JP4880784B2 - Optical waveguide type optical modulator with output light monitor - Google Patents

Description

  The present invention relates to an optical waveguide type optical modulator with an output light monitor. In particular, the present invention utilizes radiation mode light emitted from an optical waveguide as output light for monitoring in an appropriate direction, and the modulator itself. It relates to an optical waveguide modulator with an output light monitor that monitors the output light by simple monitoring means without changing the substantial structure, feedback controls the operating point of the optical modulator, and reduces noise in the monitor signal. is there.

In general, in order to maintain the output light of an optical modulator using an optical waveguide element in a constant output state, the output light of the optical modulator is monitored, and optical modulation is performed in response to changes in the output light. It is necessary to change the magnitude of the modulation voltage applied to the device.
As a method of monitoring the output light of an optical modulator using an optical waveguide element, conventionally, a directional coupler (coupler) or the like is disposed in the optical waveguide element to monitor light separately from the optical waveguide for optical signal output. A method of providing an output optical waveguide is generally performed. In this method, it is necessary to newly provide an optical circuit for monitoring light splitting in the optical waveguide element, and it is necessary to connect the optical fiber for monitor output to the optical waveguide element separately from the optical fiber for optical output signal There is.

  As another monitoring method, as disclosed in Patent Document 1, an inclined hole is provided in the clad portion on the optical waveguide, or a diffractive lens is disposed on the optical waveguide element, so that the signal in the optical waveguide is A system is known in which a part of the output light is taken out of the element substrate by this lens or the like. In this method, it is necessary to newly attach a monitor light extraction lens or the like on the optical waveguide type optical waveguide element. Since the monitor light is extracted above the optical waveguide element, The light receiving member must be attached to the optical waveguide element after it is mounted in the housing case, and this attachment requires considerable labor.

  Further, Patent Document 2 discloses a method in which the element end of an optical waveguide element is formed obliquely, a part of light output from the waveguide is reflected in an oblique direction, and the reflected light is received as monitor light. . In this method, it is necessary to select the inclined shape of the element end face within a range that does not adversely affect the main output light from the element. Therefore, there is a problem with the practicality of this method.

  Patent Document 3 describes a device in which a light receiving element is installed directly on an optical waveguide element, and a part of signal output light in the optical waveguide is directly received and monitored. In this device, it is necessary to mount the light receiving element mounting means on the optical waveguide element, and the mounting of the mounting means and the work for connecting the light receiving element to it and the adjustment work are performed by attaching the optical waveguide element to the optical waveguide element. This is done after mounting in the housing case. For this reason, the attachment and adjustment work of the light receiving element becomes considerably difficult, and the possibility of damaging the optical waveguide element increases.

  Based on the above, the present applicant, as shown in FIG. 1, in Patent Document 4, appropriately outputs radiation mode light 20 radiated from the union point of the branched optical waveguide portions in the optical waveguide 2 as output light for monitoring. The light intensity of the output light by the monitoring means using the capillary 4 having a simple structure and excellent workability and optical fiber operability without changing the substantial structure of the light modulator itself. We proposed an "optical waveguide modulator with output light monitor" that can monitor the output. In FIG. 1, 1 is a dielectric substrate, 3 is an optical fiber, 5 is a reflecting means for reflecting radiation mode light 20 formed in a capillary, 6 is a light receiving element for receiving radiation mode light, 7 is an optical waveguide element, and A housing containing a light receiving element or the like, 10 indicates incident light, and 11 indicates outgoing light.

  In addition to the radiation mode light described above, various stray light leaks from the substrate of the optical waveguide type optical modulator, and the light incident on the inside of the capillary is reflected by the inner surface of the capillary and part of them. Is incident on the photoelectric conversion element, and the monitor signal output from the photodetector also has a waveform containing a lot of background noise. For this reason, in the patent document 5, the applicant of the present invention is provided with reflection means for reflecting only a part of the radiation mode light toward the photodetector on the end surface opposite to the dielectric substrate side of the capillary. Proposed an “optical waveguide type optical modulator with an output light monitor” in which a light absorption layer is formed except for the reflection means and the optical path where the light reflected by the reflection means is directed to the photodetector.

