JP4439244B2 - Optical communication system and method - Google Patents

Description

  The present invention relates to an optical communication system and method applied to wavelength division multiplexing communication, and more particularly to an optical communication system and method suitable for mobile communication performing code division multiple access.

  In an optical communication system capable of transmitting a large amount of data information, a higher-speed and higher-density information transmission technology has been conventionally desired. Among them, wavelength division multiplexing (WDM: Wavelength Division Multiplexing) and DWDM (Dense Wavelength Division Multiplexing), which effectively use a wide low-loss wavelength region of an optical fiber, are attracting attention as a communication method that can greatly increase transmission capacity. ing. In this wavelength multiplexing communication system, a large number of lights having different wavelengths are multiplexed by a wavelength synthesizer, coupled to a single optical fiber, transmitted over a long distance, and then separated for each wavelength by a wavelength demultiplexer to extract a signal. It is. In other words, by multiplexing light of different wavelengths of n waves, a transmission capacity n times the transmission capacity of each wavelength can be obtained.

  In recent years, a mobile communication system that performs code division multiple access based on the WDM communication system as described above has also been proposed (see, for example, Non-Patent Document 1).

  As shown in FIG. 14, the mobile communication system 7 performs communication by transmitting and receiving wireless signals between a mobile communication device 71 as a mobile terminal that can be carried by a pedestrian and a mobile communication device 71. A plurality of base stations 72a, 72b, 72c for relaying, and a host control device 73 for controlling communication in the entire network including the base stations 72 via a connected optical fiber communication network 74 are provided.

  The mobile communication device 71 is a mobile phone or PHS (Personal Handyphone System) for performing a voice call, and is mounted on a vehicle or mobile phone to transmit and receive radio signals to and from the base station 72 provided in each area. It is configured to be able to.

  As shown in FIG. 15, each base station 72 is equipped with an optical modulator 81 that modulates the light intensity of the laser light propagating in the waveguide by changing the refractive index of the crystal by an external electric field. . The optical modulator 81 is formed with an electrode (not shown) so that the direction of the modulation electric field is perpendicular to the propagation direction of the laser light. Is connected to an antenna 82. The optical modulator 81 modulates the laser beam transmitted from the host controller 73 via the optical fiber communication network 74 according to the radio signal received via the antenna 82. Then, the modulated laser light is transmitted again to the host controller 73 via the optical fiber communication network 74.

  As shown in FIG. 15, the host control device 73 photoelectrically converts the laser light generating unit 86 that generates laser light to be transmitted to the base station 72 and the laser light that is intensity-modulated in the base station 72, and outputs a detection output. And a light detection unit 87 for obtaining the detection output, and the detection outputs from the base stations 72a, 72b, 72c can be collectively managed.

  In such a mobile communication system 7, the laser beam generated from the laser beam generator 86 in the host controller 73 is transmitted to each base station 72 a, 72 b, 72 c via the optical fiber communication network 74. Each base station 72 propagates the transmitted laser light into the waveguide in the optical modulator 81 and transmits it to the host controller 73 again. Here, when a call is made by a pedestrian via the mobile communication device 71, if the mobile communication device 71 is in the vicinity of the base station 72a, a radio signal is transmitted to the base station 72a. The optical modulator 81 in the base station 72a receives the wireless signal via the antenna 82 and supplies it to the electrode (not shown) described above. A modulation electric field is applied to the laser light propagating through the waveguide in the optical modulator 81 in accordance with a radio signal supplied to an electrode (not shown), and desired light intensity modulation is performed.

  That is, when the laser beam transmitted to the base station 72 is called from the mobile communication device 71 in the vicinity of the base station 72, the light intensity is modulated according to the content of the call included in the radio signal described above. Will be. On the other hand, the laser beam transmitted to the base station 72 is not subjected to the above-described phase modulation when it is not called from the mobile communication device 71 around the base station 72. In the host control device 73, when the light intensity modulation is applied to the laser light transmitted from the base station 72 via the optical fiber communication network 74, this is photoelectrically converted to detect the signal according to the content of the call. An output can be obtained.

  In the mobile communication system 7 having such a configuration, the base station 72 and the host controller 73 can be connected by an optical fiber, so that a low-loss and high-speed system can be constructed. Further, since it is not necessary to provide each component such as the laser light generator 86 and the light detector 87 for each base station 72, a small and simple system can be constructed.

  In the mobile communication system 7 as described above, a Mach-Zehnder optical modulator (hereinafter referred to as an MZ optical modulator) is used as the optical modulator 81 mounted on the base station 72, and Fabry-Perot resonance. In some cases, a Fabry-Perot type optical modulator (hereinafter referred to as an FP optical modulator) to which an optical device is applied is used.

  The MZ optical modulator, which has a low modulation efficiency compared to the FP optical modulator, receives a signal with high sensitivity at the base station 72, in other words, receives a signal with a high S / N ratio. In order to ensure, a microwave amplifier had to be provided before and after the MZ optical modulator. For this reason, there is a problem that the cost required for power distribution and the cost required for maintenance such as battery replacement are excessive, and because the structure is not insulated from the outside, the risk of lightning strikes is avoided. There was also a problem that it was not possible.

  On the other hand, the FP optical modulator has a much steeper reflectance / voltage characteristic as compared with the MZ optical modulator, as shown in FIGS. For this reason, the amplitude of the laser beam modulated in accordance with the input signal can be increased, and the modulation efficiency can be greatly improved, so that a more sensitive operation can be realized.

  That is, the FP optical modulator can be driven at a lower voltage than the MZ optical modulator, and it is not necessary to provide a microwave amplifier before and after the FP optical modulator. It can be greatly reduced. Furthermore, since this FP light modulator is configured to be completely insulated from the outside, it is possible to improve maintainability against lightning strikes.

A Study of Base Station without Electrical Power Supply using an Optically-Resonant Modulator

  Incidentally, in order to perform optical modulation in such an FP optical modulator, the DC component of the voltage applied to the FP optical modulator is set to the operating voltage shown in FIG. It is necessary to control the frequency of the laser light to be controlled so that the DC component of the operating voltage applied to the FP optical modulator becomes 0V.

  FIG. 17 shows the change in transmittance when the DC component of the voltage applied to the FP optical modulator is 0 V and the frequency of the laser light is changed. The frequency of the laser beam output from the laser beam generator 86 is controlled so as to come to the transmittance / laser beam frequency slope position shown in FIG. The host control device 73 detects the intensity of the laser light transmitted through the FP light modulator via the light detection unit 87, and controls the frequency of the laser light output from the laser light generation unit 86 based on this. That is, a signal obtained by removing the bias component from the DC component is detected, and the frequency of the laser beam is feedback-controlled so that the signal becomes zero.

  However, even in the case where the FP optical modulator as described above is applied as the optical modulator 81 in the mobile communication system 7, the optical fiber disposed in the optical fiber communication network 74 is deteriorated and the polarization direction is rotated. Furthermore, the intensity of the laser beam fluctuates due to the deterioration of the laser light source itself in the laser beam generator 86. As a result, the lock point for locking the DC component of the voltage applied to the FP light modulator to 0V varies. As a result, there arises a problem that the detection sensitivity cannot be controlled to be constant.

