JP5212203B2 - Optical receiver - Google Patents

Description

  The present invention relates to an optical receiver, and more particularly to an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal.

  In a next-generation long-distance large-capacity optical communication system that requires high speed and large capacity as communication traffic increases, the introduction of multilevel modulation / demodulation coding technology is being studied. One typical example is a differential quadrature phase shift keying (DQPSK modulation) system. In this method, compared to the conventional binary intensity modulation (OOK) method, the signal band is narrow, the frequency utilization efficiency can be improved and the transmission distance can be increased, and higher sensitivity can be expected.

First, quadrature phase shift keying (QPSK modulation, quadrature phase shift)
keying) method is “θ”, “θ + π / 2”, “θ + π”, and “θ + 3π” for each symbol “00”, “01”, “11”, and “10” composed of 2-bit data. / 2 "is assigned. Here, “θ” is an arbitrary phase. Then, the receiver reproduces the transmission data by detecting the phase of the received signal. As a means for realizing the QPSK modulation method relatively easily, there is a DQPSK modulation method. In the DQPSK modulation, the phase change amount of the carrier wave between the value of the symbol transmitted first and the value of the symbol transmitted next (“0 ”,“ Π / 2 ”,“ π ”, and“ 3π / 2 ”) are associated with 2 bits of transmission information. Therefore, the receiver can recover the transmission data by detecting the phase difference between two adjacent symbols.

  As shown in Patent Document 1 or 2, demodulating a DQPSK modulated optical signal requires two delay interferometers for generating an I (In-phase) signal and a Q (Quadrature) signal, and It is necessary to demodulate the phase difference with high accuracy. However, the optical path length in the delay interferometer changes due to the influence of the ambient temperature of the optical receiver and the phase is not stable, making it difficult to perform highly accurate demodulation. In addition, there is a problem that which interferometer cannot identify which signal component is demodulated. In addition, it is necessary to optimally adjust the optical path length difference between the branching of the optical signal and introduction into the two interferometers, and the difference in optical path length constituting each interferometer, and the control system becomes extremely complicated. Was produced.

  Patent Document 3 discloses that a DQPSK-modulated optical receiver is configured with a spatial optical system using a half mirror, as shown in FIG. The DQPSK-modulated optical signal A passes through the lens 100 and enters the bifurcated prism 101. One of the branched light waves a1 is further divided into two by the half mirror 102, and becomes a light wave a2 reflected by the first reflector 103 and a light wave a3 reflected by the second reflector. The light wave a3 is adjusted so that the phase is shifted by + π / 4 by passing through the phase adjusting unit 105.

  The light waves a2 and a3 are combined again by the half mirror 102 to form two combined light (a4, a5) and combined light (a6, a7), reflected by the mirror 105 or 106, and converged by the lens 110 or 111. Thus, the signal lights a8 and a9 become incident on the balanced photodetector 112.

  On the other hand, the other branched light wave b1 is further divided into two by the half mirror 102, and becomes a light wave b2 reflected by the first reflector 103 and a light wave b3 reflected by the second reflector. The light wave b <b> 3 is adjusted so that the phase is shifted by −π / 4 by passing through the phase adjustment unit 105.

  The light waves b2 and b3 are combined again by the half mirror 102 to form two combined light (b4, b5) and combined light (b6, b7), reflected by the mirror 105 or 106, and converged by the lens 107 or 108. Thus, the signal lights b8 and b9 are incident on the balanced photodetector 109.

  Thus, when a DQPSK-modulated optical receiver is assembled with a spatial optical system, the phase adjustment unit 105 needs to apply different phase shifts to the light waves a3 and b3, so that the configuration of the phase adjustment unit 105 is complicated. In addition, it is difficult to reduce the size of the device. Moreover, the optical components such as the half mirror 102, the two reflectors (103, 104), and the two mirrors (105, 106) need to be arranged with high precision, and their positional relationship is easily affected by temperature changes. There are problems such as complicated adjustment.

