JP2008160707A - Optical receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical receiver in which interference of signal light itself is prevented and the signal light is hardly deteriorated. <P>SOLUTION: In an optical receiver in which (n) stages of interference light eliminators are connected in series ((n) is a positive integer of ≥2), the optical receiver according to the present invention is characterized in that when an optical path length difference of the interference light eliminators on an i<SP>th</SP>stage ((i) is a positive integer of >(n)) is defined as L<SB>i</SB>, a set of all combinations of the optical path length differences L<SB>i</SB>of the interference light eliminators on the i<SP>th</SP>stage and sums thereof is defined as U, arbitrary different elements in the set U are defined as u<SB>i</SB>, u<SB>j</SB>, and a distance that the signal light moves forward during a time in which emission of the signal light is continued is defined as lx, a difference between the arbitrary different elements u<SB>i</SB>, u<SB>j</SB>in the set U becomes equal with or more than the distance lx that the signal light moves forward during the time in which the emission of the signal light is continued, and the set U satisfies a formula 1. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical receiver used in an optical CDM (Code Division Multiplex) optical communication system that transmits and receives signal light encoded by an optical transmitter.

  An optical code multiplexing system is an optical communication system that performs asynchronous communication using the same optical frequency band. The signal lights to which the codes are applied can remove the signal lights of the codes that are not desired due to the orthogonality of the codes. However, signal light to which no code is applied cannot be removed because there is no orthogonality. Such a situation occurs, for example, when a new service is added using an existing optical fiber that provides the service without affecting the existing service. In this case, the signal light to which the code is applied is spread over a wide optical frequency width, and communication is performed with a weak signal light that does not affect the existing service due to the coding gain. The signal light intensity of the additional service that does not affect the existing service is required to be sufficiently small as compared with the existing service assumed to have an extinction ratio of 8 to 12 dB.

  Ensuring sufficient coding gain for signal light for existing services with high intensity (hereinafter, “signal light for existing services” is referred to as “jamming light”) requires expansion of the optical frequency width. There has been a problem that is limited by the deterioration of the signal waveform due to the transmission line chromatic dispersion. Furthermore, the hard limiter for increasing the saturation and anti-jamming properties of the light receiver has a problem that a complicated processing circuit is required.

As means for solving such a problem, for example, there is a technique disclosed in Patent Document 1. In this prior art, a wide optical frequency width and short coherence length of a light source used in optical code multiplexing, a short duration of light emission of the light source used in optical code multiplexing (narrow pulse width), or an interference light canceller The multiple relationship between the repetition period of the transmittance and the period of the optical frequency width for one code of the encoder and decoder is used. As the interference light canceller, for example, a Mach-Zehnder (MZI) interferometer having an optical path length difference that satisfies any of the following three conditions is used.
(1) The optical path length difference of the interfering light remover is not less than the coherence length of the signal light and below the coherent length of the interfering light. (2) The optical path length difference of the interfering light remover is obtained when the signal light is multiplexed after branching. (3) The optical path length difference of the interfering light remover is subtracted from the portion added by the signal light that is encoded with a code that is not subject to decoding by the decoder and is canceled by addition and subtraction during decoding. Length that is even

The jamming light remover satisfying these conditions is adjusted so that the jamming lights transmitted through both arms interfere with each other, and the jamming light is removed from the output connected to the decoder. In the above (1) and (2), the signal light does not interfere because the optical path length difference is equal to or greater than the coherence length (condition (1)) and the branched signal lights do not overlap (condition (2)). Are output to the decoder. In (3) above, the signal light may be interfered by the interference light remover, but the portion added by the signal light other than the desired signal and the portion to be subtracted are removed by interference with a balanced intensity. Therefore, the interfering orthogonality does not occur due to the interference light canceller, and interference of other codes does not increase. In this way, it is possible to remove interfering light that does not degrade interference between codes.
JP 2006-74557 A

However, in the prior art, when there are a plurality of interfering lights, the optical frequencies of the interfering lights are close enough to be removed by the interfering light remover, or the repetition period of the transmittance of the interfering light remover There was a problem that could not be removed unless the interval was equivalent to a multiple. The ratio of signal light to interference light intensity (SIR (IN)) before passing through the interference light canceller can be expressed as SIR (IN) = I ₀ / ΣI _i . Furthermore, an ideal signal light after passing through the interference light stripper-interference light intensity ratio (SIR (OUT)) _{is, SIR (OUT) = (I} 0/2) / Σ (I i / CMR i) I can express. Here, the intensity of the signal light before passing through the interference light remover is I ₀ , the intensity of the i-th interference light is I _i (i is 1 or more), and the i-th interference light removal by the interference light remover is performed. The ratio was CMR _i .