  On the other hand, in order to realize a broadband optical modulation frequency, it is important to match the speed of the modulation signal microwave and the light wave, and various methods have been devised so far. Specific examples include thicker buffer layers, higher aspect ratios of electrodes, and ridge structures. In particular, in Patent Document 6 or 7 below, an optical waveguide and a modulation electrode are incorporated in an extremely thin substrate (hereinafter referred to as “thin plate”) having a thickness of 30 μm or less, and another substrate having a lower dielectric constant than that of the thin plate. In order to achieve speed matching between the microwave and the light wave, the effective refractive index for the microwave is lowered.

However, as the thickness of the substrate forming the optical waveguide becomes thinner, the light wave propagating through the optical waveguide tends to spread in a direction parallel to the substrate surface (a direction perpendicular to the thickness direction, hereinafter referred to as “lateral direction”). . For this reason, since the lateral mode diameter of the light wave output from the substrate is wider than the lateral width of the optical fiber, the light out of the output light from the output end of the optical waveguide 2 as shown in FIG. The light wave 21 (hereinafter referred to as “non-coupled light”) that leaks to the outside without being coupled to the fiber 3 increases, which causes a problem that it propagates through the reinforcing member such as the capillary 4 and enters the photodetector.
2, 3, 6, 7 and 11 below, the optical fiber 3 connected to the output end of the optical waveguide 2 is held in the capillary 4 in order to clarify the description of the uncoupled light 21. Description of the part to be performed is omitted.

FIG. 3 is a side view of the optical waveguide device shown in FIG. 2, and FIG. 4 shows a state where the side surface of the capillary of the optical waveguide device (particularly, the portion indicated by the dotted line 12 in FIG. 3) is observed. In FIG. 3, 1 is a dielectric substrate, 2 is an optical waveguide, 8 is an adhesive layer, and 9 is a reinforcing plate.
FIG. 4A shows a state in which the emitted light is in an ON state, and uncoupled light and some radiation mode light are observed. On the other hand, FIG. 4B shows a state in which the emitted light is in an OFF state, and radiation mode light and some uncoupled light are observed.

  As shown in FIG. 5A, normally, the radiation mode light B changes in a phase opposite to the signal light (main light) A output from the optical modulator, but as shown in FIG. When uncoupled light enters a light receiving element that receives radiation mode light, the signal observed as radiation mode light shifts from the reverse phase state of signal light A as shown by waveform B ′ in FIG. It becomes. However, the horizontal axis of FIG. 5 indicates the amount of change in the voltage applied to the optical waveguide element, and the vertical axis indicates the detected light intensity.

As shown in the waveform B ′ of FIG. 5B, the cause of the phase shift is that the optical path length until the radiation mode light reaches the light receiving element and the optical path length until the non-coupled light reaches the light receiving element. This is because a phase difference due to the difference in optical path length between the two occurs.
Further, the influence of the uncoupled light is not only the phase shift described above, but also receives the uncoupled light when the original radiation mode light does not come, so that the amplitude value of the waveform B ′ decreases, and the radiation mode light is detected. It also causes the deterioration of the extinction ratio.

  When the radiation mode light is detected by the light receiving element as the monitor light and the operating point of the signal light is controlled by adjusting the DC bias applied to the optical waveguide element based on the detection signal from the light receiving element, Since the element also receives uncoupled light, the relationship between the signal light and the monitor light shifts from the reverse phase state, and the extinction ratio of the monitor output deteriorates, so the state of the signal light to be controlled can be accurately determined Difficult to do.

JP 11-194237 A JP-A-5-34650 JP-A-5-53086 JP 2001-281507 A JP 2004-294708 A JP-A 64-18121 JP 2003-215519 A

  The problem to be solved by the present invention is to solve the above-described problems, and even when a thin plate is used as a dielectric substrate, the radiation mode light emitted from the optical waveguide as the output light for monitoring is more accurately obtained. An optical waveguide type optical modulator with an output light monitor for detection is provided.

