KR100400842B1 - In-line fiber-optic intensity modulator - Google Patents

Abstract

The present invention relates to an in-line optical modulator using side polished fiber (SPF), and a method of manufacturing and driving the same.

The present invention is based on a device using a mode coupling phenomenon between a side polished optical fiber and a ferroelectric thin film waveguide formed on the optical fiber. The thickness of the ferroelectric thin film used here can be from several tens of micrometers to 1 micrometer, and an electrode layer is attached to both surfaces of the thin film to cause a refractive index change in the dielectric. Since the refractive index change shifts the channel drop curve in the wavelength axis when the mode is combined, light of a specific wavelength applied induces a change in the amount of transmitted light passing through the optical fiber by the shift. do. On the other hand, the refractive index change is due to the polarization change accompanying the change in the electric field applied to the ferroelectric, and in the present invention, the electrically modulated signal is changed to the modulation of the optical signal intensity using the slope of the P-E history curve.

Description

In-line optical modulator and its manufacturing and driving method {IN-LINE FIBER-OPTIC INTENSITY MODULATOR}

The present invention relates to an inline type optical fiber modulator used in optical communication, and more particularly, to an optical modulator using a mode combination of a side polished fiber (SPF) and an overlay layer in an external modulation method. And an inline optical fiber modulator related to a driving method.

In general, an optical modulator refers to an apparatus for modulating an electrical signal into an optical signal when an optical fiber or a free space is used as a transmission medium.

As the modulation method of the optical modulator, it can be considered to be divided into direct modulation and external modulation. Direct modulation is a modulation method in which an injection current that determines an output signal of a laser diode is used as an input signal, and an output optical signal is directly controlled by an input signal. However, in the direct modulation method, the refractive index of the laser diode active region is changed by the change of the carrier density according to the change of the injection current, so that the emission wavelength is rapidly changed, causing optical signal distortion. This phenomenon is called chirping and is a factor that drastically degrades the performance of optical transmission system in the 1550 nm wavelength band. Therefore, to eliminate the above-mentioned chirped outside using an integrated optical system, a modulation scheme is the external modulation method above has been required in the long-distance transmission lithium niobate (LiNbO _3), lithium tantalate (LiTaO _3), using a ferroelectric, such as electro-optic polymer Light modulators are now mainstream.

Ferroelectrics were first known and discovered in 1920 in a substance called Rochelle's salt, and many studies began when the KDP-based single crystals were grown between 1935 and 1938. The characteristic properties of these materials not only have spontaneous polarization, but also the phenomenon of spontaneous polarization reversal by an electric field.

A conventional external modulation type ferroelectric light modulator will be described with reference to FIGS.

The optical signal introduced into the input terminal 101 is divided into two signals having the same power in the Y-type optical power splitter 102, and then each of the two signals by the optical combiner 112 through the modulation region 116 again. It is combined into one modulated signal and outputs to the output stage 114. When no electric field is applied to the modulation region 116 by the electrodes 108 and 110, the optical signal introduced into the input optical waveguide 101 is divided into two optical signals, and then both signals are again combined without the phase change. An optical signal identical to the optical signal combined and input at 112 is output. Meanwhile, as shown in FIG. 1B, when the two optical waveguides 104 and 106 in the modulation region 118 are applied with the electric field by the first electrode 108 and the second electrode 110, the optical waveguide ( Each optical signal passing through 104 and 106 has a phase difference from each other. Therefore, the light intensity modulated by the phase difference according to the electric field applied to the output waveguide 114 is output.

This type of modulator is capable of 10 GHz modulation with drive voltage variations of less than 10V. Optical modulators using semiconductors also have an interference absorption type and an electric field absorption type using a phenomenon in which the degree of absorption changes at a given wavelength according to the change in the electric field applied to the semiconductor layer. It is reported that the interferometer type can modulate from 1.5V change to 14GHz and the electric field absorption type can modulate from 3V voltage change to 20GHz.

However, the integrated optical modulator has an inevitable problem such as high loss efficiency when combined with the optical fiber.

Another way to use external modulation is in-line fiber modulators. In-line fiber modulators have an overlay layer of polymeric material or electro-optic crystals on the side polished optical fiber. The optical signal mode passing through the optical fiber is mode-coupled in a specific matching condition with the guided mode of the overlay layer, which appears as a channel drop phenomenon. The change in the refractive index of the overlay layer is controllable by the change in the applied electric field, and the accompanying shift in the channel drop curve results in the change in light intensity at a particular wavelength (eg, diode laser wavelength).

MOEC's product in the United States reports that 40GHz modulation is possible using an organic crystal called DAST. In general, optical crystals including DAST used in modulators generally have large electro-optic constants. However, this crystal overlay must be accompanied by a thin flake process to lower the modulation voltage. However, there is a limit (20-30μm) in processing the thin thickness of the crystal flakes, and there is a difficulty in maintaining the flatness and parallelism precisely during the polishing process.