  Further, in order to obtain the maximum modulation of the FP optical modulator, it is necessary to control the laser light frequency to the resonance frequency by locking the maximum of the transmittance / laser light frequency shown in FIG. In such a case, the sideband component due to modulation can be maximized.

  However, when the FP optical modulator as described above is applied as the optical modulator 81, there is a problem that this cannot be realized. Even if the transmittance / laser light frequency can be locked to the maximum, the voltage differential of the transmittance / voltage characteristic is 0, and detection based on light intensity modulation cannot be performed. It was.

  Further, the modulation characteristic of the FP optical modulator as the optical modulator 71 depends on the polarization direction of the laser light transmitted from the host control device 73, the temperature environment, etc., but the light disposed in the optical fiber communication network 74. Since the fiber is not necessarily a polarization-maintaining fiber, and the base station 72 is not necessarily installed in a constant temperature environment, there are some restrictions in realizing high-precision modulation that eliminates these effects. There is also the problem that it takes.

  Furthermore, the sensitivity of detection on the host controller 73 side varies depending on the phase noise and intensity noise of the propagated laser beam, and there is a problem that a desired S / N ratio cannot be obtained.

  Accordingly, the present invention has been devised in view of the above-described problems, and an object of the present invention relates to an optical communication system and method applied to wavelength division multiplexing communication, without depending on a polarization state or a temperature environment. Another object of the present invention is to provide an optical communication system and method capable of obtaining higher signal detection sensitivity.

In the present invention, in order to solve the above-described problems, the laser light transmitted from the host control device to the base station via the optical fiber communication network is modulated based on the electric signal fm from the portable communication device, and this is modulated. When transmitting to the control device and detecting, the generated laser light is modulated in accordance with the modulation signal of frequency fm2 and transmitted to the base station, and the laser light transmitted from the base station is photoelectrically converted to be electrically A signal is generated, the interference signal | fm2-fm | or the interference signal | fm2 + fm | is detected for detection from the generated electrical signal, and the generated laser light is generated according to the component of the frequency fm2 detected from the electrical signal. Control.

That is, the present invention modulates the laser light transmitted from the host control device to the base station via the optical fiber communication network based on the signal of the frequency fm supplied from the portable communication device, and sends this to the host control device. In the optical communication system for transmitting and detecting, the base station has an optical modulator comprising a Fabry-Perot resonator that modulates the transmitted laser light based on an electric signal fm from a portable communication device, and the host control The apparatus includes: laser light generating means for generating laser light; oscillation means for oscillating a modulation signal having a frequency fm2 determined based on a reflectance characteristic of the optical modulator in the base station; and generated in the laser light generating means. Modulation means for modulating the received laser light in accordance with the modulation signal supplied from the oscillation means and transmitting the modulated signal to the base station; and the laser signal transmitted from the base station. Photoelectric conversion means for generating an electrical signal to laser light by photoelectrically converting the interference signal from the electrical signal generated by the photoelectric conversion means | fm2-fm | a detection means which detects a frequency fm2 detected from the electric signal Laser light control means for controlling the laser light generation means in accordance with the component of the light source, and the oscillation means sets the frequency fm2 of the modulation signal to a frequency difference corresponding to the minimum value of the reflectance in the optical modulator. Based on FSR, the following formula
fm2 = FSR (N + 0.5)
(N is an integer)
It is characterized by determining by .
The present invention also modulates the laser beam transmitted from the host control device to the base station via the optical fiber communication network based on the signal of the frequency fm supplied from the portable communication device, and sends this to the host control device. In the optical communication system for transmitting and detecting, the base station has an optical modulator comprising a Fabry-Perot resonator that modulates the transmitted laser light based on an electric signal fm from a portable communication device, and the host control The apparatus includes: laser light generating means for generating laser light; oscillation means for oscillating a modulation signal having a frequency fm2 determined based on a reflectance characteristic of the optical modulator in the base station; and generated in the laser light generating means. Modulation means for modulating the received laser light in accordance with the modulation signal supplied from the oscillation means and transmitting the modulated signal to the base station; and the laser signal transmitted from the base station. Photoelectric conversion means for photoelectrically converting the light to generate an electric signal, detection means for detecting an interference signal | fm2 + fm | from the electric signal generated by the photoelectric conversion means, and a component of the frequency fm2 detected from the electric signal Laser light control means for controlling the laser light generation means in response to the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator. Based on the following formula
fm2 = FSR (N + 0.5)
(N is an integer)
It is characterized by determining by.

The present invention also provides an optical modulator comprising a Fabry-Perot resonator in the base station based on an electric signal fm from a portable communication device for transmitting a laser beam transmitted from a host controller to a base station via an optical fiber communication network. In the optical communication method of modulating and transmitting the detected signal to the host control device, the host device, based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, formula
fm2 = FSR (N + 0.5)
(N is an integer)
Is modulated in accordance with the modulation signal having the frequency fm2 determined by the above and transmitted to the base station, and the laser light transmitted from the base station is photoelectrically converted to generate an electrical signal. From the generated electrical signal, interference signal | fm2-fm | were detected for detection, further characterized by controlling the laser beam to generate the in accordance with the component of the frequency fm2 detected from the electric signal.
Furthermore, the present invention provides an optical modulator comprising a Fabry-Perot resonator in the base station based on an electric signal fm from a portable communication device for transmitting laser light from a host control device to a base station via an optical fiber communication network. In the optical communication method of modulating and transmitting the detected signal to the host control device, the host device, based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, formula
fm2 = FSR (N + 0.5)
(N is an integer)
Is modulated in accordance with the modulation signal having the frequency fm2 determined by the above and transmitted to the base station, and the laser light transmitted from the base station is photoelectrically converted to generate an electrical signal. From the generated electrical signal, The interference signal | fm2 + fm | is detected for detection, and the generated laser light is controlled in accordance with the component of the frequency fm2 detected from the electric signal.

In the present invention, the laser light transmitted from the host controller to the base station via the optical fiber communication network is modulated by the optical modulator comprising a Fabry-Perot resonator in the base station based on the electric signal fm from the portable communication device. Then, when this is transmitted to the host controller and detected, the generated laser light is based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, and the following formula
fm2 = FSR (N + 0.5)
(N is an integer)
The signal is modulated in accordance with the modulation signal having the frequency fm2 determined by the above and transmitted to the base station, and the laser light transmitted from the base station is photoelectrically converted to generate an electrical signal, and the interference signal is generated from the generated electrical signal. | Fm2-fm | or the interference signal | fm2 + fm | is detected for detection, and the generated laser light is controlled according to the component of the frequency fm2 detected from the electric signal.

  As a result, even if the intensity of the laser beam fluctuates due to deterioration in the optical fiber disposed in the optical fiber communication network, rotation of the polarization direction, and further deterioration of the laser light source itself in the laser light generation unit, detection is possible. Sensitivity can be controlled constant.

  Hereinafter, as a best mode for carrying out the present invention, a case where the mobile communication system 1 shown in FIG. 1 is applied will be described as an example.