  In order to solve the above-described problem, in the invention according to claim 1, the dielectric substrate is formed on the front surface or the back surface of the dielectric substrate. An optical waveguide device including an optical waveguide having an extended optical waveguide output section, an optical fiber connected to an output end of the optical waveguide output section, an output end of the optical waveguide output section, and the optical fiber An optical fiber reinforcing member that reinforces the connection portion with the end face, and a part of the radiation mode light that is radiated from a joint point of the branched optical waveguide portion and propagates through both sides of the optical waveguide output portion. An optical waveguide type optical modulator with an output light monitor having a body substrate and a photodetector for receiving light through the optical fiber reinforcing member, wherein the optical fiber reinforcing member is a hollow housing the connecting end portion of the optical fiber And the radiation module A reflection means for reflecting a part of the radiation mode light toward the photodetector is formed on the end surface of the capillary opposite to the dielectric substrate side, The hollow portion is provided with non-coupled light shielding means for preventing uncoupled light between the optical waveguide output portion and the optical fiber from entering the photodetector.

  According to a second aspect of the present invention, in the optical waveguide type optical modulator with an output light monitor according to the first aspect, the thickness of the dielectric substrate is 30 μm or less.

  According to a third aspect of the present invention, in the optical waveguide type optical modulator with an output light monitor according to the first or second aspect, the non-coupled light shielding means is a metal film disposed on the inner surface of the hollow portion of the capillary, a reflective film It is either a film or a light absorption film.

  According to a fourth aspect of the present invention, in the optical waveguide type optical modulator with an output light monitor according to the first or second aspect, the non-coupled light shielding means has a shape of a hollow portion of the capillary on the dielectric substrate side. It is characterized by expanding the hole on the end face.

  According to a fifth aspect of the present invention, in the optical waveguide type optical modulator with an output light monitor according to the first or second aspect, the non-coupled light shielding means is inserted between the hollow portion of the capillary and the optical fiber. It is a tubular insertion material.

  According to the first aspect of the present invention, it becomes possible to prevent the uncoupled light between the optical waveguide output section and the optical fiber from entering the photodetector. It becomes possible to detect more accurately.

  According to the second aspect of the present invention, since the thickness of the dielectric substrate is 30 μm or less, the structure of the first aspect is adopted even though the light modulator is likely to generate non-coupled light. Only mode light can be detected accurately.

  According to the invention of claim 3, the non-coupled light shielding means is any one of a metal film, a reflective film, and a light absorbing film disposed on the inner surface of the hollow portion of the capillary. Light can be effectively shielded.

  According to the invention of claim 4, the non-coupled light shielding means is such that the shape of the hollow portion of the capillary expands the hole on the end face on the dielectric substrate side, so that the end portion of the optical waveguide from which uncoupled light is emitted It is possible to dispose the non-coupled light shielding means in the vicinity of the lens and efficiently shield the uncoupled light before it propagates and spreads. Further, since the non-coupled light shielding means can be formed only by the process of expanding a part of the hollow portion of the capillary, it is possible to suppress the manufacturing process from becoming complicated and costly.

  According to the invention of claim 5, the non-coupled light shielding means is a tubular insert inserted between the hollow portion of the capillary and the optical fiber, so that it is close to the end of the optical waveguide from which the non-coupled light is emitted. The non-coupled light shielding means can be arranged, and by selectively propagating the non-coupled light in the tubular insert, it becomes possible to efficiently shield the non-coupled light before it propagates and spreads. .