In the case of a modulator using a polymer waveguide as an overlay, the above-described problems such as thickness, flatness, and parallelism can be solved through spin coating, but a spin coated polymer has a process called polling. It is necessary to align the electric dipoles in one direction. However, the aligned dipoles relax over time, and the polymer itself has many problems that are still to be solved because of its thermally and mechanically unstable characteristics.

An object of the present invention is to provide a device using a mode coupling phenomenon between the side polished optical fiber and the ferroelectric thin film waveguide formed on the optical fiber. It also aims to provide a compact optical modulator that has a low insertion loss (0.2 dB or less), operates at a low drive modulation voltage (a few volts), and is thermally, mechanically stable, and stable in time.

Another object of the present invention is to provide a method of driving an optical modulator using an inclined slope of a ferroelectric P-E history curve.

1 is a cross-sectional view of an optical modulator using a side polished optical fiber according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line B-B of FIG. 1. FIG.

3 is another embodiment according to FIG. 2;

4 is a P-E hysteresis loop.

5 is a channel drop curve shifted in the wavelength axis due to the overlay refractive index according to the practice of the present invention.

6 is a change in intensity of transmitted light accompanying an external electric signal according to an embodiment of the present invention;

Figure 7 is a process flow chart for explaining the manufacturing method of the optical modulator using the side polishing optical fiber according to the practice of the present invention.

8 is a cross-sectional view of a conventional optical modulator.

9 is a sectional view of FIG. 8;

In order to achieve the above object, the present invention realizes an in-line type optical modulator by adopting a side polished fiber (SPF) having extremely low insertion loss. The SPF has a structure as shown in FIG. 1 such that the optical fiber mode enables optical energy exchange with the upper overlay layer by evanescent coupling.

The present invention includes a ferroelectric thin film having a polarization direction perpendicular to and horizontal to the polishing surface when the thin film is manufactured. The thickness of the ferroelectric thin film was determined to be effective mode coupling at the wavelength of the input light signal, the refractive index changed when the electric field is applied causes the shift of the mode coupling wavelength, thereby changing the intensity of the transmitted light.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

1 is a cross-sectional view of an optical modulator according to an exemplary embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along line B-B of FIG. 1. 1 and 2, the optical modulator 10 of the present invention includes a substrate 20 made of a quartz block or a silicon wafer, and a predetermined curvature R on the substrate 20. And an optical fiber 30 planted with epoxy or a similar adhesive along the grooves formed to have. The optical fiber 30 includes a core 40 and a cladding 50.

One cross section 65 of the cladding 50 shows the surface polished by general polishing or chemical mechanical polishing. The remaining thickness of the cladding can be modulated with respect to the incident wavelength by the liquid drop method. Make it thick.

The substrate 20 is composed of an overlay layer 70 having ferroelectric properties, such as lithium niobate (LiNbO ₃ ), but the first electrode layer 60 functioning as an electrode is first stacked, and then sequentially As an overlay layer, the overlay layer 70 and the optical fiber 30 are formed as a transparent electrode layer so that the mode coupling is performed. The thickness of the overlay layer is determined as a thickness capable of dissipating wave coupling. After that, on the layer 70, a current leakage prevention film 80 such as SiN _x , SiO _x, etc. is prevented as necessary, and finally, another electrode layer 90 is stacked.

The shape of the electrode to be applied to the ferroelectric overlay layer may be a vertical electrode form as shown in FIG. 1, or a horizontal electrode form as shown in FIG. However, in the transverse electrode type, the above-described leakage preventing film 80 is not necessary. At this time, the gap between the electrodes 60 is minimized and adjusted so that the refractive index change is maximized.

4 is a curve showing the change of polarization with respect to the electric field applied on the PE history curve. In most ferroelectric thin films, the voltage for reversing polarization is several to several tens of volts, and the minimum reversible time is several hundred nanoseconds. An important point of the ferroelectric thin film used in the modulator of the present invention is to use a ferroelectric thin film having a PE history curve shown in FIG. 4 (Kwang-Ho Kim, "Metal-Ferroelectric-Semiconductor (MFS) FET's Using LiNbO ₃ / Si ( 100) Structures for Nonvolatile Memorry Application ", IEEE Trans. Electron Device, vol 19, No. 6, pp204-206, June 1998), using slopes (a) (b) shown in bold solid lines in the PE history curve of the above-mentioned thin film. Is the point. The larger the inclination of the oblique line, the larger the polarization change is possible even with the small change of the applied voltage, and thus the larger refractive index can be changed. Therefore, it is desirable that the polarization curve be as large as possible with the slope of the slope as possible. On the other hand, since the modulation in the present invention does not use polarization inversion as described above, it can be modulated with a small voltage difference, and much higher frequency modulation is possible than the polarization inversion.