  The mobile communication system 1 is applied as, for example, a system that performs code division multiple access based on a WDM communication system, and as shown in FIG. 1, a mobile communication device 11 as a mobile terminal that can be carried by a pedestrian, Including a plurality of base stations 12a, 12b, 12c for relaying communication by transmitting and receiving radio signals to and from the portable communication device 11, and each base station 12 via a connected optical fiber communication network 14 And a host control device 13 for controlling communication in the entire network.

  The portable communication device 11 is configured to be mounted on the vehicle or portable so as to transmit and receive radio signals to and from the base station 12 provided in each district. That is, the mobile communication device 11 includes, for example, a device that is mounted on a facsimile communication, a personal computer, or the like and performs data communication. In general, however, the mobile communication device 11 is a mobile phone or PHS (personal handyphone system) for performing voice communication. ), Etc., and is configured as a device that is particularly compact and lightweight and specialized in portability.

As shown in FIG. 2, each base station 12 is equipped with an optical modulator 21 that modulates the phase of laser light propagating in the medium by changing the refractive index of the crystal by an external electric field. The optical modulator 21 has the maximum modulation efficiency when fm is an integral multiple of fSR. This optical modulator 21 is a so-called Fabry-Perot type optical modulator to which a Fabry-Perot resonator is applied, and is a dielectric in order to resonate laser light transmitted from the host controller 13 via the optical fiber communication network 14. The first mirror 31 and the second mirror 32 composed of a multilayered body film and the like, and a bulk crystal such as lithium niobate (LiNbO ₃ ), for example, phase-modulate the light passing through the supplied electric signal An optical phase modulation unit 33 and electrodes 34 formed on the top and bottom surfaces of the optical phase modulation unit 33 are provided so that the direction of the modulation electric field is perpendicular to the light propagation direction. An antenna 4 for transmitting / receiving a radio signal to / from the mobile communication device 11 is connected to the electrode 34.

  In the optical modulator 21 having such a configuration, a part of the laser light transmitted from the host control device 13 via the optical fiber communication network 14 is incident from the first mirror 31. The incident laser light resonates by the first mirror 31 and the second mirror 32 disposed substantially in parallel, and a modulation electric field is applied in accordance with a radio signal supplied to the electrode 34. The phase modulation is performed.

  The host control device 13 generates laser light to be transmitted to the base station 12 and photoelectrically converts the laser light modulated in the base station 12 to obtain a detection output. That is, the host control device 13 can collectively manage detection outputs from the base stations 12a, 12b, and 12c. Details of the configuration of the host control device 13 will be described later.

  In such a mobile communication system 1, the laser beam output from the host control device 13 is transmitted to each base station 12 a, 12 b, 12 c via the optical fiber communication network 14. Each base station 12 propagates the transmitted laser light through the optical phase modulation unit 33 in the optical modulator 21, further resonates, and then reflects the laser light (reflected light) reflected from the first mirror 31. The data is transmitted to the host control device 13.

  Here, when a call is made by a pedestrian via the mobile communication device 11, if the mobile communication device 11 is in the vicinity of the base station 12a, a radio signal having a frequency fm is transmitted to the base station 12a. Become. The optical modulator 11 in the base station 12 a receives such a radio signal via the antenna 4 and supplies it to the electrode 34. A modulation electric field is applied to the laser light propagating through the optical phase modulation unit 33 in the optical modulator 21 in accordance with a radio signal having a frequency fm supplied to the electrode 34, and desired phase modulation is performed. .

  That is, when the laser beam transmitted to the base station 12 is called from the mobile communication device 11 around the base station 12, the phase modulation is performed according to the content of the call included in the above-described radio signal. It will be. On the other hand, the laser beam transmitted to the base station 12 is not subjected to the above-described phase modulation when it is not called from the mobile communication device 11 around the base station 12. In the host control device 13, when the phase modulation is applied to the laser light transmitted from the base station 12 via the optical fiber communication network 14, the detection output corresponding to the content of the call is obtained by photoelectrically converting the laser light. Can be obtained.

  Next, the configuration of the host control device 13 will be described in detail with reference to FIG.

  The host controller 13 includes a laser light generator 41 that generates laser light for transmission to the base station 12, a phase modulator 42 that performs phase modulation on the laser light emitted from the laser light generator 41, and phase modulation. The optical circulator 43 disposed in the optical path of the laser light that is phase-modulated and output by the detector 42, the light detection unit 44 that photoelectrically converts the laser light modulated in the base station 12, and the photoelectric conversion in the light detection unit 44 First band filter (first BPF) 45 and second band filter (second BPF) 46 for detecting each frequency component from the generated electrical signal, and an oscillator for oscillating a modulation signal of a predetermined frequency 48, a phase adjustment unit 49 that adjusts the phase of the modulation signal output from the oscillator 48 in accordance with the phase modulation that the phase modulator 42 should apply to the laser beam, and a first A double balanced modulator (DBM) 50 that multiplies the electrical signal of the frequency component detected by the PF 45 and the modulation signal from the oscillator 48, and controls the laser light generator 41 according to the control signal output from the DBM 50. And a control amplifier 51.

  The laser light generator 41 is a light source for generating laser light having a predetermined frequency under the control of the control amplifier 51. For example, a solid-state laser such as Nd: YAG, a semiconductor laser such as GaAs, or a gas laser such as ArF. And a light source that emits light such as an LED or a xenon lamp. Incidentally, when a semiconductor laser or the like is used as the laser light generator 41, the configuration of the phase modulator 43 may be omitted by performing phase modulation by current modulation applied to the semiconductor laser.

  In the phase modulator 42, phase modulation can be applied to the laser light by drivingly inputting a modulation signal synchronized with the time when the laser light propagates through the phase modulator 42 from the phase adjustment unit 49. Thereby, the sideband centering on the frequency of a laser beam is generable. Further, the frequency interval between adjacent sidebands is equivalent to the frequency fm2 of the input modulation signal. In order to set the frequency of the modulation signal to fm2 according to the frequency interval, the oscillator 48 generates a modulation signal of the frequency fm2, and the phase adjustment unit 49 adjusts the phase of the electric signal of the frequency fm2.

  The optical circulator 43 transmits the laser light modulated by the phase modulator 42 and guides it to each base station 12, and reflects the laser light returning from each base station 12 and guides it to the light detection unit 44. Incidentally, the optical circulator 43 may be replaced with a non-polarizing beam splitter, a polarizing beam splitter, or the like.

  The light detection unit 44 is a light receiving element that reflects the optical circulator 43 and further receives and photoelectrically converts a laser beam adjusted for the size of the beam spot by a cylindrical lens (not shown).

  The first BPF 45 detects only the component of the frequency fm2 oscillated in the oscillator 48 from the electric signal generated in the light detection unit 44 and outputs this to the DBM 50. The second BPF 46 detects only the component of the frequency | fm2 ± fm | from the electric signal generated in the light detection unit 44, and uses this as the detection output.

  The DBM 50 multiplies the electrical signal having the frequency fm2 detected by the first BPF 45 by the modulation signal having the frequency fm2 transmitted via the oscillator 48, thereby allowing the laser light generated from the laser light generating unit 41 to be amplified. A control signal from which the phase change component is extracted is generated. The control signal generated in the DBM 50 is transmitted to the control amplifier 51.

  The control amplifier 51 controls the frequency of the laser beam output from the laser beam generator 41 based on the control signal transmitted from the DBM 50.

