JP3573180B2 - Polling method for Mach-Zehnder interferometer arm - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an integrated optical device having an optical waveguide disposed on a substrate, and more particularly, to a method for manufacturing a waveguide type optical device such as an optical switch used in the field of optical communication and the like.
[0002]
[Prior art]
As a single-mode silica-based optical waveguide mainly made of silica-based glass manufactured on a planar substrate, for example, “M. Kawachi“ Silica waveguides on silicon and their application to integrated-optical component ”J. Electronic Materials. , Vol. 22, 1990, pp. 391-416 (Literature 1).
[0003]
Such an embedded quartz optical waveguide can produce a waveguide as designed, due to the excellent workability of the quartz glass, and is excellent in mass productivity. In addition, since the silica-based optical waveguide has low loss and excellent connection matching with commonly used silica-based single-mode optical fibers, it is expected as a means for realizing practical integrated optical devices. Many optical devices such as a wavelength multiplexer / demultiplexer and an optical switch have been developed.
[0004]
Examples of an optical switch realized using a silica-based optical waveguide include, for example, “N. Takato, et al.,“ Silica-Based Single-Mode Waveguides on Silicon and their Application Application to Guide-Opt. 6, 1988, pp. 1003 to 1010 (Reference 2) ”and the like, there is a“ thermo-optic switch (TΟ switch) ”utilizing the thermo-optic effect.
[0005]
In a TO switch using a silica-based optical waveguide, a good switch with low loss and excellent integration can be realized, but its response speed is about 1 ms, and a silica-based optical switch capable of higher-speed response is required. .
On the other hand, recently, a refractive index change (electro-optic effect) caused by applying a voltage has been reported for a silica-based fiber that has been subjected to a poling treatment.
[0006]
Usually, quartz-based glass is a random glass and is considered to have pseudo-central symmetry, and the first-order electro-optic effect (Pockels effect: change in refractive index in proportion to the applied electric field intensity) can be viewed in principle. Can not.
[0007]
However, the Pockels effect can be induced by performing a “thermal polling” process on such a glass system such as “increase the temperature while applying an electric field and decrease the temperature while applying the electric field”. Induction of the Pockels effect by this poling treatment is also observed in optical fibers containing silica-based glass as a main component, and recently, a refractive index change (electro-optic effect) caused by voltage application has been reported for a silica-based fiber subjected to poling treatment. (See, for example, "PG Kazansky, et al." Pockels effects in thermally polled silica optical fibers ", Electronics Lett., Vol. 31, 1995, pp. 62-63.).
[0008]
The response speed due to the Pockels effect in the quartz glass induced by the poling process is very high, and has a response speed of 10 ns or less. That is, it is shown that, by inducing the Pockels effect in the silica glass waveguide, the refractive index of the waveguide can be controlled at a high speed of 10 ns or less (100 MHz or more) by applying a voltage.
[0009]
In order to realize a high-speed optical switch or a light intensity modulator having a high extinction ratio by using the electro-optic effect, it is useful to configure an interferometer such as a Mach-Zehnder interferometer (MZI). A Mach-Zehnder interferometer composed of a silica-based fiber, a bulk optical component, or the like is susceptible to external disturbances such as temperature fluctuations, is unstable, and has a problem that it cannot be a practical optical device.
[0010]
On the other hand, the MZI composed of a quartz-based waveguide on a Si substrate described in Document 1 described above is more resistant to external disturbances than the MZI composed of an optical fiber or a bulk optical component, and has a higher transmission. Optical characteristics such as light intensity are stable. Further, the interferometer of the silica-based waveguide has an advantage that the processing of the silica-based glass is easy, so that high-precision processing can be performed, and the fabrication according to the design value is possible.
[0011]
A thermal poling process is applied to the arm waveguide part of the Mach-Zehnder interferometer composed of this silica-based waveguide to induce the electro-optic effect. And a practical optical switch and an optical intensity modulator having the above.
[0012]
However, the efficiency of the Pockels effect induced by "thermal poling" as shown in the aforementioned document 3 is about 0.05 pm / V in terms of the electro-optical constant r, which is not a very large value. Was.
[0013]
As means for improving the efficiency of the Pockels effect, a Ge-doped silica-based optical fiber is externally irradiated with an ultraviolet laser beam (wavelength: 193 nm) while applying an electric field, that is, "optical poling"("optically-excitedpoling"). It has been reported that a large electro-optical effect of an electro-optical constant r = 6 pm / V was induced by performing “induced polling” or “optically assisted polling” (for example, “T”). Fujiwara, D. Wong, Y. Zhao, S. Fleming, S. Pool and M. Seats, Electron. Lett., 31, 1995, 573 (Reference 4)).
