.38 (or respectively power values of 0.85 and 0.15, because cos [(π/4)/2]=0.92 and sin [(π/4)/2]=0.38; the squares of which are 0.85 and 0.15, respectively). These signal levels can be called high-level and low-level signals although their precise numerical values differ from those used in connection with BPSK modulation.
It can be seen that the DQPSK modulated signal is transformed to a phase/amplitude modulated signal having two distinctive amplitude levels. (In case of a φ=0 delay interferometer, there would be three amplitude levels.) After the limiting amplification there is some improvement at both levels, such that the Q value of the received signal is improved. A further improvement can be obtained if the optional first bandpass filter BPF1 and second bandpass filters BPF2 are employed and when the limiting amplifier induces SPM broadening and/or frequency shifting into the signal spectra. Contrary to the teaching of reference 1, limiting amplification alone, or optionally combined with Mamyshev regeneration, is sufficient for signal improvement. It must be noted, however, that discriminative gain does not harm the operation of the invention.
Actual measurements were carried out using an amplifying medium exhibiting the gain curve shown in
The isolators 9-35, 9-36, and circulators 10-31, 10-32 shown in
The embodiments described in connection with
Operation of the circuit 11-0 differs from the previous embodiments in that the one or more limiting amplifiers 11-33, 11-34 perform amplitude regeneration while the optical signal is in its first modulation format, in which useful information is conveyed by modulating at least one physical parameter other than amplitude. The optical signal is not converted to the second modulation format until the second coupler 11-13, which in the present embodiment is followed by the photo detector stage 4-5 without any intervening conversion or regeneration elements. A difference to the previous employments of Mamyshev regeneration is that now the spectrally broadened and/or frequency-shifted signal does not have high-level signals and low-level signals. Instead, both spectrally broadened and/or frequency-shifted signals are in the first modulation format. When properly employed, the Mamyshev regeneration is known to remove noise, and particularly ASE noise (ASE=amplified spontaneous emission) of the signal.
Assuming that phase is the physical parameter being modulated, the optical signal exiting from the output port of the second coupler 12-13 to the Sagnac loop 12-33, 12-35, is phase/amplitude modulated having high-level pulses and low-level pulses. These pulses propagate along the Sagnac loop and collide with one another in the SOA 12-33. After circulating through the loop, the pulses re-enter the coupler 12-13, which splits them to the A and B arms via the circulators 12-42, 12-43. Thereafter the A and B arm signals are directed to a third coupler 12-44. The second and third couplers 12-13, 12-44 form a Mach-Zehnder interferometer, whose output is again phase/amplitude modulated. The optical signals are terminated into a photo detector stage 4-5 without any intervening conversion or regeneration elements. Mamyshev regeneration can be included by introducing the first bandpass filter BPF1 and the second bandpass filters BPF2, and by ensuring that the limiting amplification provides the needed SPM into the signal. The bandpass filters BPF2 can be located after the circulators 12-42, 12-43, or in one or both output arms of the 3 dB coupler 12-44.
The insensitivity to internal reflections arises from the following reason. Signals that first enter the Sagnac loop from outside, then counter-propagate through the full loop, and then exit the loop, are treated differently from signals that do not make a full roundtrip in the loop. Internal reflections within the Sagnac loop do not make a complete roundtrip, thus they do not follow the strict symmetrical operation that is the basic Sagnac principle for signals entering the loop from outside. In general, parasitic reflections are exhibited by SOA facets. Internal SOA chip facets at the transition from the SOA chip to waveguide, fiber or free-space optical elements is a special problem that cannot be influenced by external engineering solutions. In implementations without the Sagnac configuration, strong parasitic reflections cause interference between the high-level pulses and low-level pulses, and this interference can disturb circuit operation and reduce noise-suppression efficiency. The Sagnac configuration described in connection with
The operating principle of sending the parasitic reflections to the output ports where they do not interfere with the other signal (such as low-level signals with high-level signals or vice versa) is based on balancing of the Sagnac loop's arm lengths on either side of the SOA, which is located at the midpoint of the loop. As usual, the arms can be realized as waveguides, optical fibres or free space paths. An additional −π/2 (or +π/2) radian phase shift element 12-35 is needed within the loop. If the reflection facets are not positioned symmetrically to the center of the Sagnac loop, the phase shift element 12-35 needs to be adjusted to a phase shift value other than −π/2 (or +π/2). The more the reflection values on either side of the SOA are alike, the better is the loop's ability to suppress reflection-induced noise. Only the difference in reflection values causes residual disturbances, even if the absolute reflection value at the facets is high.
The Sagnac loop arrangement of the circuit 14-0 is identical with that of the circuit 12-0B shown in
The embodiments of
The two parallel modulation conversion stages are followed by respective photo-electric conversion stages 4-5A, 4-5B, which are again implemented similarly to the corresponding element 4-5 in
Reference numeral 19-0 generally denotes a signal regenerator or noise suppression element positioned in front of the optical receiver. The signal regenerator 19-0 comprises a demultiplexer 19-2, which separates the optical channels from one another, similarly to the multiplexer 19-12 of the optical receiver. For each channel, the signal regenerator 19-0 comprises a delay interferometer 4-1, a SOA component 19-4 and a 3 dB coupler 19-6. The SOA component 19-4 and the 3 dB coupler 19-6 generally correspond to respective elements 4-33 and 4-32, which were shown and described in connection with
The arrangement shown in
This arrangement is susceptible of variations. For instance, the elements 19-8 through 19-12, that is, the MUX1 and DEMUX2 plus the optical connection 19-10 can be replaced by an optical path, such as an optical fiber and, optionally, with a band pass filter which acts as the second band pass filter BPF2 of Mamyshev regeneration. This variation is applicable to conversion of DQPSK signal into two amplitude-modulated signals, whereby a DPSK regenerator as described in connection with
The above description of the various embodiments of the invention is not restricted to any particular implementation of the optical components. Instead the optical components, including but not limited to optical paths, delay elements, phase shifters, couplers, interferometers, saturable absorbers, nonlinear amplifiers, etc., can be constructed by means of any of the available technologies, including optical fibers, waveguides, free-space optical components (such as lenses, mirrors, or gratings), or some other types of optical path construction known to those skilled in the art, or any combinations of such technologies. The optical medium in the optical paths may include glass, such as silica; semiconductor, such as silicon; fluid, such as liquid, gas or gas mixture (eg air), or vacuum.