  As a result, even if the intensity of the laser beam fluctuates due to deterioration in the optical fiber disposed in the optical fiber communication network, rotation of the polarization direction, and further deterioration of the laser light source itself in the laser light generation unit, Since only the change component can be taken out and it is difficult to be affected by the change in light intensity, the detection sensitivity can be controlled to be constant.

Further, in the mobile communication system 1 to which the present invention is applied, the laser light output from the laser light generating unit 41 is controlled based on the bound driver method described below. In this bounded driver method, the frequency of the laser light is controlled so as to be the bottom of the reflectance in the optical modulator 21. In general, the optical modulator 21 using a Fabry-Perot resonator has a much steeper reflectivity / voltage characteristic as compared with the case of using a Mach-Zehnder optical modulator, thereby realizing highly efficient optical modulation. Can be made. In this optical modulator 21, in order to actually perform high-efficiency optical modulation on the laser light, it is necessary to determine the frequency according to the resonance frequency in the optical modulator 21 by controlling the frequency fm2 of the modulation signal. is there. For this reason, in order to reduce the influence of the drift of the DC component in the reflected light, the phase modulation in the optical modulator 21 is maximized as shown in FIG. Signal) and control the laser frequency to lock at the zero cross point. In FIG. 4, when the frequency of the laser light is changed with the DC component of the voltage applied to the optical phase modulation unit 33 being 0 V, the frequency difference corresponding to the minimum value of the reflectance in the optical phase modulation unit 33 is represented by FSR. FIG. 5 shows the relationship of each frequency fm2 with respect to the reflectance / laser light frequency characteristics of the optical phase modulation section 33. Here, when the frequency of the modulation signal is set to fm2 = FSR (N + 0.5) with respect to the reflectance / laser light frequency characteristics of the optical phase modulator 33 as shown in FIG. 5A (FIG. 5B). ), Fm2 = FSR (N + 0.4) (FIG. 5C), and fm2 = FSR (N + 0.1) (FIG. 5D), the reflectance / laser light A zero cross occurs at the bottom of the frequency characteristic. This suggests that in any case, the optical phase modulation unit 33 can be locked at the bottom of the reflectance / laser light frequency at which maximum modulation is obtained.

  In particular, when the frequency of the modulation signal is fm2 = FSR (N + 0.5) (FIG. 5 (b)), the point L corresponds to the bottom of the reflectance / laser light frequency in the optical phase modulator 33. By adjusting the point as a stable point, it is possible to lock at the bottom of the reflectance / laser beam frequency. It is also possible to adjust M point as a stable point by inverting the sign of feedback.

  On the other hand, in the case of fm2 = FSR (N + 0.4) (FIG. 5C) and fm2 = FSR (N + 0.1) (FIG. 5D), the reflectance / laser light A stable point is also generated at a location indicated by “x” not corresponding to the bottom of the frequency, and in order to prevent this, a complicated control system using a computer or the like is required.

  That is, by setting the frequency fm2 of the modulation signal to (N + 0.5) times the FSR in the optical phase modulation unit 33, it is possible to reliably lock at the bottom of the reflectance / laser light frequency without constructing a complicated control system. be able to. Based on the frequency fm of the radio signal received from the mobile communication device 11 in such a state, the laser beam resonating with the optical phase modulation unit 33 can be modulated with higher efficiency.

  In the mobile communication system 1 including the host control device 13 having the above-described configuration, the following operation is realized.

  The frequency characteristic of the laser beam generated in the laser beam generator 41 is composed of only a single carrier component as shown in FIG. The laser light is modulated by the modulation signal having the frequency fm2 in the phase modulator 42, and the side band having the frequency interval fm2 is generated over a wide band as shown in FIG. 6B. Incidentally, in FIG. 6B, the modulation index of the phase modulation is assumed to be 1 or less, and the second-order or higher sideband components are weak and ignored.

  The laser beam in which such a sideband is generated passes through the optical circulator 43 as it is and is transmitted to each base station 12 via the optical fiber communication network 14. When a call is made from the mobile communication device 11 around each base station 12, a radio signal having a frequency fm is transmitted to the base station 12 and propagates through the optical modulator 21 mounted on the base station 12. The laser light is phase-modulated by applying a modulation electric field according to a radio signal having a frequency fm.

  FIG. 6C shows the frequency characteristic of the laser light that is phase-modulated based on the radio signal having the frequency fm in the optical modulator 21. As a result of the modulation based on the radio signal having the frequency fm as shown in FIG. 6C, a side band having a frequency interval fm is newly generated. Further, as shown in FIG. 6C, the sideband interval of the frequency interval fm2 is set to (N + 0.5) times the FSR in the optical phase modulation unit 33 because the frequency fm2 is set. It does not coincide with the bottom of the reflectance / laser light frequency shown in FIG. On the other hand, the carrier component becomes 0 because it coincides with the bottom of the reflectance / laser beam frequency shown in FIG.

  As described above, the laser light emitted from the optical phase modulation unit 33 via the second mirror 32 includes a sideband component based on the modulation signal having the frequency fm2 and a sideband component based on the radio signal having the frequency fm from the mobile communication device 11. Is the main frequency component. The host control device 13 photoelectrically converts the laser beam having the frequency component via the light detection unit 44, so that the detection output is an interference signal between the sideband component at the frequency fm2 and the sideband component at the frequency fm. The | fm2 ± fm | component and the 2 × fm2 component are main. There is a 2 × fm component, but it is negligible and can be ignored.

  Here, the | fm2-fm | component will be described. Since the same applies to the | fm2 + fm | component, the description of the | fm2-fm | component is cited.

  As shown in FIG. 6C, the laser light emitted from the optical phase modulation unit 33 through the second mirror 32 has a high-frequency sideband component and a low-frequency sideband component. When locked to the bottom of the reflectivity / laser light frequency, the optical phase modulator 33 and the phase modulator 42 work equivalently, so that the modulation component of fm2 and the modulation component of fm are on the high frequency side and the low frequency side. Each is in phase. That is, as shown in FIG. 6C, the frequency components of the sidebands generated on the high frequency side and the low frequency side are symmetric with respect to the frequency of the carrier component, and the positive and negative of the electric field are opposite to each other. . As a result, when viewed from the beat fm2-fm, the high-frequency sideband component and the low-frequency sideband component are in phase with each other and do not cancel each other because they strengthen each other. Since the component fm2-fm includes the frequency fm of the radio signal transmitted from the mobile communication device 11, the host control device 13 uses the fm2- from the electric signal photoelectrically converted via the light detection unit 44. fm can be detected via the second BPF 46 and used as a detection output.

  That is, in such a mobile communication system 1, it is possible to simultaneously realize the laser control for matching the carrier component to the bottom of the reflectance / laser light frequency and the detection in the frequency component of fm2-fm. Normally, it is not necessary to perform phase modulation used for laser control and phase modulation used for detection under the same conditions, but the system can be greatly simplified by modulating at the same frequency.