At this time, in the conventional interference light remover, the CMR _i becomes small due to the relationship between the optical frequencies of the interference light, and thus the interference light may not be sufficiently removed. For example, if the difference in optical frequency between two stages of interfering light having substantially the same intensity is half the period of the interfering light canceller, SIR (IN) = I ₀ / {I ₁ + I ₂ } = I ₀ / {2I _{1} becomes, SIR (OUT) = (I} 0/2) / {2I 1/2} = SIR (iN) , and the signal light to interference light intensity ratio by the interference light stripper has a problem that does not improve.

Moreover, the interference light can be removed by simply connecting the conventional interference light removers in series. However, if the optical path length differences of the respective interference light cancellers are close to each other and pulses passing through different optical paths overlap or the sum of the optical path length differences is less than the coherence length, the signal light also interferes. The problem is that the intensity I _{0 of the} signal light that passes through the interference light canceller and reaches the decoder due to interference decreases, and the signal light to interference light intensity ratio SIR (OUT) after passing through the interference light canceller decreases. there were.

  An object of the present invention is to provide an optical receiver that prevents interference of signal light itself and prevents the signal light from being deteriorated.

  The inventors of the present invention have described the difference between the optical path length difference of the interference light canceller, the optical path length difference of the decoder, and any combination of these optical path length differences as the coherence length or the time during which signal light emission continues (pulse width). It has been found that the above problem can be solved by setting the distance that the signal light travels to be equal to or longer than the length that is the coherence length of the signal light, and the present invention has been completed.

Specifically, the optical receiver according to the present invention removes the interfering light included in the input light by interference through a plurality of optical paths having a predetermined optical path length difference, and sets the intensity ratio of the signal light to the interfering light. In an optical receiver having n (where n is a positive integer of 2 or more) stages of interfering light cancellers to be improved connected in series, the optical path of the interference light canceler at the i-th stage (i is a positive integer of n or less) A length difference L _i , a set U composed of all combinations of the optical path length difference L _i of the interference light canceller at the i-th stage and their sum, any different elements u _i , u _{j of} the set U, and the signal Assuming that the distance that the signal light travels during the time that light emission continues or the length lx that is the coherence length of the signal light, the difference between any different elements u _i and u _j of the set U is the signal light The distance that the signal light travels during the time when the light emission continues or the length that is the coherence length of the signal light Becomes more x, the set U is characterized by satisfying Equation 1.

  The above optical receiver can prevent the signal light itself from interfering and make the signal light difficult to deteriorate.

  In the optical receiver according to the present invention, it is preferable that at least one of the optical frequency width of one chip of the signal light and the optical frequency width of the interfering light is less than half of the free spectral range of the optical receiver. .

  Here, the free spectrum range is a frequency interval at which transmission loss of the signal light and the interference light is minimized.

  The above optical receiver can take into account the conduction of the signal light and the sufficient removal of the interfering light.

In the optical receiver according to the present invention, a decoder that splits the input light and multiplexes it through a plurality of optical paths having a predetermined optical path length difference is connected to an output of the interfering light remover, and the decoder A set U ′ consisting of all combinations of the optical path length difference L _{n + 1 of} the interference light remover, the optical path length difference L _{i + 1} of the decoder and the optical path length difference L _{n + 1} of the decoder, and their sum, any different element of the set U ′ If u ′ _i , u ′ _j , the difference between any different elements u ′ _i and u ′ _j of the set U ′ is the distance traveled by the signal light during the time when the signal light emission continues or the signal light It is preferable that the coherence length is equal to or longer than the length lx, and the set U ′ satisfies the expression (2).

  The above optical receiver can avoid unintended deterioration of the signal light due to interference caused by the interference light canceller and the decoder.

  In the optical receiver according to the present invention, it is preferable that the interference light remover has the predetermined optical path length difference not less than a coherence length of the signal light and not more than a coherence length of the interference light.

  In the optical receiver described above, since the optical path length difference of the interfering light cancellers at each stage and an arbitrary difference between the elements of the set consisting of the sum are equal to or greater than the coherence length of the signal light, the interfering light of the signal light itself Interference in the light remover can be prevented.

  In the optical receiver according to the present invention, the interference light remover is configured such that the predetermined optical path length difference is longer than a distance traveled by the signal light during a time period during which the signal light is continuously emitted, and the interference light The distance is preferably shorter than the distance traveled by the interfering light during the time that light emission continues.

  In the above optical receiver, the optical path length difference of the interfering light cancellers at each stage and an arbitrary difference between the elements of the set consisting of the difference are determined from the distance traveled by the signal light during the time that the signal light emission continues. For this reason, the signal lights output from the interfering light remover do not overlap at the time of multiplexing, so that interference of the signal lights themselves can be prevented.

  In the optical receiver according to the present invention, the decoder decodes the signal light encoded with an optical frequency width that is a multiple or a divisor of the repetition period of the transmittance of the interfering light remover, and converts the signal light into the input light. The signal light that is included and encoded by a code that is to be decoded by the decoder, or at least one light of the signal light that is encoded by a code that is not to be decoded by the decoder and is canceled by addition and subtraction during decoding The frequency width is preferably equal to or greater than the least common multiple of the repetition period of the transmittance of the interfering light remover and the optical frequency width of one code of the decoder.

