JP2012150327A - Two-beam multiplexing circuit and demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a two-beam multiplexing circuit that has no requirement for fusing an optical fiber for connecting between an interferometer module and a PD (or Photo Detector) module, and that has little occurrence of loss of interference light, deterioration of an extinction ratio, and brightness and darkness of interference fringes caused by angle deviation and/or optical axis misalignment.SOLUTION: In a two-beam multiplexing circuit, two luminous fluxes interfering each other are multiplexed, and an electrical signal according to the intensity of light is output from a photodetector. The two-beam multiplexing circuit includes: a condensing lens for condensing the two luminous fluxes interfering each other; an optical waveguide for multiplexing the two luminous fluxes condensed by the condensing lens by making them incident on one end thereof; and a photodetector on which emitted light from the other end of the optical waveguide is perpendicularly made incident.

Description

The present invention is for inputting output signal light, such as a delay interferometer, used for demodulation of a differential phase modulation signal in optical fiber communication, particularly in optical fiber communication adopting wavelength division multiplexing (WDM), to a photodetector. And a demodulator using the same.

  In recent years, with the rapid development of the Internet, optical fiber communication that transmits information not by electrical signals but by optical signals using optical fibers as transmission paths has been developed and developed in order to realize the demand for higher speed and larger capacity of networks. It has been put into practical use. In such optical fiber communication, in order to realize higher speed and larger capacity, the optical property that “lights with different wavelengths do not interfere with each other” is used to provide a plurality of optical fibers with different wavelengths. Dense Wavelength Division Multiplexing (DWDM) that multiplexes and transmits optical signals is attracting attention.

In optical fiber communication employing the DWDM method, an optical signal modulated by a differential phase modulation method (DPSK: Differential Phase Shift Keying) or a four-phase differential phase modulation method (DQPSK: Differential Quadrature Phase Shift Keying) is generally transmitted. The optical signal received by the demodulator having the delay interferometer is demodulated.
As shown in FIG. 4 of Patent Document 1, a conventional DPSK and DQPSK receiver generally has one light beam for each light beam between a delay interferometer module and a PD (Photo Detector) module. It has the structure connected via a fiber.

  On the other hand, as shown in FIG. 2 of Patent Document 1, there is a configuration in which the output light of the delay interferometer is directly output to the photodetector without going through the optical fiber.

  FIG. 4 shows an example of the configuration of a conventional two-beam combining circuit, which is a cross-sectional view shown in FIG. The light beam S10 incident from the incident means (not shown) is branched into two at the branching portion 11 to become S11 and S12. The light beams S11 and S12 are incident on two delay interferometers composed of the same components. Here, the two delay interferometers are a first lens 10, a bifurcated prism 11, a half mirror 12, a first reflector 13, a phase adjusting unit 14, a second reflector 15, a first mirror 16, and a second mirror. The mirror 17, the second lens 18, the third lens 19, the fourth lens 20, and the fifth lens 21 are provided with a phase difference of π / 2. Four outputs from the two delay interferometers based on the branched lights S11 and S12 are received by the photodetector 22 or 23 via the mirror 16 or 17, and the lenses 18, 19, 20 or 21, respectively. With such a configuration, light in which the two light beams on the short path side and the long path side of the delay interferometer interfere with each other is input to the photodetector, and a DQPSK optical signal demodulator can be configured.

  The alignment of the input light and output interference light that is performed when manufacturing the delay interferometer blocks the light on the short path side of the delay interferometer and reflects the long path so that the optical power on the long path side is maximized. The plate is positioned (e.g., tilt adjustment), and the short path side blocks the light on the long path side and positions the reflector on the short path side.

  The following patent documents are known as prior art of the demodulator.

JP 2007-151026 A

In order to configure as shown in FIG. 4 of Patent Document 1, the optical path length of the optical fiber between the output channels of 2 channels in the DPSK delay interferometer and the 4 channels in the DQPSK delay interferometer is aligned, and the input on the PD module side is similarly performed. It is necessary to align the optical path length between optical fiber channels. The optical path length difference between channels usually needs to be adjusted at a very small level of about 200 μm. That is, a special design and manufacturing method for aligning the optical path length of the optical fiber is required.