[0014]
[Problems to be solved by the invention]
However, when the above-described poling method is applied to a waveguide manufactured on a flat substrate, there is a problem that light emitted from the outside often damages an electrode manufactured near the waveguide. Was. In addition, since the core portion is irradiated with light from the outside, an electric field is applied in a direction perpendicular to the substrate, and there is a problem in electrode fabrication that an electrode cannot be arranged above the core in the vertical direction.
[0015]
An object of the present invention is to provide an optical waveguide polling method capable of manufacturing a silica-based waveguide optical device having low loss, excellent workability and integration, and high-speed response.
[0016]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides an optical waveguide formed by surrounding a core made of silica-based glass on a substrate with a clad made of silica-based glass and having a lower refractive index than the core . And a method of performing a polling process on an optical waveguide including two arm waveguides connecting the two directional couplers , wherein the polling process is completed while propagating ultraviolet light or visible light to the optical waveguide . A poling method for a Mach-Zehnder interferometer arm that increases an electro-optic constant of the arm waveguide by applying an external electric field to the arm waveguide by an electrode disposed on an over clad that controls the refractive index of the arm waveguide. suggest.
[0017]
`` Propagation irradiation light polling, '' which applies an electric field while `` propagating irradiation '' of ultraviolet light or visible light to an optical waveguide, is an optical waveguide that extends over 10 cm or more by high intensity light confined in the core part, Light irradiation can be performed at once. Also, when performing "external irradiation", the electrode for voltage application and the clad portion provided near the core were often damaged, but according to "propagation irradiation", the electrode was not damaged. Light irradiation can be performed. In addition, since the light is emitted by propagating through the core portion, an electric field can be applied in a direction perpendicular to the substrate (TM direction), and an electrode can be arranged above the core in the vertical direction. There are few restrictions on the above.
[0018]
An electro-optic effect is induced in the portion of the MZI arm waveguide that has been subjected to the “optical poling process”, and the refractive index changes when an electric field is applied.
[0019]
For example, the magnitude Δn of the refractive index change that occurs when an external electric field E _ex is applied in the TM direction is:
Δn _TE = (１ ／) r ₁ n _TE ³ E _ex
Δn _TM = (１ ／) r ₂ n _TM ³ E _ex (1)
(For example, see “Nishihara et al.“ Optical Integrated Circuit ”(Ohm Du))”. Here, r ₁ and r ₂ indicate the electro-optic constants in the TE and TM directions corresponding to the case where an external electric field is applied in the TM direction, and n _TE and n _TM indicate the refractive indexes in the TE and TM directions, respectively.
[0020]
Therefore, the greater the external electric field intensity, the greater the change in the refractive index can be obtained.
[0021]
This electric field can be applied by directly applying a voltage to the electrode used at the time of polling. The MZI can be operated as an optical switch or an optical intensity modulator by utilizing the change in the refractive index (electro-optic effect) caused by the application of the electric field in the arm waveguide. At this time, since the MZI is manufactured on the substrate, it is a practical optical component that operates more stably against external disturbances such as temperature fluctuations than the MZI composed of optical fibers and bulk optical components. .
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
1A and 1B show an example of a waveguide type optical device manufactured by the method of the present invention, in which an optical switch having a Mach-Zehnder interferometer is shown. FIG. 1A is an overall perspective view, and FIG. It is a fragmentary sectional view. In the figure, 11 is a Si substrate, 12 and 13 are waveguides (GeO ₂ -doped silica glass core), 14 is under cladding, 15 is over cladding, 16 is a thin film electrode, and 17 and 18 are waveguides 12 and 13 in proximity. This is a directional coupler configured by the above.
[0023]
Here, the fabrication of the optical waveguide was performed in the same manner as in, for example, the method described in the above-mentioned document 2. That is, a glass film layer mainly composed of silica-based glass to be the under clad 14 and the cores 12 and 13 is formed on the Si substrate 11 by a flame deposition (FHD) method, and then the core is formed by reactive ion etching (RIE). A ridge structure was formed in a portion, and the ridge structure was buried again by the FHD method with the over clad 15 mainly composed of quartz glass to produce an optical waveguide. Here, the core was formed of Ge-added quartz glass, the relative refractive index difference Δ between the core and the clad was 0.7%, and the core structure was rectangular and 7 μm × 7 μm.
[0024]
The Mach-Zehnder interferometer is constituted by the two directional couplers 17 and 18 described above and the arm waveguide that connects between the directional couplers 17 and 18 of the waveguides 12 and 13.
[0025]
After fabrication of the waveguide, chromium Cr and gold Au were deposited near the core of one of the arm waveguides and patterned into a desired shape to form the electrode 16. Here, the length L of the electrode part parallel to the core was 6.5 cm, and the electrode interval d was 45 μm. The material used for forming the electrode may be any material having high conductivity, such as Pt, NiCr, Ta ₂ N, and Al.