  The optical receiver enables signal transmission of the signal light from which the interfering light is removed, and the signal light encoded by a code not to be decoded by the interfering light remover satisfies an optical frequency width condition. When the signal light encoded with the code to be decoded does not increase and the signal light encoded with the code to be decoded satisfies the condition of the optical frequency width, the code with the code to be decoded The intensity of the converted signal light does not vary, and the optical path length difference with which the signal light interferes can be selected.

  INDUSTRIAL APPLICABILITY The present invention can provide an optical receiver that prevents interference of signal light itself and hardly deteriorates signal light.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment. Moreover, the same code | symbol was attached | subjected to the same apparatus and the same member.

(First embodiment)
The optical receiver according to the first embodiment removes the interference light included in the input light by interference through a plurality of optical paths having a predetermined optical path length difference, thereby improving the intensity ratio of the signal light to the interference light. In an optical receiver in which n (n is a positive integer of 2 or more) stages of optical eliminators connected in series, the optical path length difference L of the interference light eliminator at the i-th stage (i is a positive integer of n or less). _i , a set U composed of all combinations of the optical path length difference L _i of the interference light canceller at the i-th stage and their sum, any different elements u _i , u _{j of} the set U, and emission of the signal light Is a distance lx traveled by the signal light during a continuous period of time, a difference between any different elements u _i and u _j of the set U is equal to or greater than a distance lx traveled by the signal light during a time when the signal light is continuously emitted. Thus, the set U satisfies Equation 1.

  The case where a new service is added to an existing service in the communication network to which the optical receiver according to the first embodiment is applied will be described as an example. FIG. 1 shows a configuration diagram of the communication network 100. In FIG. 1, a light source 111 for an existing service and a modulator 112 that modulates data of the existing service and a transmitter 110 that outputs signal light (jamming light) for the existing service are added. An optical transmitter 105 that includes a light source 106 that outputs a short pulse light for service and a modulator 107 that modulates with the data of the service to be added and outputs a signal light for the service to be added is shown. Here, the configuration including the modulators 107 and 112 that modulate outside the light sources 106 and 111 has been described. However, a direct modulation light source that directly modulates the output intensity of the light sources 106 and 111 without using the modulators 107 and 112 is used. May be. Also, the jamming light for the existing service and the signal light for the service to be added are multiplexed by the optical splitter 102, and are transmitted to the optical receiver 115 for the service to be added to the existing optical receiver 120 for the service after transmission. The light is branched by the optical branching device 104. Here, the configuration including the optical branching units 102 and 104 before and after the transmission is described. However, the combination of the transmission path and the two optical branching units 102 and 104 is replaced, and a single optical branching unit combines and separates the signals. It is good also as a structure to wave (not shown).

  On the receiving side, the existing service optical receiver 120 including the detector 121 for detecting signal light from the existing service, and the service receiver to be added according to the first embodiment. There is an optical receiver 115. The service optical receiver 115 to be added includes a two-stage interference light removers 116a and 116b for removing signal light from the existing service, and a detector 117 for detecting the light after removal. As the two-stage interfering light removers 116a and 116b, for example, the input light is branched, a plurality of branched lights adjusted to a predetermined optical path length difference are combined, and the interference is caused by the optical path length difference of the branched light. A Mach-Zehnder interferometer that changes the output optical path according to the output intensity or optical frequency of the branched light can be used. The intensity of the transmitted light with respect to the optical frequency in the output optical path of the Mach-Zehnder interferometer is comb-like, and the intensity of the portion corresponding to the comb-like teeth can be expressed by a sine function. In FIG. 1, the two-stage interference light removers 116a and 116b are shown. However, the interference light remover 116 is not limited to two stages, and the interference light remover 116 may have three or more stages (not shown). The same applies to the following.

  The jamming light remover 116 removes the jamming light from the existing service by adjusting the optical frequency so that the jamming light from the existing service is not transmitted. In the optical receiver according to the first embodiment, the interference light remover is configured such that the predetermined optical path length difference is longer than a distance traveled by the signal light during a time during which the signal light is continuously emitted, and the interference light is removed. The distance is preferably shorter than the distance traveled by the disturbing light during the time that light emission continues.