It is the schematic of the conventional optical waveguide type optical modulator with an output light monitor. It is a top view which shows the state of the conventional capillary vicinity. It is a side view which shows the state of the conventional capillary vicinity. It is an observation figure which shows the state of the conventional capillary vicinity. It is a graph which shows the relationship between signal light and the monitor light of a light receiving element. It is a figure which shows the 1st Example (reference example) which concerns on this invention. It is a figure which shows the 2nd Example (reference example) which concerns on this invention. It is a figure which shows the 3rd Example based on this invention. It is a figure which shows the 4th Example which concerns on this invention. It is a figure which shows the 5th Example (reference example) which concerns on this invention. It is a figure which shows the 6th Example (reference example) which concerns on this invention. It is a figure which shows the 7th Example which concerns on this invention.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 6 shows a first embodiment (reference example) of an optical waveguide type optical modulator with an output light monitor according to the present invention.
The present invention has a dielectric substrate 1, two or more branch optical waveguide portions formed on the front surface or the back surface of the dielectric substrate, and an optical waveguide output portion extending from a joint point of the branch optical waveguide portions. An optical waveguide element including the optical waveguide 2, an optical fiber 3 connected to the output end of the optical waveguide output unit, and a connection part between the output end of the optical waveguide output unit and the end face of the optical fiber are reinforced. A part of the radiation mode light 20 radiated from the converging point of the optical fiber reinforcing member 4 and the branched optical waveguide portion and propagated through both sides of the optical waveguide output portion is transferred to the dielectric substrate and the optical fiber. An optical waveguide type optical modulator with an output light monitor having a photodetector (not shown) that receives light through a reinforcing member, wherein the optical fiber reinforcing member has a hollow portion that accommodates a connection end portion of the optical fiber. And having the radiation mode light propagated Reflecting means for reflecting a portion of the radiation mode light toward the photodetector is formed on the end surface of the capillary opposite to the dielectric substrate side, and the optical waveguide output A non-coupled light shielding means 30 is provided for preventing the uncoupled light 21 between the optical fiber and the optical fiber from entering the photodetector.

  In particular, in the optical modulator to which the present invention is applied, the thickness of the dielectric substrate is 30 μm or less, and in the optical modulator using such a thin plate, uncoupled light 21 that is not coupled to the optical fiber 3 increases. Therefore, the present invention can be effectively applied.

As the overall configuration of the optical waveguide type optical modulator with an output light monitor according to the present invention, the configuration of the conventional optical modulator shown in FIG. 1 can be adopted. In particular, a non-coupled light shielding means is formed on the capillary 4 or the like. Except for this point, it can be configured similarly.
Also in the present invention, the substrate 1 made of a ferroelectric material such as LiNbO ₃ is fixed in the case (housing) 7. For example, the Mach-Zehnder type optical waveguide 2 is formed on the surface portion. This Mach-Zehnder type optical waveguide 2 has an optical waveguide input section, a branched optical waveguide section branched therefrom, a multiplexing section of the branched optical waveguide section, and an optical waveguide output section, and a control (not shown) is provided on each branch section. Electrodes (signal electrodes, ground electrodes, etc.) are arranged. An input-side capillary is joined to the input end (right side in FIG. 1) of the substrate 1. An input-side optical fiber is introduced through the hollow portion of the capillary, and the tip end face thereof is connected to the input end face of the optical waveguide input portion.

Similarly, an output side capillary 4 and an output side optical fiber 3 are connected to the output end side of the optical waveguide 2.
The light wave enters the optical waveguide input section from the input side optical fiber as indicated by an arrow 10 and is distributed to the branch section. When an applied electric signal is applied to the control electrode via, for example, a connector (not shown) disposed on the side surface of the case 7, the optical phase of the light wave propagating through the branch optical waveguide portion changes according to the applied electric signal. . The light waves that have undergone the phase change are multiplexed at the multiplexing unit and interfere with each other to generate signal light. The signal light after the interference passes through the optical fiber 3 reinforced by the capillary 3 and is output to the outside of the case 7 as indicated by an arrow 11.