  As for noise, phase noise at a point separated by | fm2-fm | on the high frequency component side in the ± 1st order sideband of fm2 affects detection. The noise component of this frequency is the frequency component fm, In the case of an integral multiple of λ, the reflection characteristic of the optical phase modulation unit 33 functions as a notch filter and is lost. For this reason, in the present invention, the influence of laser noise is reduced by designing the minimum reflectance in the first mirror 31 and the second mirror 32 as small as possible to realize so-called Fabry-Perot resonance. Also good.

  FIG. 7 shows the relationship of the S / N ratio with respect to the modulation index β2 of the phase modulation by the modulation signal of the frequency fm2. The curve k in FIG. 7 shows the relationship of the S / N ratio with respect to the modulation index β2 when no thermal noise is included. A curve l shows the relationship of the S / N ratio to the modulation index β2 when thermal noise is included and the point L in FIG. 5B is adjusted as a stable point. A curve m shows the relationship of the S / N ratio with respect to the modulation index β2 when thermal noise is included and the point L in FIG. 5 (c) is adjusted as a stable point.

  As shown in FIG. 7, the S / N ratio of the photoelectrically converted electric signal is optimized by utilizing the fact that the signal intensity is proportional to the product of the sideband amplitude of fm2 and the sideband amplitude of fm. Is possible.

  In the mobile communication system 1 to which the present invention is applied, when the frequency of the modulation signal is fm2 = FSR (N + 0.5), the point L is a stable point even when the point M is a stable point. It can be detected almost in the same way. As shown in FIG. 5 (b), zero crossing occurs at the bottom of the reflectance / laser light frequency in the optical phase modulation unit 33, while also at an M point corresponding to an intermediate portion having no bottom of the reflectance / laser light frequency characteristic. Zero cross occurs. Even when the point M is controlled as a stable point, the sideband SB1 generated by the phase modulation by the modulation signal of the frequency fm2 coincides with the bottom of the reflectance / laser beam frequency. The sideband SB1 is modulated by the radio signal having the frequency fm from the mobile communication device 11 in the optical modulator 21 in the base station 12.

  In such a case, the laser light generated over the wide band of the side band SB1 having the frequency interval fm2 as shown in FIG. 8A is transmitted to each base station 12 via the optical fiber communication network 14, and each base station. When a call is made from the portable communication device 11 in the vicinity of 12, the optical modulator 21 performs phase modulation based on the radio signal having the frequency fm. Here, when the M point is controlled as a stable point, deeper phase modulation is applied based on the sideband SB1. In other words, the sideband SB1 having the frequency interval fm2 is modulated by the frequency fm in the optical modulator 21. As a result, a new sideband SB2 is generated at a frequency interval of ± fm centered on the sideband SB1. Further, a side band SB3 is also generated.

  Incidentally, since the reflectance in the optical phase modulation unit 33 is high in the band of the carrier component, the carrier component itself is not attenuated. As a result of interference between the carrier component and the sideband SB2, the components of the interference signal fm2-fm are strengthened. That is, when the M point is adjusted as a stable point, the frequency component of fm2-fm can be detected in the same manner, and furthermore, an interference signal having almost the same intensity as that when the L point is adjusted as a stable point can be obtained. Can do.

  In addition, regarding noise, intensity noise in a band away from fm2-fm from the normal carrier component on the high frequency side or low frequency side affects the detection. The noise component in such a band has a reflection characteristic of the optical modulator 21 so-called notch filter. Will be lost to work as. Therefore, the influence of noise at the time of detection can be reduced by designing the minimum reflectance in the optical phase modulation unit 33 as small as possible.

  In the mobile communication system 1 to which the present invention is applied, a polarization-independent ring type electro-optic modulator described below may be used as the optical modulator 11 mounted on each base station 12.

  As shown in FIG. 9, the ring-type electro-optic modulator 80 includes an optical phase modulator 111, and a first mirror 112 and a second mirror that are disposed so as to face each other via the optical phase modulator 111. 113, an optical resonator 110 formed on the top and bottom surfaces of the optical phase modulator 111 such that the direction of the modulation electric field is substantially perpendicular to the light propagation direction, and each of the above-described components. And a temperature compensation housing 116 to be incorporated. Further, the first optical path 121 and the second optical path 122 made of optical fibers for propagating light to enter and exit the first mirror 112, respectively, are disposed at the end of the first optical path 121. The first collimator 131, the second collimator 132 disposed at the end of the second optical path 122, and the polarization maintaining coupler 141 connected to the first optical path 121 and the second optical path 122, respectively. And.

  The optical phase modulator 111 is an optical device that phase-modulates light that passes through based on a supplied electrical signal. The optical phase modulator 111 may be made of the same material as that of the waveguide 12. Incidentally, the reflectance of each side surface of the optical phase modulator 111 is controlled so as to totally reflect the incident light. For this reason, as shown in FIG. 9, the light incident from the oblique direction via the first mirror 112 is totally reflected by the side surface of the optical phase modulator 111 and also reflected by the second mirror 113. Therefore, a path as if to draw a ring in the optical phase modulator 111 is taken.

  The first mirror 112 and the second mirror 113 are provided to resonate light incident on the optical resonator 110, and resonate by reciprocally reflecting light passing through the optical phase modulator 111.

  The first mirror 112 is disposed on the light incident side of the optical phase modulator 111, and light is incident from the first collimator 131 or the second collimator 132. In addition, a part of the light reflected by the second mirror 113 and passed through the optical phase modulator 111 is reflected, and a part of the light is emitted to the outside.

  The electrode 115 is connected to an antenna 4 for transmitting and receiving radio signals. Phase modulation is applied to the light propagating in the optical phase modulator 111 with respect to the electrode 115 by a radio signal having a frequency fm supplied from the outside.

  The polarization preserving coupler 141 separates horizontal linearly polarized light and vertical linearly polarized light from the light transmitted from the optical fiber communication network 440 connected to the optical fiber communication network 14. The separated light including the linearly polarized light component in the horizontal direction propagates through the first optical path 121, and the light including the linearly polarized light component in the vertical direction propagates through the second optical path 122.

  The first optical path 121 rotates the optical fiber and rotates the polarization component of light from the horizontal direction to the vertical direction. Thereby, the polarization direction of the light propagating through the first optical path 121 and the light propagating through the second optical path 122 can be made the same. Note that the second optical path 122 may also have a function of rotating the polarization direction.

  The first collimator 131 and the second collimator 132 convert the light propagating through the first optical path 121 and the second optical path 122 into parallel light, respectively, to the first mirror 112 constituting the optical resonator 110. Exit.

  In such a ring type electro-optic modulator 80, the light propagating from the optical fiber communication network 440 is first incident on the polarization maintaining coupler 141. Even if the incident light includes various polarization components, the incident light is separated according to the direction of each linearly polarized light and propagates through the first optical path 121 and the second optical path 122. The separated light is emitted as parallel light through the first collimator 131 and the second collimator 132 after the polarization direction is controlled to rotate. Each emitted light passes through the first mirror 112 from the A direction and the B direction, and propagates while being totally reflected by the side surface of the optical phase modulator 111. Then, each light reflected by the second mirror 113 returns to the first mirror 112 again, a part of the light is reflected, and a part of the light is emitted to the outside through the first mirror 112.