Here, the time during which signal light emission continues will be described by taking a binary signal as an example. Examples of the duration of signal light emission include the following.
(1) Time during which signal light emission continues within the time corresponding to 1 bit of signal light (2) Time during which light emission of pulse when 1-bit signal light is generated from one or more pulses (3) Wavelength Time during which signal light for one chip that can interfere with each other is emitted with signal light encoded with at least one of code and optical frequency code (4) Code with at least one of wavelength code and optical frequency code Time (pulse width) of the emission of pulses for one chip that can interfere with each other with the converted signal light
(5) Time during which emission of signal light for one chip that can interfere with each other with signal light encoded by at least one of wavelength-time code and optical frequency-time code continues (6) wavelength-time code or The time (pulse width) in which the emission of pulses for one chip that can interfere with each other with signal light encoded with at least one of the optical frequency and time codes continues
(7) Time during which emission of signal light for one chip that can interfere with each other with signal light encoded by at least one of wavelength-polarization code and optical frequency-polarization code continues (8) wavelength-polarization The duration (pulse width) of light emission for one chip that can interfere with each other with signal light encoded by at least one of the wave code and optical frequency-polarization code

  The optical path length difference between the two optical paths of the interfering light remover 116 is equal to or longer than the distance traveled by the signal light during the time when the emission of the signal light such as an RZ (Return to Zero) signal continues. That is, it is an optical path length difference that does not overlap when the signal lights branched by the interference light remover 116 are combined, and an NRZ (Non Return to Zero) signal is a signal light (interference light) for general existing services. This is the distance traveled by the disturbing light during the continuous light emission, or the optical path length difference that at least partly overlaps when the light beams branched by the disturbing light remover 116 are combined. Further, the optical path length difference of the interfering light canceller 116 at each stage and an arbitrary difference between the elements of the set composed of the sums of the optical path lengths do not overlap the duration of the emission of the signal light branched by the respective Mach-Zehnder interferometers. In addition, it is longer than the distance traveled by the signal light during the time when the signal light continues to be emitted.

An expression relating to the optical path length difference of the interfering light remover 116 is shown below. When the jamming light remover 116 has two stages, that is, n = 2, the optical path length difference of the jamming light remover 116 is expressed by Equation 3. Also, ∀ | u _i −u _j | in Equation 3 is expressed by Equation 4.

Further, when the interference light remover 116 has three stages, that is, n = 3, the optical path length difference of the interference light remover 116 is expressed by Equation 5. In addition, ∀ | u _i −u _j | in Expression 5 is expressed by Expression 6.

The description will be continued as an example in the case where the jamming light remover 116 has two stages. The input light is input to the interference light remover 116a, and one of the interference lights is removed by adjusting the optical path length difference so as to remove one of the two interference lights. The input light is further input to the interference light remover 116b whose optical path length difference is adjusted so as to remove the other interference light, and the other interference light is removed. The signal light has an optical path length difference between each of the interference light removers 116a and 116b and an arbitrary difference between the elements of the set formed by the sum thereof longer than the distance traveled by the signal light during the time when the signal light emission continues. Since the output pulses of the optical remover 116 do not overlap at the time of multiplexing, they do not interfere with each other, and are branched in half at each stage of the interfering light remover 116, and the remaining quarter is an optical receiver 115 for additional services. Is input. At this time, the signal light to interference light intensity ratio (SIR (IN)) before passing through the interference light remover can be expressed by SIR (IN) = I ₀ / ΣI _i . Also, ideal after passing through the interference light stripper signal light pair of interfering light intensity ratio (SIR (OUT)) _{is, SIR (OUT) = (I} 0/2 n) / Σ (I i / CMR i) It can be expressed as Here, the intensity of the signal light before passing through the interference light remover is I ₀ , the intensity of the i-th interference light is I _i (i is 1 or more), and the i-th interference light removal by the interference light remover is performed. The ratio was CMR _i . In the above formula, the jamming light remover 116 removes one of the two jamming lights and does not remove the other jamming light, but the effect of removing the other jamming light is considered. For example, the value of the signal light to interference light intensity ratio (SIR (OUT)) after passing through the interference light remover becomes larger. If two interference light beams can be simultaneously removed by the one-stage interference light remover 116, they may be removed by the one-stage interference light remover 116. Here, the interference light remover 116 has two stages, but if the signal light to interference light intensity ratio (SIR (OUT)) after passing through the interference light remover is within the required range, this The number of stages above may be used.