  On the other hand, in Patent Document 4, the present applicant has proposed an optical receiver that focuses on the plane of polarization as shown in FIG. Specifically, the DQPSK modulated light A is introduced into the 1-bit delay circuit through the polarization controller 1 in a state B in which the plane of polarization is aligned in one direction. The 1-bit delay circuit is composed of polarization-maintaining fiber couplers (21, 22, 24), and a half-wave plate 23 that rotates the plane of polarization by 90 degrees is disposed in one of the branched light paths. . As a result, the combined light wave is combined with two signal lights delayed by 1 bit from each other in a state in which the planes of polarization are orthogonal, as indicated by reference numeral C.

  Further, the signal light C is split into two through the half mirror 31 and the reflecting plates 32 to 34, and is polarized through the +45 degree quarter wavelength plate 41 and the -45 degree quarter wavelength plate, respectively. The means 51 and 52 further divide into two and enter the balanced light receiving elements 61 and 62 to generate the I component signal and the Q component signal.

  However, the polarization-maintaining fiber coupler is very sensitive to disturbances such as temperature changes and external stresses, and has a problem of poor stability as a device. In addition, since the half mirror is also affected by the polarization dependence, an error is likely to occur in the optical path length, and there is a high possibility that a time lag occurs between the I component signal and the Q component signal.

  Further, when a half mirror is used as a branching means in a 1-bit delay circuit as in Patent Document 3, a polarization-dependent half mirror is used on the branch side indicated by reference numeral 21 in FIG. On the multiplexing side indicated by 24, it is necessary to prepare half mirrors called polarization synthesis, which have different functions, and there is a problem that the apparatus becomes complicated.

JP 2006-295603 A JP 2007-158852 A JP 2007-151026 A Japanese Patent Application No. 2008-255528 (filed on September 30, 2008)

  The problem to be solved by the present invention is to solve the above-mentioned problems, in an optical receiver that demodulates a DQPSK-modulated optical signal into a multi-level phase-modulated signal, using a simple spatial optical system, An object of the present invention is to provide an optical receiver that is not easily affected by disturbances such as temperature changes.

In order to solve the above-mentioned problem, an invention according to claim 1 is directed to an optical receiver that demodulates a DQPSK-modulated optical signal into a multi-level phase-modulated signal, and converts the DQPSK-modulated optical signal into two different polarization planes. An optical system that branches into a light wave and generates a delay of one bit in one of the branched lights and combines them is configured using at least a polarization beam splitter. A quarter-wave plate for relatively fixing the phase relationship between the component signal and the Q component signal is provided, and a single light wave is incident on the polarization beam splitter via a polarization controller, and the polarization beam The light wave emitted from the splitter is branched into two, and one of the branched lights passes through the +45 degree ¼ wavelength plate, the 22.5 degree polarization plane rotating means, and the polarization separating means. It generates, the other branched light and generates a Q component signals through the polarization rotation means and the polarization separating means of the quarter-wave plate and 22.5 degrees -45 degrees.

The invention according to claim 2 is an optical receiver that demodulates a DQPSK-modulated optical signal into a multi-level phase-modulated signal, and branches the DQPSK-modulated optical signal into two optical waves having different polarization planes, An optical system for generating a delay of 1 bit in the split light and combining the two is configured using at least a polarization beam splitter, and an I component signal and a Q component signal for the light wave emitted from the polarization beam splitter A quarter-wave plate is provided to relatively fix the phase relationship between the two, a single light wave via the polarization controller is separated into two by the polarization separation means, and passes through the 45-degree polarization plane rotation means After that, one of the two light waves incident on the polarization beam splitter and emitted from the polarization beam splitter is rotated by +45 degrees of the quarter wavelength plate and 22.5 degrees of polarization plane rotation. The I component signal is generated through the stage and the polarization separation means, and the other light wave is converted to the Q component signal through the quarter wave plate of −45 degrees, the polarization plane rotation means and the polarization separation means of 22.5 degrees. It is characterized by generating.