It is readily apparent to a person skilled in the art that the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims. Individual features from various embodiments can be used in combinations which are not described in the present document. For instance,
While the terms QPSK and DQPSK imply encoding in which two symbol pairs are evenly distributed among the four quadrants of a circle, the invention is not restricted to such encoding schemes.
The invention is applicable to a variety of modulation formats, the reception of which involves comparison of two consecutive symbols with one another. The physical parameter being modulated may be phase, frequency or polarization state, or a combination of these parameters. The signal may be polarization multiplexed, ie, it may carry two streams of optical information in orthogonal polarization states, and the polarization demultiplexing may be performed after the limiting amplification. Alternatively or additionally the signal may be wavelength division multiplexed, ie, the delay interferometer and the limiting amplifier may process optical signals with more than one carrier wavelength simultaneously, such that the wavelength division demultiplexing is performed after the limiting amplification.
Phase shifting or a phase shifter refers to any means or technique for controlling mutual phase shift between two electromagnetic waves travelling in two respective optical paths. One exemplary technique involves altering the index of refraction of the optical path, by using a temperature difference between the two optical paths. For instance, the optical fiber may be locally heated in one of the optical paths. The heating alters the index of refraction, which in turn alters the optical path length of the electromagnetic wave travelling in the heated optical path. Any phase shift control may take place virtually anywhere along the optical path, or the phase shift control may take place in a distributed manner. Any phase shift of a given sign (plus or minus) in one optical path (A or B) may be replaced by a phase shift of the opposite sign (minus or plus) in the other optical path (B or A). Yet further, the non-linear elements, such as saturable absorbers or semiconductor optical amplifiers (SOA) may be used for integrated phase shift control by adjusting their temperature, bias current or the optical power traversing the non-linear element.
The number of limiting amplification and optional Mamyshev regeneration stages is not limited to one limiting amplification stage and one Mamyshev regeneration stage, but the invention may contain several limiting amplification and several Mamyshev regeneration stages. For example, in
The location of the first and second bandpass filters BPF1 and BPF2 may vary within a receiver. For instance, the first bandpass filter BPF1 may be installed at any one or more locations before the location of the spectral broadening and/or frequency shifting. The first bandpass filter BPF1 can thus be located before the modulation conversion means (such as the delay interferometer), after the modulation conversion means, or inside the modulation conversion means. Each of the bandpass filters BPF1 and/or BPF2 can be a wavelength multiplexer or demultiplexer used in WDM systems to separate different wavelength channels from each other. The second bandpass filter BPF2 can be located anywhere between the location of the spectral broadening and/or frequency shifting and photoelectric conversion means.
The first and second bandpass filters BPF1, BPF2 may be provided by means of thin film coatings, waveguide gratings, finite impulse response (FIR) filters, such as asymmetric Mach-Zehnder interferometers, resonators, arrayed waveguide gratings, or any combination of these or other filters known in the art. The filters may be tunable, such that the transmission spectrum may be altered by changing a voltage (as the case is with liquid crystal filters), temperature, or some other physical parameter. For example, the filters may have a periodic transmission function at a frequency of 50 GHz or 100 GHz, in which case a single filter component may be used to filter many different wavelengths separately or simultaneously. The frequency grid may be one provided by a standardization body, such as the International Telecommunication Union (ITU).
The limiting amplification and/or spectral broadening and/or frequency shifting may be provided by means other than semiconductor optical amplifiers. For example, the limiting amplification may be obtained by parametric amplification in glass or semiconductor waveguide. The spectral broadening can likewise be obtained in appropriately dimensioned glass or semiconductor waveguides.
The photoconversion may be obtained by photodiodes, phototransistors, or metal-semiconductor-metal detectors, or by other means known in the art.
The optical couplers used in the various embodiments of the present invention, including the 3 dB couplers and couplers with different coupling ratios, can be constructed by using any of several construction techniques, including but not limited to partially reflecting mirrors, waveguides coupled to one another via an evanescent field, or gratings.
As stated in several contexts above, the drawings are intended to be schematic in the sense that they primarily illustrate the logical arrangement of the novel elements of the invention. Those skilled in the art will understand that practical working implementations based on such schematic drawings may include additional components which are not specifically illustrated or described. Optical isolators or circulators that were described in connection with
| Number | Date | Country | Kind |
|---|---|---|---|
| 20095288 | Mar 2009 | FI | national |
This application is a national-phase continuation of PCT/FI2010/050206, published as WO2010/106231A1, which application claims priority from Finnish Patent Application 20095288, filed 19 Mar. 2009, and from U.S. provisional patent application 61/174,053, filed 30 Apr. 2009.
| Number | Date | Country | |
|---|---|---|---|
| 61174053 | Apr 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/FI2010/050206 | Mar 2010 | US |
| Child | 13227140 | US |