A guideline for the lower limit of an arbitrary difference between elements of a set consisting of the optical path length difference of the interfering light cancellers at each stage and their sum will be described. In an ideal pulse, the base is widened to any extent, and it is difficult to define a lower limit of an arbitrary difference between elements of a set composed of an optical path length difference and a sum of optical path length differences. Therefore, the lower limit is defined with an overlap that can be ignored. The case of a rectangular wave will be described as a simple example with reference to FIG. FIG. 6 shows a portion b where the interference light is removed from the output route due to interference. In the one-stage interference light canceller 116, the light branched into the two optical paths having the optical path length difference L _i is recombined. A portion where the signal lights do not overlap when re-multiplexing is ideally output from both output optical paths evenly. Assuming that all overlapping portions a of the signal light removed from the output path due to interference are output from one optical path, the output ratio of the one optical path is Z + (1−Z) / 2, and the output ratio of the other optical path is The ratio is (1-Z) / 2. Since the interference light remover 116 is set so as to remove the interference light from the optical path output to the decoder, the ratio of the remaining power is ideally (1−Z _J ) / 2. The output optical path of the overlapping portion a of the signal light removed from the output path due to interference is unknown, and is (1−Z _S ) / 2 to (1 + Z _S ) / 2. Therefore, when the interference light canceller has one stage, the signal light to interference light intensity ratio (SIR) is {(1-Z _S ) / 2} / {(1-Z _J ) / 2} or more {(1 + Z _S ) / 2} / {(1-Z _J ) / 2} or less. Here, the overlapping ratio of the signal light is Z _S , and the overlapping ratio of the interference light is Z _J. If the influence that the interference light cannot be removed without overlapping is expressed by I _i / CMR _i , it is added when the light constituting the signal light partially overlaps when remultiplexing after branching by the interference light remover 116 The minimum value I ′ ₀ of the intensity of the signal light output from the first-stage interference light canceller is I ′ ₀ = I ₀ (1−Z _S ) / 2. Therefore, if the overlapping ratio of the i-th interference light remover is Z _Si , the signal light to interference light intensity ratio (SIR (OUT)) after passing through the interference light remover 116 is SIR (OUT) = I ₀ {1/2 ⁿ −Σ (Z _S / 2 ^{(n + 1−i)} )} / Σ (I _i / CMR _i ).

Next, the case of a pulse will be described with reference to FIG. Assuming a Gaussian pulse, the optical path length difference is desirably a distance that the signal light travels at least four times the temporal spread σ of the Gaussian pulse. In the Gaussian pulse, the power at the base from a distance of 2σ from the center is 0.05. Approximating a state in which the skirts on one side of each of the branched pulses are all overlapped, Z _{S which} is a power that allows the pulses to overlap each other and interfere with each other is 0.05, and after passing through the first stage jamming light remover The worst value of the signal light is (1−Z _S ) /2=0.475. Compared to the case where the pulses do not overlap each other, the numerator of the signal light to interference light intensity ratio (SIR (OUT)) after passing through the first stage interference light remover is 50% to 47.5%, That is, the deterioration is 1/20.

  Since the lower limit of the optical path length difference of each stage of interfering light cancellers and any difference between the elements of the set consisting of the sum is determined from the allowable range of degradation of the signal light to interfering light intensity ratio (SIR), for example, If the signal light intensity is allowed to be degraded by 1/10 due to interference between the signal lights in the entire interfering light remover, the upper limit of the ratio of the overlapping portions is approximately 1/20, and the interference light canceller of each stage The lower limit of the optical path length difference and any difference between the elements of the set consisting of these sums is four times the time spread σ of the Gaussian pulse where the ratio of overlapping parts at each stage of the interference light canceller is 1/20 or less. This is the distance traveled by the signal light.

Next, the upper limit of the stage number (n) of the interference light eliminator will be described. One of the upper limits is defined, for example, because the intensity of the signal light that has passed through the interference light canceller does not fall below the reception sensitivity. In addition, another upper limit of this is that, for example, signal lights that can interfere with each other after being branched and re-multiplexed by the interference light remover do not overlap, including the time during which the emission of adjacent signal light continues. It is prescribed from. When the signal light emission duration is 700 fs in binary transmission at 1 Gbit / s and seven stages of two-branch interference light cancellers are provided, each optical path length difference is a minimum optical path length difference of 2 For example, 50 ps, 25 ps, 12.5 ps, 6.25 ps, 3.125 ps, 1.5625 ps, and 0.78 125 ps. That is, the maximum value of n satisfying (time for one bit of signal light) / 2 ⁿ is ^equal to or longer than the time during which signal light emission continues is the upper limit of the number (n) of interference light removers. Here, the time for one bit of signal light is used because it overlaps with adjacent bits of signal light and does not interfere.

In consideration of the conduction of signal light and sufficient removal of interference light, in the optical receiver according to the first embodiment, the optical frequency width (Δf ₀ ) of one signal light of the signal light or the optical frequency width (Δf of the interference light) It is preferable that at least one of _i ) is less than half of the free spectral range of each interfering light canceller constituting the optical receiver. Furthermore, it is more preferable that the optical frequency width of the interfering light is much smaller than half of the free spectral range of the optical receiver, for example, less than 1/10. Here, the optical frequency width preferably takes into account the spectral line width of the light source itself and the spread of the optical frequency width due to modulation.

  As described above, the optical receiver according to the first embodiment can remove interfering light from additional service signal light having a narrow pulse width.

In the optical receiver according to the first embodiment, a decoder that splits the input light and multiplexes it through a plurality of optical paths having a predetermined optical path length difference is connected to an output of the interference light canceller, An optical path length difference L _{n + 1 of} the decoder, an optical path length difference L _{i of the} interference light canceller, an optical path length difference L _{n + 1} of the decoder, and a total combination of them, and an arbitrary set of the set U ′ different elements u _'i, u' When _j, 'any different elements u of' _i and u 'difference between _j is the signal light travels the distance or the time the emission of the signal light continues the set U It is preferable that the length is equal to or longer than the length lx which is the coherence length of the signal light, and the set U ′ satisfies the formula 2.