FIG. 5 is a characteristic graph of an optical signal transmitted through an optical fiber and shows a periodic channel drop appearing when the optical fiber is combined with an overlay mode. The change in the refractive index of the overlay layer induces the shift of the peak wavelength (λ ₁ ↔ λ ₂ ) as shown in FIG. 5 and is seen as a change in the transmitted light intensity when viewed at a specific wavelength (eg, λ ₁ ).

FIG. 6 illustrates the optical modulation principle using the shift of the resonance wavelength at the specific incident laser wavelength of FIG. 5. The incident modulation field c induces modulation of the resonance wavelength at the same frequency by the principle of Fig. 5, which leads to modulation d of the light output. Since the refractive index change of the present invention uses the vibration of the polarization rather than the polarization inversion of the overlay layer, the modulation frequency can be made much higher than the limit frequency of the polarization inversion.

7 is a process flowchart for explaining a process for manufacturing the light modulator of the present invention.

Forming a groove having curvature R on the side of a silicon wafer or a quartz block by etching or string polishing (S1), and then planting an optical fiber 30 in the groove, and then applying an adhesive Attaching the optical fiber to the grooves (S2), polishing the upper part of the substrate 20 to which the optical fiber is attached to make the cladding of the optical fiber to a specific thickness (S3), and for applying the electric field on the substrate. Depositing a first electrode layer (transparent electrode) to a specific thickness (S4), depositing a ferroelectric thin film such as lithium niobate (LiNbO3) on the first electrode layer (S5), and a current in the ferroelectric thin film as necessary. The manufacturing process is performed by stacking a current leakage prevention film for preventing leakage on the ferroelectric thin film (S6), and finally depositing a second electrode layer for applying an electric field (S7).

It is possible to solve the insertion loss which is a problem in the integrated circuit modulator by realizing the in-line modulator using the lateral abrasive optical fiber provided by the present invention, and by using the ferroelectric thin film as an overlay, in the modulator using the crystal The problem of polishing limit thickness, flatness and parallelism was solved.

In addition, the use of inorganic material as an overlay material solves the problem of deterioration in modulation and thermal and mechanical instability, which is a problem in polymer in-line modulators, and the side of the PE history curve as an overlay material. Since the ferroelectric thin film having a linear and large slope is used, the low modulation voltage and the high modulation frequency can be realized.

Meanwhile, although illustrated and described with respect to embodiments of the present invention, modifications and changes may be made by those skilled in the art. Accordingly, the following claims are intended to cover all such alterations and modifications without departing from the spirit and scope of the invention.

Claims (10)

delete An optical fiber installed on the substrate with a predetermined curvature and including a core and a cladding for transmitting an optical signal; And a ferroelectric thin film formed by polishing one side of the cladding, the ferroelectric thin film formed of an inorganic material and an electro-optic polymer and causing a refractive index change by applying an electric field, and an electrode layer applying an electric field to the thin film. line modulator. delete delete An optical fiber installed on the substrate with a predetermined curvature and including a core and a cladding for transmitting an optical signal; And an electrode layer which is installed by polishing one side of the cladding and is installed in a horizontal shape with a ferroelectric thin film that causes a refractive index change by applying an electric field and applies an electric field to the thin film. Fluorescent modulator. An optical fiber installed on the substrate with a predetermined curvature and including a core and a cladding for transmitting an optical signal; And an electrode layer configured to polish one side of the cladding, the ferroelectric thin film causing a refractive index change by applying an electric field, and an electrode layer applying an electric field to the thin film, wherein the electrode side is installed in a vertical shape, and the first electrode layer and the second electrode layer. An in-line type optical modulator, characterized in that the first electrode layer is a transparent electrode. delete An optical fiber installed on the silicon wafer substrate with a predetermined curvature and including a core and a cladding for transmitting an optical signal; And an electrode layer configured to polish one side of the cladding, the ferroelectric thin film causing the refractive index change by applying an electric field, and an electrode layer applying the electric field to the thin film. Forming a groove having a predetermined curvature R on one side of the silicon wafer by etching or string polishing (S1), and then planting the optical fiber 30 in the groove, and then using the adhesive to groove the optical fiber Attaching to the substrate (S2), polishing the upper portion of the substrate 20 to which the optical fiber is attached to make the cladding of the optical fiber to a specific thickness (S2), a first electrode layer (ITO) for applying an electric field on the substrate E) to a specific thickness (S4), depositing a ferroelectric thin film on the first electrode layer (S5), if necessary, a current leakage prevention film for preventing current leakage in the ferroelectric thin film on the ferroelectric thin film The deposition method (S6), and finally the step of depositing a second electrode layer for applying the electric field (S7) of the manufacturing method of the in-line (in-line) light modulator. Light modulation of the electrical signal input through the ferroelectric thin film by changing the refractive index of the ferroelectric thin film by changing the voltage corresponding to the slope of the PE history curve showing the change in polarization degree according to the electric field applied to the ferroelectric thin film. A method for driving an in-line type optical modulator characterized by the above-mentioned.