  That is, the light incident from the A direction propagates in the optical phase modulator 111 so as to draw a ring counterclockwise as shown in FIG. 9 and is emitted in the B direction through the first mirror 112. The light emitted from the B direction propagates through the second optical path via the second collimator 132 and returns to the polarization preserving coupler 141. Similarly, light incident from the B direction propagates in a clockwise direction so as to draw a ring, and is emitted through the first mirror 112 in the A direction. Then, the light emitted from the A direction propagates through the second optical path via the second collimator 132 and returns to the polarization preserving coupler 141. Incidentally, the lights returned to the polarization preserving coupler 141 are combined with each other and transmitted to the optical fiber communication network 440 again.

  In such a ring type electro-optic modulator 80, if the refractive index and the modulation efficiency of the material constituting the optical phase modulator 111 are strongly dependent on a specific polarization direction, the polarization is changed according to the polarization direction. The polarization direction of each light separated in the storage coupler 141 can be controlled in the same direction. Thereby, even if the supplied light has any polarization component, highly efficient phase modulation can be realized without depending on this.

  Further, even if the optical fiber disposed in the optical fiber communication network 440 is not a polarization preserving fiber, highly efficient phase modulation can be performed while controlling the polarization direction. For this reason, by using this ring type electro-optic modulator 80 as the optical modulator 81 mounted on each base station 12, the general versatility of the entire system can be enhanced.

  Note that the material and structure of the temperature compensation housing 116 of the ring type electro-optic modulator 80 may compensate for changes in the thermal expansion coefficient and thermal refractive index of the crystal constituting the optical phase modulator 111. As a result, it is possible to realize high-precision modulation that eliminates the temperature environment in which the base station 12 is installed.

  Incidentally, the ring type electro-optic modulator 80 to which the present invention is applied is not limited to the above-described embodiment. For example, the ring type electro-optic modulator 80 shown in FIG. You may apply to the optical modulator 90. FIG. In the ring-type electro-optic modulator 90 shown in FIG. 10, the same components as those of the ring-type electro-optic modulator 80 are denoted by the same reference numerals and description thereof is omitted.

  The ring-type electro-optic modulator 90 includes an optical phase modulator 111 and an optical resonator including a first mirror 212 and a second mirror 213 that are disposed so as to face each other with the optical phase modulator 111 interposed therebetween. 210, and electrodes 115 formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction. The ring-type electro-optic modulator 90 includes a collimator lens 231 that converts the light from the optical fiber communication network 440 into parallel light, and a birefringence element 241 that separates the light emitted from the collimator lens 231 according to each polarization component. The half-wave plate 232 disposed on the optical path of light having one polarization component separated in the birefringent element 241 (hereinafter referred to as the first optical path 221), the first optical path 221 and And a plano-convex lens 233 provided on an optical path of light having another polarization component separated in the birefringent element 241 (hereinafter referred to as a second optical path 222).

  The first mirror 212 and the second mirror 213 are provided to resonate the light incident on the optical resonator 210, and resonate by reciprocally reflecting the light passing through the optical phase modulator 111.

  The first mirror 212 is disposed on the light incident side of the optical phase modulator 111, and light is incident from the plano-convex lens 233. The second mirror 213 is configured as a so-called concave mirror so that the light focused by the plano-convex lens 233 becomes parallel light again.

  The birefringent element 241 separates horizontal linearly polarized light and vertical linearly polarized light from the light that has been converted into parallel light by the collimator lens 231 by using birefringence whose refractive index changes depending on the polarization direction. The separated light including the linearly polarized component in the horizontal direction propagates through the first optical path 221, and the light including the linearly polarized component in the vertical direction propagates through the second optical path 222.

  The half-wave plate 232 is disposed on the first optical path 221 so that the high speed axis is inclined by 45 ° from the polarization direction of the light. Thereby, the polarization component of the light can be rotated from the horizontal direction to the vertical direction, and the polarization direction of the light propagating through the first optical path 221 and the light propagating through the second optical path 222 can be made the same. . Note that the second optical path 222 may also have a function of rotating the polarization direction.

  In such a ring type electro-optic modulator 90, light propagated from the optical fiber communication network 440 is incident on the birefringent element 241 through the collimator lens 231. The incident light is separated into the first optical path 221 and the second optical path 222 according to the direction of each linearly polarized light, even if it includes various polarization components. Among these, the light separated into the first optical path 221 passes through the half-wave plate 232, the rotation direction of the polarization is controlled, and the light propagating through the second optical path 222 is converged by the plano-convex lens 233. Thus, the light is incident on the optical phase modulator 111 from different directions (A direction and B direction).

  The light incident from the A direction propagates in the optical phase modulator 111 so as to draw a ring clockwise as shown in FIG. 10, and is emitted in the B direction via the first mirror 212, and is a plano-convex lens 233. And then returns to the birefringent element 241. Similarly, the light incident from the B direction propagates in a counterclockwise manner so as to draw a ring, is emitted in the A direction via the first mirror 212, and is further birefringent element 241 via the half-wave plate 232. Return to.

  Even in such a ring-type electro-optic modulator 90, even if the light propagating through the optical fiber communication network 440 has any polarization component, high-efficiency phase modulation can be realized without depending on this. Can do.

  In the ring-type electro-optic modulator 90, each component may be incorporated in the temperature compensation casing 116 as described above.

  Further, in the ring type electro-optic modulator 80 to which the present invention is applied, the optical phase modulator 111 may be further arranged in the configuration shown in FIG.

  The configuration shown in FIG. 11 is a so-called loop configuration in which the first optical path 121 is connected to the second optical path 122 via the optical resonator 110 and the optical phase modulator 111. As a result, the first mirror 112 is disposed near the terminal end of the first optical path 121, and the second mirror 113 is disposed near the terminal end of the second optical path 122.

  The light containing the linearly polarized component in the horizontal direction separated by the polarization preserving coupler 141 propagates through the first optical path 121, enters the optical phase modulator 111 via the first mirror 112, and is modulated. It returns to the polarization preserving coupler 141 via the second optical path 122. The light including the linearly polarized light component in the vertical direction separated by the polarization preserving coupler 141 propagates through the second optical path 122, is incident on the optical phase modulator 111 via the second mirror 113, and is modulated. The light returns to the polarization preserving coupler 141 via the first optical path 121. By the way, the distance between the first path 121 and the second path 122 is the same in order to multiplex the lights returning from the first optical path 121 and the second optical path 122 to the polarization preserving coupler 141. To do.

  That is, in the configuration shown in FIG. 11, the input and output of light to the optical phase modulator 111 can be realized through different optical paths 121 and 122. Further, in this configuration, even if the input / output of light is switched between the optical paths 121 and 122, the same phase modulation can be performed.

  Further, the ring type electro-optic modulator 80 to which the present invention is applied is not limited to the above-described embodiment. For example, a ring type electro-optic having a birefringent element inside a resonator as shown in FIG. The modulator 10 may be applied. In the ring electro-optic modulator 10, the same components and members as those of the ring electro-optic modulator 80 described above are denoted by the same reference numerals and description thereof is omitted.