  Here, the decoder (not shown) divides the input light and multiplexes it via a plurality of optical paths having a predetermined optical path length difference. In this case, the minimum value of the difference between the optical path lengths of the interfering light cancellers and decoders at each stage, and the difference between the elements of the set consisting of any combination thereof, is the emission of the signal light for the service to be added. It becomes more than the distance which advances to the time (pulse width) to continue. Because of this setting, the optical receiver according to the first embodiment can avoid unintended deterioration of signal light due to interference caused by the interference light canceller and the decoder even when the optical receiver includes the decoder.

  FIG. 2 shows a diagram of an optical receiver having two stages of interference light cancellers. The interference light remover 116a is, for example, a Mach-Zehnder interferometer having a free spectral range of 20 GHz. The interference light canceller 116b is, for example, a Mach-Zehnder interferometer having a free spectral range of 40 GHz. The decoder 118 is, for example, a Mach-Zehnder interferometer having a free spectrum range of 80 GHz. The minimum value of the optical path length difference between these two-stage interfering light cancellers 116a and 116b and the decoder 118 and the length difference between the elements of the set consisting of the sum of any combination thereof is 50 ps. This is the distance traveled.

  The light source 106 is, for example, a short pulse light source such as a mode-locked semiconductor laser (MLLD), and the pulse width is shorter than the minimum value of the length difference between all elements constituting the above set. For example, a mode-locked semiconductor laser that outputs a pulse having a pulse period of about 10 GHz (pulse interval is about 100 ps), a pulse width of about 10 ps, and a wavelength width of about 4 nm. The interference light sources 111a and 111b are, for example, external resonant wavelength tunable semiconductor lasers having respective wavelengths of 1560 nm and 1561 nm and an optical frequency width of approximately 1 GHz or less when output from the optical transmitter. The frequency interval is other than multiples of 20 GHz and 40 GHz which are free spectrum ranges of the interference light removers 116a and 116b. The three lights output from the light source 106 and the light sources 111a and 111b are modulated with, for example, a 1 Gbit / s pseudo-random pattern.

  FIG. 3A shows an optical spectrum before passing through the jamming light remover, FIG. 3B shows an optical spectrum after passing through the first stage jamming light remover, and FIG. 3C shows a second spectrum. The optical spectrum after passing through an interference light canceller is shown. It can be seen that the interference light is sequentially removed by the two-stage interference light remover.

  FIG. 4A shows an eye pattern when there is no interfering light, and FIG. 4B shows an eye pattern after removing the interfering light. Since the phase of the data and the phase of the mode-locked semiconductor laser are not synchronized, noise appears in the eye pattern. FIG. 4A and FIG. Since it is the same type, it can be seen that the eye pattern of the signal light has not deteriorated.

  For example, the Mach-Zehnder interferometer used as the interfering light remover may have one output optical path. Examples of the interference light canceller include an arrayed waveguide diffraction grating (AWG, Arrayed Waveguide Grating) that splits input light into a plurality of optical paths and interferes with each other, a Fabry-Perot interferometer, and a Mach-Zehnder interferometer with a ring.

  The optical transmitter may include a light source, a modulator, and an encoder in order, or may include a light source, an encoder, and a modulator in order. If the frequency characteristic of the output light intensity directly modulates the light source corresponding to the code, the light source may serve as both the encoder and the modulator. The optical receiver may include an interference light canceller and a decoder in order, or may include an interference light canceller, a decoder, and a detector in order.

  The optical receiver according to the first embodiment has shown the signal light for the existing service other than the signal light for the service to be added. However, the optical signal is not the signal light for the existing service but the interference light by a malicious user. This is also effective when superposed on the transmission path by means such as an optical branch path. In this case, as shown in FIG. 5, there is no optical transmitter and optical receiver that provide the existing service. In FIG. 5, only one interference light remover 116 is shown for simplicity.

(Second Embodiment)
In the optical receiver according to the second embodiment, the jamming light remover preferably has the predetermined optical path length difference equal to or greater than the coherence length of the signal light and equal to or less than the coherence length of the jamming light. .

  The difference between the optical receiver according to the second embodiment and the optical receiver according to the first embodiment is in the setting of the characteristics of the light source for service to be added and the optical path length difference of the interference light canceller. In the second embodiment, in the relational expression used in the first embodiment, the distance lx that travels during the time that signal light emission continues is replaced with the length lx that is related to the coherence length of the signal light. The optical receiver according to the second embodiment has the same configuration as the optical receiver according to the first embodiment, and may include a decoder. The light source is an incoherent light source having a wide optical frequency width. In the second embodiment, the interference light has a narrower optical frequency width and a longer coherence length than the signal light. Therefore, for example, a Mach-Zehnder interferometer having an optical path length difference equal to or greater than the coherence length of the signal light and equal to or less than the coherence length of the jamming light is used as the jamming light remover. The jamming light remover adjusts the jamming light transmitted through both arms to interfere with each other and removes the jamming light from the output connected to the decoder. Further, when the interference light canceller and the decoder are used, the optical path length difference between the interference light canceller and the decoder and an arbitrary element between the elements of the set consisting of the sum of the optical path length differences between the interference light canceller and the decoder. The difference is equal to or greater than the coherence length of the signal light for service to be added. Because of this setting, even when a plurality of interfering light cancellers and decoders are provided, unintentional signal light degradation due to interference can be avoided.