The invention according to claim 3 is an optical receiver that demodulates a DQPSK-modulated optical signal into a multi-level phase-modulated signal, and branches the DQPSK-modulated optical signal into two light waves having different polarization planes, An optical system for generating a delay of 1 bit in the split light and combining the two is configured using at least a polarization beam splitter, and an I component signal and a Q component signal for the light wave emitted from the polarization beam splitter A quarter wave plate for relatively fixing the phase relationship is provided, and a single light wave is incident on the polarization beam splitter via the polarization controller, and the light wave emitted from the polarization beam splitter is − After passing through the quarter-wave plate of 45 degrees, it is branched into two, and one of the branched lights generates an I component signal through a polarization plane rotating means and a polarization separating means of 22.5 degrees. The other branched light and generates a Q component signals through the polarization rotation means and the polarization separating means of the half-wave plate and 22.5 degrees.

According to a fourth aspect of the present invention, in the optical receiver according to any one of the first to third aspects, a signal based on the detected light intensity is detected by detecting a light intensity of a part of the light wave emitted from the polarization beam splitter. Thus, the position of the reflecting means for generating the delay of 1 bit is adjusted in cooperation with the polarizing beam splitter.

According to the first to third aspects of the present invention, in an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal, the DQPSK-modulated optical signal is branched into two optical waves having different polarization planes. An optical system that generates a delay of one bit in one branched light and combines them is configured using at least a polarization beam splitter, and an I component signal and a Q component with respect to the light wave emitted from the polarization beam splitter Since a quarter wave plate for relatively fixing the phase relationship with the signal is provided, polarization separation / combination can be realized with a simple spatial optical system using a polarization beam splitter. By combining the / 4 wavelength plate, the I component signal and the Q component signal can be accurately separated. Furthermore, since there are fewer adjustment points compared to conventional spatial optical systems, such as a 1-bit delay circuit using a polarizing beam splitter, it is necessary to adjust the optical components. It becomes possible to improve the stability with respect to.

Furthermore , according to the first aspect of the present invention, a single light wave is incident on the polarization beam splitter via the polarization controller, and the light wave emitted from the polarization beam splitter is branched into two. The I component signal is generated through the +45 degree quarter wave plate and the 22.5 degree polarization plane rotating means and the polarization separating means, and the other split light is -45 degree quarter wave plate and 22 Since the Q component signal is generated through the polarization plane rotation means and the polarization separation means of 5 degrees, a DQPSK-modulated optical receiver can be realized with a simple spatial optical system.

Further, according to the invention according to claim 2 , the single light wave via the polarization controller is separated into two by the polarization separation means, and after passing through the polarization plane rotation means of 45 degrees, is made incident on the polarization beam splitter. Of the two light waves emitted from the polarization beam splitter, one of the light waves generates an I component signal through the +45 degree quarter wave plate, the 22.5 degree polarization plane rotating means, and the polarization separating means. The other light wave generates a Q component signal through the -45 degree ¼ wavelength plate, 22.5 degree polarization plane rotating means and polarization separating means, so that DQPSK modulated light can be obtained with a simple spatial optical system. A receiver can be realized.

Furthermore, according to the invention of claim 3 , a single light wave is incident on the polarization beam splitter via the polarization controller, and the light wave emitted from the polarization beam splitter passes through the quarter wave plate of −45 degrees. After being split, the light is split into two, one split light passes through a 22.5 degree polarization plane rotating means and a polarization splitting means to generate an I component signal, and the other split light is a half-wave plate and 22.5 Since the Q component signal is generated through the polarization plane rotating means and the polarization separating means, a DQPSK-modulated optical receiver can be realized with a simple spatial optical system.