  This ring-type electro-optic modulator 10 includes an optical phase modulator 111 and an optical resonator composed of a first mirror 112 and a second mirror 113 which are disposed so as to face each other via the optical phase modulator 311. 110 and electrodes (not shown) formed on the top and bottom surfaces of the optical phase modulator 111 so that the direction of the modulation electric field is substantially perpendicular to the light propagation direction. The ring-type electro-optic modulator 10 includes a first Faraday rotator 321 having a 45 ° polarization rotation disposed between the first mirror 112 and the optical phase modulator 111, a second mirror 113, and an optical phase modulation. A second Faraday rotator 322 is provided between the container 111 and the container 111.

  The first mirror 112 receives the light of each polarization component separated by the polarization preserving coupler 141. The first mirror 112 reflects a part of the incident light and transmits a part thereof.

  The first Faraday rotator 321 rotates the polarization direction of the light passing through the first mirror 112 by −45 ° and emits the light to the optical phase modulator 111. Further, the first Faraday rotator 321 further rotates the polarization direction of the light from the optical phase modulator 111 by −45 ° and emits the light to the first mirror 112.

  The second Faraday rotator 322 rotates the polarization direction of the light that has passed through the optical phase modulator 111 by −45 ° and emits the light to the second mirror 113. The second Faraday rotator 322 rotates the polarization direction of the light from the second mirror 113 by −45 ° and emits the light to the optical phase modulator 111.

  That is, in the ring type electro-optic modulator 10, when light having a polarization component shifted by 45 ° from the vertical direction (hereinafter referred to as 45 ° polarization) is supplied as shown in FIG. The first Faraday rotator 321 is rotated by −45 ° to make the polarization direction vertical. The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and then is rotated by −45 ° with respect to the polarization direction by the second Faraday rotator 322 and is shifted by −45 ° from the vertical direction. Component (hereinafter referred to as -45 ° polarized light). The −45 ° polarized light is reflected by the second mirror 113 and then rotated again by −45 ° with respect to the polarization direction by the second Faraday rotator 322 to become a horizontal polarization component. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and is rotated again by −45 ° with respect to the polarization direction by the first Faraday rotator 321 to become 45 ° polarization. It will be emitted through 112.

  Similarly, in the ring type electro-optic modulator 10, when -45 ° polarized light is supplied as shown in FIG. 12B, the light is rotated by −45 ° by the first Faraday rotator 321. The polarization direction is set to the horizontal direction. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and then is rotated by −45 ° in the polarization direction by the second Faraday rotator 322 to become 45 ° polarization. This 45 ° -polarized light is reflected from the second mirror 113 and then rotated again by −45 ° with respect to the polarization direction by the second Faraday rotator 322 to become a vertically polarized component. The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and is rotated again by −45 ° with respect to the polarization direction by the first Faraday rotator 321 to become −45 ° polarization. The light is emitted through the mirror 112.

  As described above, in the ring type electro-optic modulator 10, by disposing the Faraday rotators 321 and 322 before and after the optical phase modulator 111, only two orthogonal polarization components are propagated in the optical phase modulator 111. Can be made. The polarization directions of the two orthogonal polarization components differ from each other depending on the propagation direction in the optical phase modulator 111, but have the same propagation path and optical distance, respectively. For this reason, the optical resonator 110 itself is degenerated for each light having two orthogonal polarization components. In such a case, high-efficiency phase modulation is possible without being governed by the polarization direction of incident light.

  Furthermore, the present invention is not limited to the ring type electro-optic modulator 10 described above, but a ring type electro-optic modulator 300 in which a quarter-wave plate is installed inside the optical resonator 110 as shown in FIG. You may make it apply to.

  This ring type electro-optic modulator 300 includes an optical phase modulator 111 and an optical resonator 110, and a quarter-wave plate 331 disposed between the first mirror 312 and the optical phase modulator 111. And a quarter-wave plate 332 disposed between the second mirror 113 and the optical phase modulator 111.

  The quarter-wave plates 331 and 332 give a phase difference of π / 2 to the light passing therethrough when they have vertical and horizontal polarization components.

  In other words, in the ring type electro-optic modulator 300, when clockwise circularly polarized light (hereinafter referred to as clockwise circularly polarized light) is supplied, it is converted by the quarter wavelength plate 331 into vertical linearly polarized light. And The light composed of the polarization component in the vertical direction passes through the optical phase modulator 111 as it is, and is converted into counterclockwise circularly polarized light (hereinafter referred to as counterclockwise circularly polarized light) by the quarter wavelength plate 332. The counterclockwise circularly polarized light is reflected from the second mirror 113 and then converted into linearly polarized light in the horizontal direction from the quarter-wave plate 322 and passes through the optical phase modulator 111 as it is. The light is converted to right-handed circularly polarized light again by the quarter-wave plate and is emitted through the first mirror 112.

  Further, in the ring type electro-optic modulator 300, when a counterclockwise circularly polarized light is supplied, it is converted into a linearly polarized light in the horizontal direction by the quarter wavelength plate 331. The light composed of the polarization component in the horizontal direction passes through the optical phase modulator 111 as it is, and is converted into clockwise circular polarization by the quarter wavelength plate 332. Then, the clockwise circularly polarized light is reflected from the second mirror 113 and then converted into linearly polarized light in the vertical direction from the quarter-wave plate 322 and passes through the optical phase modulator 111 as it is. The light is converted to counterclockwise circularly polarized light again by the ¼ wavelength plate and is emitted through the first mirror 112.

  As described above, in the ring-type electro-optic modulator 300, by arranging the quarter wavelength plates 331 and 332 before and after the optical phase modulator 111, two polarization components orthogonal to each other in the optical phase modulator 111. Can only propagate. The polarization directions of the two orthogonal polarization components differ from each other depending on the propagation direction in the optical phase modulator 111, but have the same propagation path and optical distance, respectively. For this reason, the optical resonator 110 itself is degenerated for each light having two orthogonal polarization components. In such a case, high-efficiency phase modulation is possible without being governed by the polarization direction of incident light.

  In particular, the ring-type electro-optic modulators 10 and 300 are selectively used according to the polarization component of the supplied light, so that the inside of the optical phase modulator 111 is independent of the polarization direction of linearly polarized light and the direction of circularly polarized light. Only two polarization components orthogonal to can be propagated, and high efficiency of modulation can be promoted.