Of the two radiation mode lights radiated into the substrate 1 at the optical waveguide multiplexing unit 6, the radiation mode light 20 passes through the inside of the capillary 3 and is a reflection surface formed in the upper half of the tip surface of the capillary 3. 5 is reflected. When a substantially cylindrical shape is used as the capillary 3, the radiation mode light is collected on the cylindrical peripheral surface and is emitted in a direction substantially perpendicular to the output portion of the optical waveguide 2. The radiation mode light 20 enters the light receiving element 6 and outputs an electric signal corresponding to the light intensity of the radiation mode light.
It should be noted that the propagating light wave (signal light or radiation mode light) is reflected at the coupling portion between the optical waveguide 2 and the optical fiber 3, the outer peripheral surface of the capillary, and the light receiving surface of the light receiving element, so that it does not become return light. In addition, it is preferable that the angle formed between the traveling direction of the propagation light and the boundary surface is slightly inclined from the vertical state as in Patent Document 4 or 5.

  In the first embodiment (reference example), as shown in FIG. 6, the non-coupled light shielding means is configured by cutting off a part 30 of the reflecting means 5 of the capillary 4 that is close to the optical fiber 3. Has been.

  As a second embodiment (reference example) of the non-coupled light shielding means, as shown in FIG. 7, light is applied to the surface of the optical fiber 3 exposed from the capillary 4 or the surface of the capillary located in the vicinity of the optical fiber. An absorbent material 31 can be arranged. In particular, it is more effective to arrange the light absorbing material 31 at a position where the non-coupled light is reflected on the reflecting surface 5 of the capillary. The light absorbing material 31 can be formed by applying a coating material containing a light absorbing material such as carbon black to the surface of the capillary 4 and drying it. Needless to say, the light-absorbing material and the forming method are not limited to those described above, and techniques known in the art can be applied.

  As a third embodiment of the non-coupled light shielding means, as shown in FIG. 8, a metal film, a reflection film or a light absorption film indicated by 32 is formed on the inner surface of the hollow portion of the capillary through which the optical fiber 3 passes. Any one of them is arranged so that uncoupled light radiated from the joint portion between the optical waveguide 2 and the optical fiber 3 is confined in the hollow portion and is not radiated in the direction of the light receiving element.

  As a fourth embodiment of the non-coupled light shielding means, as shown in FIG. 9, the shape of the hollow portion of the capillary 4 expands the hole 33 on the end face on the dielectric substrate 1 side. The expanded inner surface of the hole 33 can prevent uncoupled light from entering the capillary. It is also possible to apply a light absorbing material to the inner surface of the expanded hole as required. Such a hole 33 can be easily formed at the time of forming a capillary.

  As a fifth embodiment (reference example) of the non-coupled light shielding means, as shown in FIG. 10, a reflection film or light absorption film such as 34 is formed between the capillary 4 and the dielectric substrate 1. Is. The shape of the reflection film or the like is arbitrary so long as it does not block the radiation mode light detected by the light receiving element while excluding the joint portion between the optical fiber 3 and the optical waveguide 2 in order to allow signal light to propagate to the optical fiber. It is possible to select the shape.

  As a sixth embodiment (reference example) of the non-coupled light shielding means, as shown in FIG. 11, the non-coupled light propagating in the capillary 4 is formed at a position 35 where it is emitted from the capillary to the outside. It is a light absorbing material provided on the cut portion of the capillary or on the capillary surface. As in the first embodiment, the shape of the cut portion can be selected as long as it prevents propagation of uncoupled light to the light receiving element. As shown in FIG. 4, the non-coupled light is radiated intensively from the vicinity of the region 35 of the capillary 4, and the shielding effect of the non-coupled light is enhanced only by providing a shielding means on the capillary surface in this region.

  A seventh embodiment of the non-coupled light shielding means is a tubular insertion member 35 inserted between the hollow portion of the capillary 4 and the optical fiber, as shown in FIG. By adopting this configuration, it becomes possible to confine uncoupled light radiated from the joint between the optical waveguide 2 and the optical fiber 3 in the tubular insertion member 35 so as not to be radiated in the direction of the light receiving element. . The tubular insertion member 35 may be attached to the outer periphery of the optical fiber 3 in advance, and then the capillary 4 may be attached, or conversely, the tubular insertion member 35 is attached in advance to the hollow portion of the capillary 4 and is inserted into the through hole of the hollow insertion member. The optical fiber 3 may be inserted and fixed.