According to the invention of claim 4 , the light intensity of a part of the light wave emitted from the polarization beam splitter is detected, and the signal based on the detected light intensity cooperates with the polarization beam splitter to delay the one bit. Therefore, it is possible to provide a DQPSK-modulated optical receiver that can always maintain a 1-bit delay circuit in an appropriate state and is not easily affected by disturbance such as a temperature change. Become.

It is a figure which shows the optical receiver of the DQPSK modulation comprised with the conventional spatial optical system. It is a figure which shows the optical receiver of the DQPSK modulation using the polarization plane which the present applicant presented by the previous application. It is a figure which shows the 1st Example which concerns on the optical receiver of this invention. It is a figure which shows the 2nd Example which concerns on the optical receiver of this invention. It is a figure which shows the 3rd Example which concerns on the optical receiver of this invention. It is a figure which shows the example which provides an optical path length adjustment means in the delay circuit for 1 bit in the optical receiver of this invention.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIG. 3, in the optical receiver that demodulates the DQPSK modulated optical signal A into a multi-level phase modulated signal, the DQPSK modulated optical signal is branched into two optical waves having different polarization planes. At the same time, an optical system that generates a delay of one bit in one of the branched lights and combines them is configured by using at least the polarization beam splitter 2, and I (In -phase) component signals and Q (Quadrature) component signals are provided with quarter-wave plates (8, 9) for relatively fixing the phase relationship.

  The optical receiver of the present invention is characterized in that an interferometer necessary for demodulating the I component signal and the Q component signal is composed of only one interference means using the polarization beam splitter 2 as shown in FIG. It is to be done. By using this polarization beam splitter, (1) the DQPSK-modulated optical signal can be branched into two light waves having two orthogonal polarization planes, and (2) the two branched lights can be split while maintaining the polarization plane relationship. The function of combining can be realized.

  In addition, by combining the polarization beam splitter and the quarter wave plate, it is possible to accurately separate the I component signal and the Q component signal. A highly accurate I component signal and Q component signal can be obtained.

The first embodiment of the optical receiver of the present invention shown in FIG. 3 will be described in detail.
The polarization plane of the optical signal A is adjusted by the polarization controller 1 and is incident on the polarization beam splitter 2. In the polarization beam splitter, the signal light is branched into two light waves, and the two branched lights have polarization planes orthogonal to each other. The polarization plane of the light wave emitted from the polarization controller 1 is set so that the two light waves branched by the polarization beam splitter 2 have substantially the same intensity.

  One branched light is directed to the prism mirror 3, reflected by the prism mirror 3 as shown in FIG. 3, and then enters the polarization beam splitter 2 again. The other branched light is directed to the prism mirror 4, is reflected by the prism mirror 4, and is incident on the polarization beam splitter 2 again. The optical path length difference between the two branched lights is set so as to produce a phase difference of 1 bit. In particular, the position of the prism mirror 4 is fixed with respect to the polarizing beam splitter 2, and the position of the prism mirror 3 is set so as to be movable so as to be close to or away from the polarizing beam splitter. Various mechanisms for moving the prism mirror 3 can be used. However, when the position is finely adjusted, the piezoelectric element 5 or the like can be preferably used.

  The light waves reflected from the prism mirrors 3 and 4 and re-entering the polarization beam splitter 2 have a phase difference of 1 bit and are combined while maintaining the orthogonal polarization plane relationship, and the polarization beam splitter 2 Exits from.

The light wave emitted from the polarization beam splitter is branched into two by the half mirror 6, and one of the branched lights is a +45 degree quarter wave plate 8, a 22.5 degree polarization plane rotating means 10, a calcite or other polarized light. is separated into signal light ^- through the wave separating means 11, I ⁺ signal light for generating I-component signal and I. The I ⁺ signal light and the I ⁻ signal light are incident on a light receiving element (not shown) such as a balanced light receiving element to generate an I component signal.