  As in the first embodiment, the lower limit of the length lx related to the coherence length of the signal light is shown. The interference power decreases exponentially according to the optical path length difference. A length that decreases to 1 / e (e is about 2.718) is defined as a coherence length lc. Similarly to the first embodiment, in order to suppress the reduction due to interference of the signal light output from the interference light canceller to 1/10 or less, the reduction per stage of the interference light canceller is performed by the same calculation as in the first embodiment. Is twice the coherence length lc at which 1/20 becomes 1/20.

(Third embodiment)
In the optical receiver according to the third embodiment, the decoder decodes the signal light encoded with an optical frequency width that is a multiple or a divisor of a repetition period of the transmittance of the interfering light remover, and the input At least one of the signal light that is included in light and encoded by a code that is to be decoded by the decoder or that is encoded by a code that is not to be decoded by the decoder and is canceled by addition and subtraction during decoding The optical frequency width is preferably equal to or greater than the least common multiple of the repetition period of the transmittance of the interfering light remover and the optical frequency width of one code of the decoder. In particular, it is more preferable that the added part and the subtracted part are equal when the signal light encoded with a code not desired to be decoded by the decoder is set as a multiple of the least common multiple.

The difference between the optical receiver according to the third embodiment and the optical receivers according to the first embodiment and the second embodiment is that the optical frequency width that is a multiple or a divisor of the repetition period of the transmittance of the interfering light remover is as follows. A decoder for decoding, the optical frequency width to be decoded by the decoder, the optical frequency of the signal light encoded by the code to be decoded by the decoder with the signal light included in the input light, and the decoder to decode When there is signal light that is encoded with a non-target code and is canceled by addition / subtraction at the time of decoding, each of the optical frequencies of the signal light is equal to the repetition period of the transmittance of the interference light remover and one code of the decoder. It is to become more than the least common multiple of the optical frequency width. Specifically, the repetition cycle of the transmittance of the interfering light eliminator satisfies any of the following.
(1) The repetition period of the transmittance of the interfering light remover is a natural number of the optical frequency width of the chip constituting the code, and the optical frequency width of the encoded signal light is equal to or greater than the optical frequency of the interfering light (2) When the signal light is encoded with a repetition code that can be divided into a plurality of repetitive parts, the optical frequency width of the repetitive part is 2m times (m is a natural number), and the optical frequency of the entire signal light including the plurality of repetitive parts Is 2- ^k (k is a natural number)

  For (1) above, an optical frequency chip width that constitutes the code of the optical CDM signal is obtained by using a Mach-Zehnder interferometer for the encoder and decoder, and the optical frequency of the repetition period of the transmittance of the interfering light remover. An explanation will be given taking an optical path length difference that is half as an example. In this example, the interfering light canceller changes the optical path length difference of the interfering light canceller so as to remove the interfering light from the output optical path of the Mach-Zehnder interferometer that alternately outputs to both outputs with a period of half the optical frequency chip width. Even then, light having substantially the same intensity as the service signal light to be added is input to the optical receiver. For this reason, signal transmission of the optical CDM signal from which the interference light is removed becomes possible. The same effect can be obtained by the interference light remover that satisfies the condition (2).

  In the third embodiment, the cycle of the repetition optical frequency of the code canceled by the decoder without being subject to decoding by the decoder and the repetition cycle of the transmittance with respect to the optical frequency of the interfering light remover are also in a multiple relationship. When there is signal light encoded with a code that is not subject to decoding that is added or subtracted at the time, the part added with the signal light and the part to be subtracted are of equal intensity and are removed by interference in the interfering light remover. Therefore, there is no disruption of orthogonality between codes, and interference due to other codes is not increased by the interference light canceller. For this reason, unlike the optical receivers according to the first and second embodiments, the optical receiver according to the third embodiment can select an optical path length difference in which signal light interferes. Furthermore, the optical receiver according to the third embodiment includes a plurality of interference light cancellers, and the minimum value of any difference between the elements of the set consisting of the optical path length difference of the interference light cancellers and their sum is obtained. This is at least one of the distance traveled by the time during which emission of the signal light for service to be added continues or the coherence length of the signal light for service included in the input light. Even when a plurality of interference light cancellers and decoders are provided for this setting, unintended deterioration of the optical signal due to interference due to the combination of the interference light canceller and the decoder can be avoided.