The size and shape of the tubular insert 35 can be arbitrarily selected within a range that does not block the radiation mode light detected by the light receiving element. Here, it is preferable that the position and the shape of the end surface of the tubular insertion material opposite to the side in contact with the dielectric substrate are different from the reflecting surface of the capillary. In particular, as shown in FIG. 12, if the length of the tubular insert 35 is made longer than that of the capillary 4, separation of uncoupled light and radiation mode light propagated in the tubular insert is facilitated, and uncoupled. It is possible to easily prevent light from entering the photodetector. Furthermore, by disposing any of a metal film, a reflective film, and a light absorbing film on the inner surface of the hollow portion of the capillary or the outer surface of the tubular insertion member, the shielding effect of non-coupled light can be further enhanced.
The refractive index of the tubular insert is preferably the same as or higher than the refractive index of the capillary.

  According to the optical waveguide type optical modulator with an output light monitor according to the present invention, even when a thin plate is used as a dielectric substrate, the radiation mode light emitted from the optical waveguide as the output light for monitoring is detected more accurately. Therefore, it is possible to provide an optical waveguide type optical modulator with an output light monitor.

DESCRIPTION OF SYMBOLS 1 Dielectric substrate 2 Optical waveguide 3 Optical fiber 4 Capillary 5 Reflecting means 6 Light receiving element 7 Case 8 Adhesive layer 9 Reinforcement board 10 Incident light 11 Output light (signal light)
20 radiation mode light 21 non-coupled light 30 excision part 31 light absorbing material 32 metal film etc. 33 extension part 34 reflection film etc. 35 tubular insert

Claims (5)

A dielectric substrate; and an optical waveguide formed on the front surface or the back surface of the dielectric substrate and having two or more branched optical waveguide portions and an optical waveguide output portion extending from a joint point of the branched optical waveguide portions. Optical waveguide elements,
An optical fiber connected to the output end of the optical waveguide output section;
An optical fiber reinforcing member that reinforces the connection between the output end of the optical waveguide output section and the end face of the optical fiber;
Light that receives a part of the radiation mode light that is radiated from the union point of the branched optical waveguide part and propagates through both sides of the output part of the optical waveguide through the dielectric substrate and the optical fiber reinforcing member A detector;
In an optical waveguide type optical modulator with an output light monitor,
The optical fiber reinforcing member has a hollow portion that accommodates a connection end portion of the optical fiber, and is a capillary that propagates the radiation mode light;
Reflecting means for reflecting a part of the radiation mode light toward the photodetector is formed on the end surface of the capillary opposite to the dielectric substrate side,
An output light monitor characterized in that non-coupled light shielding means for preventing uncoupled light from the optical waveguide output section and the optical fiber from entering the photodetector is provided in the hollow portion. Attached optical waveguide type optical modulator.   2. The optical waveguide type optical modulator with output light monitor according to claim 1, wherein the dielectric substrate has a thickness of 30 [mu] m or less.   3. The optical waveguide type optical modulator with an output light monitor according to claim 1, wherein the non-coupled light shielding means is any one of a metal film, a reflective film, and a light absorbing film disposed on the inner surface of the hollow portion of the capillary. An optical waveguide type optical modulator with an output light monitor.   3. The optical waveguide type optical modulator with an output light monitor according to claim 1 or 2, wherein the non-coupled light shielding means is such that a shape of a hollow portion of the capillary extends a hole on an end face on the dielectric substrate side. An optical waveguide type optical modulator with an output light monitor.   3. The optical waveguide type optical modulator with an output light monitor according to claim 1, wherein the non-coupled light shielding means is a tubular insertion member inserted between the hollow portion of the capillary and the optical fiber. An optical waveguide type optical modulator with an output light monitor.

Images