The other branched light is reflected by the mirror 7 and passes through the −45 degree quarter wave plate 9 and the 22.5 degree polarization plane rotating means 10 and polarization separating means 11 to generate a Q component signal. Q ⁺ signal light and Q ^- is separated into signal light. Similar to the I component signal, the Q ⁺ signal light and the Q ⁻ signal light generate a Q component signal by a light receiving element (not shown).

  Note that the Q component signal light is propagated in excess of the optical path length between the half mirror 6 and the mirror 7 as compared with the I component signal light. This optical path length difference may cause a phase difference between the I component signal and the Q component signal. In order to eliminate this, a half mirror 31 and reflectors 32 to 34 as shown in FIG. 2 are used. It is also possible to match the optical path lengths of both. In addition, it is also possible to suppress generation | occurrence | production of the phase difference peculiar to a half mirror by using birefringent members, such as a polarization beam splitter and a calcite, instead of a half mirror.

Next, a second embodiment of the optical receiver of the present invention will be described with reference to FIG.
In FIG. 4, the signal light A passes through the polarization controller 1 and then is separated into two light waves by the polarization separation means 12. Each light wave passes through the 45-degree polarization plane rotating means 13 and then enters the polarization beam splitter 2, and, as in the first embodiment, the polarization beam splitter 2 and the prism mirrors 3 and 4 are equivalent to one bit. It is converted into light waves having a phase difference and orthogonal polarization planes. Reference numeral 5 denotes a piezoelectric element for adjusting the position of the prism mirror 3. In order to adjust the position, it is possible to change the optical path length by interposing a polymer, silicon or the like in the optical path.

After two light waves having a phase difference of 1 bit and whose polarization planes are orthogonal to each other are generated, one of the light waves is a +45 degree quarter wave plate 8 and a 22.5 degree polarization plane rotating means 10 and The light is separated into I ⁺ signal light and I ⁻ signal light for generating an I component signal through the polarization separation means 11.

The other light wave emitted from the polarization beam splitter 2 passes through a -45 degree quarter wave plate 9 and a 22.5 degree polarization plane rotating means 10 and polarization separating means 11 to generate a Q component signal. Q ⁺ signal light and Q ^- is separated into signal light.

  In the second embodiment, since no means for branching light waves such as a half mirror is used, there is no influence of the polarization dependency of the half mirror, and an error in the optical path length hardly occurs, and the I component signal and the Q component signal. It is possible to suppress the occurrence of a time lag between the two.

Next, a third embodiment of the optical receiver of the present invention will be described with reference to FIG.
FIG. 5 shows an application example of the first embodiment of FIG. 3, and both have the same configuration until they exit the polarization beam splitter 2. In FIG. 5, the light waves emitted from the polarization beam splitter 2 and having a phase difference of 1 bit and whose planes of polarization are orthogonal to each other pass through the −45 degree quarter-wave plate 14, and then are transmitted by the half mirror 15. One of the branched lights is split into I ⁺ signal light and I ⁻ signal light for generating an I component signal via a polarization plane rotating means and a polarization separating means of 22.5 degrees. .

The other branched light passes through the half mirror 15 and the mirror 16, and further passes through the half-wave plate 17 and the polarization plane rotating means 10 and the polarization separating means 11 of 22.5 degrees to generate a Q component signal. Q ⁺ signal light and Q ^- is separated into signal light.

  Since the quarter-wave plate 14 is more expensive than the half-wave plate 17, in the third embodiment, one quarter-wave plate is used to reduce the manufacturing cost.

Next, a method for more accurately adjusting the delay circuit for 1 bit constituted by the polarization beam splitter 2 and the prism mirrors 3 and 4 will be described with reference to FIG.
The basic configuration of FIG. 6 is substantially the same as the configuration of FIG. 4 showing the second embodiment. A part of the light wave emitted from the polarization beam splitter 2 is extracted by the half mirrors 71 and 72, synthesized by the mirror 73 and the half mirror 74, and the light intensity is detected by the light receiving element 75. The dither circuit 77 monitors the change in light intensity from the light receiving element 75 while changing the displacement of the prism mirror 3, and the controller 76 determines the optimum position where a delay of 1 bit can be realized. For example, the optimal position can be determined by setting the prism mirror 3 so that the light intensity becomes the maximum light amount.