It is a figure which shows the structure of the mobile communication system to which this invention is applied. It is a figure for demonstrating the structure of the optical modulator mounted in each base station. It is a figure for demonstrating per structure of a host control apparatus. It is a figure for demonstrating the relationship of the reflectance with respect to the laser frequency in an optical modulator. It is a figure which shows the relationship of each frequency fm2 with respect to the reflectance / laser-light frequency characteristic of the optical phase modulation part 33. FIG. It is a figure which shows the frequency characteristic of the laser beam containing the carrier component generated in the laser beam generation part. It is a figure which shows the relationship of S / N ratio with respect to modulation index (beta) 2 of the phase modulation by the modulation signal of frequency fm2. It is a figure shown about the frequency characteristic of the laser beam in the case of making M point stable. It is a figure shown about the structure of a ring type | mold electro-optic modulator. It is a figure shown about the other structure of a ring type | mold electro-optic modulator. It is a figure which shows about the structure of a ring type | mold electro-optic modulator which implement | achieves the input and output of a laser beam via a different optical path. It is a figure shown about the ring type | mold electro-optic modulator which provided the Faraday rotator inside the resonator. It is a figure shown about the structure of the ring-type electro-optic modulator which installed the quarter wavelength plate inside the optical resonator. It is a figure for demonstrating about the mobile communication system which performs code division multiple access based on a WDM communication system. It is a figure for demonstrating about the mobile communication system which performs code division multiple access based on a WDM communication system. It is a figure which shows the reflectance / voltage characteristic in a FP light modulator and a MZ light modulator. It is a figure which shows the change of a reflectance when the DC component of the voltage applied to an FP light modulator is 0V, and the frequency of a laser beam is changed.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Mobile communication system, 4 Antenna, 8 Ring type electro-optic modulator, 11 Portable communication apparatus, 12 Base station, 13 Host control apparatus, 14 Optical fiber communication network, 21 Optical modulator, 31 1st mirror, 32 1st 2 mirrors, 33 optical phase modulator, 34 electrodes, 111 optical phase modulator, 112 first mirror, 113 second mirror, 115 electrodes, 116 temperature compensation housing, 121 first optical path, 122 second Optical path 131 First collimator 132 Second collimator 141 Polarization preserving coupler

Claims (14)

Light that modulates laser light transmitted from a host control device to a base station via an optical fiber communication network based on a signal having a frequency fm supplied from a portable communication device, and transmits the modulated light to the host control device for detection. In a communication system,
The base station includes an optical modulator including a Fabry-Perot resonator that modulates transmitted laser light based on an electric signal fm from a portable communication device,
The host controller is
Laser light generating means for generating laser light;
Oscillating means for oscillating a modulation signal of frequency fm2 determined based on the reflectance characteristic of the optical modulator in the base station;
Modulation means for modulating the laser light generated in the laser light generation means in accordance with the modulation signal supplied from the oscillation means and transmitting it to the base station;
Photoelectric conversion means for photoelectrically converting the laser light transmitted from the base station to generate an electrical signal;
Detection means for detecting an interference signal | fm2-fm | from the electric signal generated by the photoelectric conversion means;
Laser light control means for controlling the laser light generation means in accordance with the component of the frequency fm2 detected from the electrical signal;
Have
The oscillating means sets the frequency fm2 of the modulation signal based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, and the following formula fm2 = FSR (N + 0.5)
(N is an integer)
An optical communication system characterized by:   Light that modulates laser light transmitted from a host control device to a base station via an optical fiber communication network based on a signal having a frequency fm supplied from a portable communication device, and transmits the modulated light to the host control device for detection. In a communication system,
  The base station includes an optical modulator including a Fabry-Perot resonator that modulates transmitted laser light based on an electric signal fm from a portable communication device,
  The host controller is
  Laser light generating means for generating laser light;
  Oscillating means for oscillating a modulation signal of frequency fm2 determined based on the reflectance characteristic of the optical modulator in the base station;
  Modulation means for modulating the laser light generated in the laser light generation means according to the modulation signal supplied from the oscillation means, and transmitting the modulated signal to the base station;
  Photoelectric conversion means for photoelectrically converting the laser light transmitted from the base station to generate an electrical signal;
  Detection means for detecting an interference signal | fm2 + fm | from the electrical signal generated by the photoelectric conversion means;
  Laser light control means for controlling the laser light generation means in accordance with the component of the frequency fm2 detected from the electrical signal;
  Have
  The oscillation means sets the frequency fm2 of the modulation signal based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator based on the following equation:
fm2 = FSR (N + 0.5)
(N is an integer)
  Determined by
  An optical communication system. The optical modulator includes: a separating unit that separates the transmitted laser light according to a polarization direction; a polarization control unit that aligns the polarization direction of each of the separated laser beams; and an incident from the polarization control unit at different angles. Resonance means for resonating each laser beam between the reflecting mirrors, and phase modulation means for modulating the phase of the light resonated by the resonance means based on the electric signal fm from the portable communication device. The optical communication system according to any one of claims 1 and 2 . The optical communication system according to claim 3 , wherein the separating means in the optical modulator is constituted by a birefringent element. 4. The optical communication system according to claim 3 , wherein the resonance means in the optical modulator is configured such that the separated laser beams are incident through different reflecting mirrors. The phase modulation means is a bulk crystal whose refractive index changes by applying an electric field,
The optical communication system according to claim 3 , wherein the polarization control means is a Faraday rotator with 45 ° polarization rotation disposed at both ends of the bulk crystal in the phase modulation means. The phase modulation means is a bulk crystal whose refractive index changes by applying an electric field,
The optical communication system according to claim 3 , wherein the polarization control means is a quarter-wave plate disposed at both ends of the bulk crystal in the phase modulation means. Laser light transmitted from the host controller to the base station via the optical fiber communication network is modulated by the optical modulator comprising a Fabry-Perot resonator in the base station based on the electric signal fm from the portable communication device, In an optical communication method of transmitting to a host control device and detecting,
In the above host device,
Based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, the generated laser light is expressed by the following formula: fm2 = FSR (N + 0.5)
(N is an integer)
Modulated according to the modulation signal of the frequency fm2 determined by the above and transmitted to the base station,
The laser beam transmitted from the base station is photoelectrically converted to generate an electrical signal,
The interference signal | fm2-fm | is detected for detection from the generated electrical signal,
Furthermore, the laser beam to be generated is controlled according to the component of the frequency fm2 detected from the electrical signal .   The laser beam transmitted from the host controller to the base station via the optical fiber communication network is modulated by the optical modulator composed of a Fabry-Perot resonator in the base station based on the electric signal fm from the portable communication device. In an optical communication method of transmitting to a host control device and detecting,
  In the above host device,
  Based on the frequency difference FSR corresponding to the minimum value of the reflectance in the optical modulator, the generated laser light is expressed by the following equation:
  fm2 = FSR (N + 0.5)
  (N is an integer)
Modulated according to the modulation signal of the frequency fm2 determined by the above and transmitted to the base station,
  The laser beam transmitted from the base station is photoelectrically converted to generate an electrical signal,
  The interference signal | fm2 + fm | is detected for detection from the generated electrical signal,
  Furthermore, the laser beam to be generated is controlled according to the component of the frequency fm2 detected from the electrical signal.
  An optical communication method characterized by the above. In the optical modulator, the transmitted laser beam is separated according to the polarization direction, the polarization directions of the separated laser beams are aligned, and the laser beams incident at different angles are placed between the opposing reflectors. The optical communication method according to claim 8, wherein the phase of the resonated light is modulated based on an electric signal fm from a portable communication device. The optical communication method according to claim 10 , wherein the optical modulator separates the transmitted laser light according to a polarization direction by a birefringent element. The optical communication method according to claim 10, wherein the laser beams incident through different reflectors are resonated. The phase of the resonated light is modulated by a bulk crystal whose refractive index changes by applying an electric field, and the polarization direction of each laser beam by a Faraday rotator with 45 ° polarization rotation disposed at both ends of the bulk crystal. The optical communication method according to claim 10 , wherein: The phase of the resonated light is modulated by a bulk crystal whose refractive index is changed by applying an electric field, and the polarization directions of the laser beams are aligned by quarter-wave plates disposed at both ends of the bulk crystal. The optical communication method according to claim 10 .

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