  In addition, in order to fuse and connect modules to each other, it is usually necessary to perform fusion again and a surplus length considering the bending radius of the optical fiber, and the length is about 1 m. The optical path length of such an optical fiber varies depending on the refractive index of the optical fiber, the temperature dependence of the refractive index, the photoelasticity, the variation in the core diameter, etc. There was a problem that the length difference became large.

  In the structure in which the output light of the interferometer is directly output to the photodetector as in the demodulator of FIG. 2 of Patent Document 1, two light beams on the short path side and the long path side of the interferometer (that is, the third reflected light S11aa and the first light beam) 5 combined with the transmitted light S11bb, combined with the third transmitted light S11ab and the fifth reflected light S11ba, combined with the fourth reflected light S12aa and the sixth transmitted light S12bb, and fourth transmitted. It is difficult for the optical axes of the light S12ab and the sixth reflected light S12ba to be aligned and incident on the photodetector. Due to manufacturing restrictions, the light is input to the photodetector in a state where there is an angle deviation of the light beam and an optical axis deviation. Therefore, there is a problem that the loss and the extinction ratio of the interference light are deteriorated, and the light fringes of the interference fringes are generated on the surface of the photodetector due to the angle shift, and the optical power of the interference light is changed.

  The present invention has been made to solve the above-described problems of the prior art, and does not require the fusion of an optical fiber for connection between the interferometer module and the PD module, and the loss of interference light due to angular deviation or optical axis deviation. An object of the present invention is to provide a two-beam combining circuit in which deterioration of extinction ratio and generation of interference fringes are small.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In a two-beam combining circuit that combines two interfering light beams and outputs an electrical signal corresponding to the light intensity from the photodetector,
A condensing lens that condenses the two interfering light beams;
An optical waveguide for entering and combining the two light beams collected by the condenser lens at one end thereof;
And a photodetector for receiving light emitted from the other end of the optical waveguide.

In order to achieve such a problem, the invention according to claim 2 of the present invention is:
The two-beam combining circuit according to claim 1,
The light receiving surface of the photodetector is attached to the other end of the optical waveguide.

The invention described in claim 3
The two-beam combining circuit according to claim 1,
A mirror that reflects the light emitted from the optical waveguide is provided, and the light reflected by the mirror is incident on the photodetector.

The invention according to claim 4
The two-beam combining circuit according to claim 1,
The light reflected under the total reflection condition on the end surface of the optical waveguide polished obliquely is incident on the photodetector.

The invention according to claim 5
The two-beam combining circuit according to claim 1,
A reflection mirror is deposited on the end surface of the optical waveguide polished obliquely, and light reflected by the reflection mirror is incident on the photodetector.

The invention described in claim 6
The two-beam combining circuit according to any one of claims 1 to 5,
The optical waveguide has a length sufficient to converge the meandering of the two light beams.

The invention described in claim 7
The two-beam combining circuit according to any one of claims 1 to 6,
An optical fiber is used as the optical waveguide.

The invention described in claim 8
The two-beam combining circuit according to claim 7,
The optical fiber is bent, and the two light beams are vertically incident on the one end thereof.

The invention according to claim 9
In a demodulator that demodulates a differential phase modulated optical signal,
A delay interferometer that emits two light beams that interfere based on the optical signal;
A condenser lens that collects the two light beams emitted from the delay interferometer;
An optical waveguide for entering and combining the two light beams collected by the condenser lens at one end thereof;
And a photodetector for receiving light emitted from the other end of the optical waveguide.

The invention according to claim 10 is:
The demodulator according to claim 9, wherein
An optical fiber is used as the optical waveguide.

  The present invention has the following effects.

That is, by combining the two interfering light beams with the optical waveguide, the photodetector can be easily positioned, and the two light beams can be incident on the photodetector without causing an axis deviation or an angle deviation, so that the loss of the interference light, It is possible to realize a two-beam combining circuit with little deterioration in extinction ratio and less occurrence of interference fringes.