  As described above, according to the present invention, in an optical receiver that demodulates a DQPSK-modulated optical signal into a multilevel phase-modulated signal, a simple spatial optical system is used to influence the influence of disturbance such as a temperature change. It is possible to provide an optical receiver that is not easily affected.

DESCRIPTION OF SYMBOLS 1 Polarization controller 2 Polarization beam splitters 3, 4 Prism mirror 8, 9, 15 1/4 wavelength plate 10 22.5 degree polarization rotation means 11, 12 Polarization separation means 13 45 degree polarization rotation means

Claims (4)

In an optical receiver that demodulates a DQPSK modulated optical signal into a multi-level phase modulated signal,
An optical system that divides a DQPSK-modulated optical signal into two light waves with different polarization planes, generates a delay of one bit in one of the branched lights, and multiplexes them is configured using at least a polarization beam splitter And
A quarter wavelength plate is provided for relatively fixing the phase relationship between the I component signal and the Q component signal with respect to the light wave emitted from the polarization beam splitter,
A single light wave is incident on the polarization beam splitter via a polarization controller, and the light wave emitted from the polarization beam splitter is branched into two, and one branched light has the 1/4 wavelength of +45 degrees. The I component signal is generated through the plate, the polarization plane rotating means and the polarization separating means of 22.5 degrees, and the other branched light is the -45 degree quarter wave plate and the 22.5 degrees polarization plane rotating means. And an optical receiver that generates a Q component signal through the polarization separation means. In an optical receiver that demodulates a DQPSK modulated optical signal into a multi-level phase modulated signal,
An optical system that divides a DQPSK-modulated optical signal into two light waves with different polarization planes, generates a delay of one bit in one of the branched lights, and multiplexes them is configured using at least a polarization beam splitter And
A quarter wavelength plate is provided for relatively fixing the phase relationship between the I component signal and the Q component signal with respect to the light wave emitted from the polarization beam splitter,
A single light wave via a polarization controller is separated into two by a polarization separation means, and after passing through a 45-degree polarization plane rotation means, is incident on the polarization beam splitter and is emitted from the polarization beam splitter 2 Of the two light waves, one light wave generates an I component signal through the +45 degree quarter wave plate, 22.5 degree polarization plane rotating means and polarization separating means, and the other light wave is -45 degree. An optical receiver characterized in that a Q component signal is generated through the ¼ wavelength plate, a 22.5 degree polarization plane rotating means and a polarization separating means. In an optical receiver that demodulates a DQPSK modulated optical signal into a multi-level phase modulated signal,
An optical system that divides a DQPSK-modulated optical signal into two light waves with different polarization planes, generates a delay of one bit in one of the branched lights, and multiplexes them is configured using at least a polarization beam splitter And
A quarter wavelength plate is provided for relatively fixing the phase relationship between the I component signal and the Q component signal with respect to the light wave emitted from the polarization beam splitter,
A single light wave is incident on the polarization beam splitter via a polarization controller, and the light wave emitted from the polarization beam splitter is branched into two after passing through the quarter wave plate of −45 degrees. One branched light passes through a 22.5 degree polarization plane rotating means and a polarization separating means to generate an I component signal, and the other branched light is a half-wave plate and a 22.5 degree polarization plane rotating means and a polarized light. An optical receiver characterized in that a Q component signal is generated through wave separation means. In the optical receiver according to any one of claims 1 to 3, detecting the portion of light intensities of light waves emitted from the polarization beam splitter, the signal based on the light intensity the detected, polarization beam splitters and co An optical receiver characterized in that the position of the reflecting means that operates to generate a delay of one bit is adjusted.

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