  Note that the light source used in the third embodiment has a discrete spectrum as long as the added part and the subtracted part are equal when decoding the signal light encoded with a code not desired to be decoded. It may be a short pulse light source such as a mode-locked semiconductor laser or an ASE light source such as a super luminescence diode having a continuous spectrum.

  In the third embodiment, the intensity of signal light does not fluctuate when the optical frequency to be removed by the jamming light remover is changed by selecting the repetition characteristics and common multiples of the jamming light remover and decoder. However, if the optical frequency width of the signal light is not the common multiple of the repetition period of the optical frequency of the interfering light canceller and the code length of the decoder, the portion of the signal light that protrudes from the optical frequency width when the optical frequency to be removed is changed There is a problem that the strength of the fluctuates. In other words, if it is a common multiple, the interference caused by the signal light encoded with the code not to be decoded is constant without increasing or decreasing, and the intensity of the signal light encoded with the code to be decoded does not vary. On the other hand, if it is not a common multiple, interference occurs in the signal light encoded with a code not to be decoded and its intensity fluctuates, and the signal light encoded with the code to be decoded has a signal light to interference light intensity ratio ( SIR) varies.

  The optical receiver according to the present invention can be used in an optical CDM optical communication system.

It is a block diagram of a communication network. It is a figure of the optical receiver of a form provided with two steps of interference light cancellers. It is a figure which shows the optical spectrum before and behind passing through an interference light removal device. It is a figure which shows the eye pattern after the case where there is no interference light and interference light is removed. Communication network without existing optical transmitters and optical receivers that provide services It is a figure explaining the standard of the minimum of the arbitrary difference between the elements of the set which consists of the optical path length difference of the interference light removal device in the case of a rectangular wave, and those sums. It is a figure explaining the standard of the minimum of the arbitrary difference between the elements of the set which consists of the optical path length difference of the interference light removal device in the case of a Gaussian pulse, and those sums.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Communication network 102,104 Optical branching device 105,110 Optical transmitter 106,111,111a, 111b Light source 107,112 Modulator 108 Encoder 115,120 Optical receiver 116,116a, 116b Interference light remover 117,121 Detection 118 Decoder a a portion where signal light output to the output route due to interference b overlap portion where interference light is removed from the output route due to interference Li optical path length difference Zs ratio of portions where signal light overlap Zi overlap of interference light Part ratio

Claims (6)

An interference light remover for removing the interference light included in the input light by interference through a plurality of optical paths having a predetermined optical path length difference and improving the intensity ratio of the signal light to the interference light is n (n is 2). A positive integer above.) In an optical receiver connected in stages,
i-th stage (i is n or less positive integer.) consisting of all combinations of the optical path length difference L _i of the interference light _stripper, the optical path length difference L _i and their sum of the interference light stripper of i-th stage Assume that the set U, any different elements u _i and u _{j of} the set U, and the distance that the signal light travels or the length lx that is the coherence length of the signal light during the time that the emission of the signal light continues. , The difference between any different elements u _i and u _j of the set U is equal to or longer than the distance lx that the signal light travels or the coherence length of the signal light during the time that the emission of the signal light continues, The optical receiver according to claim 1, wherein the set U satisfies Equation (1).

  2. The light according to claim 1, wherein at least one of the optical frequency width of one chip of the signal light and the optical frequency width of the interfering light is less than half of a free spectral range of the optical receiver. Receiver. A decoder that splits the input light and multiplexes it through a plurality of optical paths having a predetermined optical path length difference, is connected to the output of the interfering light remover;
Optical path length difference L _{n + 1} of the _decoder, the interference optical path length difference of the optical path length difference L _i and the decoder of the light stripper L _{n + 1} and consisting of all combinations of their union U ', the set U' any Are different elements u ′ _i , u ′ _j , the difference between any different elements u ′ _i and u ′ _j of the set U ′ is the distance traveled by the signal light during the time that the emission of the signal light continues or 3. The optical receiver according to claim 1, wherein a length lx which is a coherence length of the signal light is equal to or longer than the length lx, and the set U ′ satisfies Formula 2. 4.

  4. The interference light canceller according to claim 1, wherein the predetermined optical path length difference is not less than a coherence length of the signal light and not more than a coherence length of the interference light. The optical receiver described.   The interfering light remover is configured such that the predetermined optical path length difference is longer than a distance traveled by the signal light during a time during which the signal light emission continues, and the interfering light during a time during which the light emission continues. The optical receiver according to claim 1, wherein the optical receiver is shorter than a distance traveled by. The decoder decodes the signal light encoded with an optical frequency width that is a multiple or a divisor of a repetition period of the transmittance of the interference light remover;
The signal light that is included in the input light and encoded by the decoder with a code to be decoded or encoded with a code that is not to be decoded by the decoder and is canceled by addition and subtraction at the time of decoding. 4. The light according to claim 3, wherein at least one of the optical frequency widths is equal to or greater than a least common multiple of a repetition period of transmittance of the interfering light remover and an optical frequency width of one code of the decoder. Receiver.

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