Further, the loss due to the angular deviation of the input light is reduced by the numerical aperture (NA) of the optical waveguide.

1 is a configuration perspective view showing an example of a two-beam combining circuit according to an embodiment of the present invention. It is a structure perspective view which shows the modification of the 2 light beam multiplexing circuit of FIG. It is a structure perspective view which shows the 2nd Example of the two light beam multiplexing circuit which concerns on embodiment of this invention. It is a cross-sectional view showing a configuration example of a conventional two-beam combining circuit.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a structural perspective view showing an example of a two-beam combining circuit according to an embodiment of the present invention.

The two-beam combining circuit in FIG. 1 includes a condensing lens 3 that receives two-beam interference light emitted from a delay interferometer, and a core portion 42 of the first end face 41 that emits light emitted from the condensing lens 3. And the PD chip surface 5 on which the light emitted from the end surface 43 is incident. The PD chip surface 5 shows a part of a photodetector (not shown). The optical fiber 4 has a minimum length of about 2 mm to 4 mm necessary for converging the meandering of two light beams incident from the condenser lens 3.

The operation of the two-beam combining circuit in FIG. 1 will be described below. Two interfering light beams emitted from the delay interferometer based on the differential phase modulated optical signal are incident on the condenser lens 3. The two light beams 1 and 2 condensed by the condenser lens 3 are incident on the core portion 42 of the one end 41 of the optical fiber 4. The two light beams 1 and 2 propagate through the optical fiber 4 and are combined, then exit from the other end 43 and enter the PD chip surface 5 perpendicularly. An electrical signal corresponding to the light intensity detected on the PD chip surface 5 is output from the photodetector as a demodulated signal of the above-described differential phase modulated optical signal.

In the case of the demodulator shown in FIG. 2 of Patent Document 1, it is difficult to adjust the light beam on the light receiving surface of the PD chip twice. When a wave circuit is used, a relatively easy operation of adjusting the axes of the condenser lens 3 and the optical fiber 4 of the two-beam combining circuit is sufficient. Further, since the positioning of the PD chip surface 5 only needs to be performed with one light beam at the output end of the optical fiber 4 in FIG. 1, the mounting of the photodetector becomes easy.

According to the two-beam combining circuit configured as described above, the two light beams that interfere with each other are combined by an optical fiber, so that the photodetector can be easily positioned without causing the two beams to be misaligned or misaligned. Since it becomes easy to enter the photodetector, it is possible to realize a two-beam combining circuit in which interference light loss, extinction ratio deterioration, and interference fringe lightness and darkness are reduced. That is, since the overlap of the two light beams becomes complete, complete interference occurs and S / N is improved.

Further, by using an optical fiber having a large numerical aperture (NA), loss due to an angular deviation of input light is reduced.

In addition, since the light focused by the optical fiber 4 is detected by a photodetector that is relatively larger than the diameter of the optical fiber, all the emitted light from the optical fiber 4 can be detected, and the influence of positioning errors is small. .

If a single mode optical fiber is used as the optical fiber 4, the multimode light wave generated by the angle shift when the output light of the delay interferometer enters the optical fiber 4 is diffused as a radiation mode. So it does not reach the photodetector. As a result, single-mode light is input to the photodetector, and a stable phase can be realized.

Further, since the two-beam interference light from the interferometer is emitted into the space, it is not necessary to fuse the optical fibers at the emission end or to align the optical path length.

Further, since the optical fiber is used, the core cross section is a circle, and there is no shift in the traveling speed of the TM wave (magnetic field) and the TE wave (electric field), so that no shift in interference occurs.

In the case of an interferometer used in a specific polarization state, not only an optical fiber but also an optical waveguide can be used.

Further, an optical waveguide having no polarization dependency, for example, an optical waveguide designed in a single mode may be used instead of the optical fiber 4.

Further, it can be combined with various delay interferometers such as a DPSK delay interferometer and a DQPSK delay interferometer.

FIG. 2 shows a modification of the two-beam combining circuit. When the optical axes of the two incident light beams are not perpendicular to the light receiving surface of the photodetector, the two light beams can be easily detected by bending the optical fiber. It is a structure perspective view which shows what can be made to inject perpendicularly with respect to the light-receiving surface of a container.

The two-beam combining circuit in FIG. 2 is different from the two-beam combining circuit in FIG. 1 in that an optical fiber 4a bent in a "<" shape is provided, and a condensing lens 3 disposed obliquely is an optical fiber 4a. This is a point facing one end 41a. The other parts are the same as in FIG. 1, and the PD chip surface 5a is attached to the other end 43a of the optical fiber 4a. The PD chip surface 5a shows a part of a photodetector (not shown). The fiber 4a has a length of at least about 2 mm to 4 mm so that the meandering of the two light beams incident from the condenser lens 3 converges.

Here, two interfering light beams emitted from the delay interferometer based on the differential phase-modulated optical signal are incident on the condenser lens 3 from a direction oblique to the PD chip surface 5. The two light fluxes 1 and 2 collected by the condenser lens 3 are incident on the core portion 42a of the one end 41a of the optical fiber 4a and propagate through an optical path bent in a “<” shape in the optical fiber 4a. Wave light is emitted from the other end 43a and incident perpendicularly on the PD chip surface 5a. From an unillustrated photodetector, an electrical signal corresponding to the light intensity detected on the PD chip surface 5a is output as a demodulated signal of the above-described differential phase modulated optical signal.

According to the two-beam combining circuit configured as described above, in addition to the advantages of the two-beam combining circuit of FIG. 1, the optical path after incidence to make the two beams incident perpendicularly to the light receiving surface of the photodetector. When it is necessary to change the direction of the optical path, that is, to change the so-called optical path, it can be easily realized by bending the optical fiber. Therefore, the number of components such as the optical path changing reflector prism can be reduced, and the manufacturing becomes easy, so that the cost can be reduced.

FIG. 3A is a configuration perspective view showing a second embodiment of the two-beam combining circuit, which has a structure in which light emitted from an optical fiber is reflected by a mirror and incident on a photodetector. 3A includes a condensing lens 3 (not shown) similar to that in FIG. 1, an optical fiber 4b that receives the two-beam interference light from the condensing lens 3, and the optical fiber. A triangular groove block 7 having a triangular groove 9 on which 4b is placed and fixed, and provided in the triangular groove block 7, an optical fiber forming an angle of 45 ° with the optical axis direction of the optical fiber 4b at the end of the triangular groove 9 A mirror 6 (not shown) that reflects the light emitted from the other end of 4b, a lens 8 (not shown) that collects the light reflected by this mirror 6, and the light collected by this lens 8 is incident And a PD chip surface 5b. The PD chip surface 5b shows a part of a photodetector (not shown). The optical fiber 4b has a length of about 2 mm to 4 mm necessary for converging the meandering of two light beams incident from the condenser lens 3 at the shortest.

Two modified examples of the two-beam combining circuit shown in the configuration perspective view of FIG. 3A are shown in FIGS. Constituent elements corresponding to the respective parts in FIG.

In the two-beam combining circuit shown in FIG. 3B, a mirror 61 is provided at the end of the triangular groove 91 on the triangular groove block 71 at an angle of 45 ° with the optical axis direction of the optical fiber 4b1. The light emitted from the other end of 4b1 is configured to reflect upward.

Two interfering light beams 1 and 2 emitted from the delay interferometer based on the differential phase modulated optical signal are incident on the condenser lens 3. The two light beams 1 and 2 collected by the condensing lens 3 are incident on the core portion of one end of the optical fiber 4b1, are propagated through the optical fiber 4b1 and are combined, and then emitted from the other end. Then, the light is condensed by the lens 8 and incident on the PD chip surface 5 perpendicularly. An electrical signal corresponding to the light intensity detected on the PD chip surface 5 is output from the photodetector as a demodulated signal of the optical signal subjected to the differential phase modulation.

In the two-beam combining circuit shown in FIG. 3C, the other end of the optical fiber 4b2 on the triangular groove 92 of the triangular groove block 72 is polished so as to form an angle of 45 ° with the optical axis direction. The combined light propagating through the optical fiber 4b2 is reflected upward by a total reflection condition or by a reflection mirror formed by vapor deposition on the polished surface.

The two interfering light beams 1 and 2 emitted from the delay interferometer based on the differential phase-modulated optical signal are guided into the optical fiber 4b2 as in the case of FIG. 3B, and the other end of the optical fiber 4b2 After being reflected upward by the polished surface, the light is condensed by the lens 8 and incident perpendicularly on the PD chip surface 5b. Other operations are similar to those in the case of FIG.

3 has the same characteristics as those shown in FIGS. 1 and 2 and is not limited by the curvature of the optical fiber when the optical path is changed. The length can be shortened, and miniaturization becomes easy. However, it is preferable that the optical fiber 4b has a length of about 2 mm to 4 mm as a minimum length necessary for convergence of the meandering of two light beams incident from the condenser lens 3.

Further, the configuration shown in FIG. 3 may be more advantageous depending on the wiring method of electric signals, and wire bonding is easy because the surface of the wiring substrate and the surface of the PD chip are parallel.

  In the embodiment of FIG. 3, the angle that the mirror makes with the optical axis direction is not limited to 45 °, but an arbitrary oblique angle is taken according to the angle formed by the optical axis direction of the optical fiber 4b and the PD chip surface 5b. be able to.

In the case of an interferometer used in a specific polarization state, not only the optical fiber 4b but also an optical waveguide can be used.

Further, an optical waveguide having no polarization dependency, for example, an optical waveguide designed in a single mode may be used instead of the optical fiber 4b.

Further, it can be combined with various delay interferometers such as a DPSK delay interferometer and a DQPSK delay interferometer.

  The present invention is not limited to the above-described embodiments and modifications, and includes many changes and modifications without departing from the essence thereof.

1, 2 Interfering two light beams 3 Condensing lenses 4, 4a, 4b, 4b1, 4b2 Optical waveguide 41, 41a Optical waveguide end 42, 42a Core portion 43, 43a Optical waveguide other end 5, 5a, 5b Photodetector 61, 62 mirror

Claims (10)

In a two-beam combining circuit that combines two interfering light beams and outputs an electrical signal corresponding to the light intensity from the photodetector,
A condensing lens that condenses the two interfering light beams;
An optical waveguide for entering and combining the two light beams collected by the condenser lens at one end thereof;
A two-beam combining circuit comprising: a photodetector that receives light emitted from the other end of the optical waveguide. 2. The two-beam combining circuit according to claim 1, wherein a light receiving surface of the photodetector is attached to the other end of the optical waveguide. 2. The two-beam combining circuit according to claim 1, further comprising a mirror that reflects the light emitted from the optical waveguide, and the light reflected by the mirror is incident on the photodetector. 2. The two-beam combining circuit according to claim 1, wherein light reflected under a total reflection condition on an end face of the optical waveguide polished obliquely enters the photodetector. 3. 2. The two-beam combining circuit according to claim 1, wherein a reflection mirror is deposited on an end face of the optical waveguide polished obliquely, and light reflected by the reflection mirror enters the photodetector.   6. The two-beam combining circuit according to claim 1, wherein the optical waveguide has a length sufficient to converge the meandering of the two light beams.   The two-beam combining circuit according to claim 1, wherein an optical fiber is used as the optical waveguide. 8. The two-beam combining circuit according to claim 7, wherein the optical fiber is bent so that the two light beams are perpendicularly incident on the one end thereof. In a demodulator that demodulates a differential phase modulated optical signal,
A delay interferometer that emits two light beams that interfere based on the optical signal;
A condenser lens that collects the two light beams emitted from the delay interferometer;
An optical waveguide for entering and combining the two light beams collected by the condenser lens at one end thereof;
A demodulator comprising: a photodetector that receives light emitted from the other end of the optical waveguide.   The demodulator according to claim 9, wherein an optical fiber is used as the optical waveguide.

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