[0026]
As shown in FIG. 2, a mode-locked (ML) Q-switch (Qsw) operation Nd ³⁺ : YAG laser (ML-Qsw-Nd ³⁺ : YAG laser) 21 for the optical switch manufactured as described above, for example, 10 A second harmonic (SH) light (wavelength: 532 nm) of light having a wavelength of 1064 nm is generated by the KTP crystal 22, and the SH light passes through the light having a wavelength of 1064 nm, and the dichroic mirror 23 reflects the light having a wavelength of 532 nm. A voltage of 5 kV was applied from the voltage source 26 for 30 minutes while being guided to the port P1 via the mirror 24 and the lens 25 having a wavelength of 532 nm and having a reflectance of 100% and propagating through the waveguide 12.
[0027]
Since the directional couplers 17 and 18 have the wavelength characteristics shown in FIG. 3, the light introduced from the port P1 is not coupled to the waveguide 13 but all propagates through the waveguide 12. After 30 minutes, the SH light was shut off and the voltage was reduced to 0V. The pulse time width of the mode-locked laser light was about 100 ps, the mode locking frequency was 82 MHz, and the Qsw repetition frequency was 800 Hz.
[0028]
After this poling process, light from a semiconductor laser having a wavelength of λ = 1.3 μm was input from the port P1 with TM polarization using a polarization maintaining fiber. A voltage is applied between the electrode 16 and the Si substrate 11 while the intensity of the output light from the ports P3 and P4 is detected by a thermal power meter (Thermal PM) 28 through the lens 27, and the output light is output. The change in intensity was measured (a photodiode may be used instead of the thermal power meter).
[0029]
FIG. 4 shows a change in the output light intensity normalized with respect to the applied voltage at this time. It is shown that the phase changes almost in proportion to the applied voltage V. That is, the refractive index change Δn is proportional to the applied electric field strength.
[0030]
The amount of phase change Δφ is
_{Δφ = 2πη (1 / λ)} (n e 3/2) r (ΔV / d) L ...... (2)
Can be represented by Here, η is the coupling coefficient, _ne is the refractive index of the core, d is the distance between the electrodes, L is the interaction length (the length of the external electric field applied to the core), ΔV is the applied voltage, and λ is the measurement wavelength. , R are electro-optic constants.
[0031]
In this _{example, λ = 1.3μm, n e =} 1.454, and d = 45 [mu] m, and L = 6.5cm. The voltage V _{π at which the} phase changes by _{π is} 180 (V), and the electro-optical constant r at this time is evaluated to be 1.6 pm / V. The extinction ratio of this electro-optical switch was 35 dB, and the loss was 1 dB.
[0032]
As described above, the present invention is very excellent as a method for realizing an optical switch having low loss, high extinction ratio, and high-speed response.
[0033]
FIG. 5 shows another example of a waveguide type optical device manufactured by the method of the present invention, here an optical intensity modulator having a Mach-Zehnder interferometer. In the figure, 31 is an Si substrate, 32 and 33 are waveguides (GeO ₂ -added quartz glass core), 34 and 35 are thin film electrodes, 36 is a thin film heater, and 37 and 38 are waveguides 32 and 33 arranged close to each other. This is a directional coupler.
[0034]
Here, the fabrication of the optical waveguide was performed in the same manner as in, for example, the method described in the above-mentioned document 2. That is, a glass film layer mainly composed of quartz-based glass to be an under clad (not shown) and cores 32 and 33 is formed on a Si substrate 31 by a flame deposition (FHD) method, and thereafter, reactive ion etching is performed. The ridge structure of the core portion was formed by (RIE), and burying was performed again by the FHD method with an over clad (not shown) mainly composed of silica glass to produce an optical waveguide. The core was formed of Ge-added quartz glass, the relative refractive index difference Δ between the core and the clad was 0.3%, and the core structure was rectangular and 8 μm × 8 μm.
[0035]
The Mach-Zehnder interferometer is constituted by the two directional couplers 37 and 38 described above and the arm waveguide connecting between the directional couplers 37 and 38 of the waveguides 32 and 33.
[0036]
After the fabrication of the waveguide, chromium Cr and gold Au were deposited near the core of one of the arm waveguides and patterned into desired shapes to form electrodes 34 and 35. Here, the length L of the electrode portion parallel to the core was 8 cm, and the electrode interval d was 40 μm. The material used for forming the electrode may be any material having high conductivity, such as Pt, NiCr, Ta ₂ N, and Al.
[0037]
Further, the chromium thin film heater 36 is patterned on the arm waveguide opposite to the arm waveguide on which the electrodes 34 and 35 are formed, and the phase of the MZI using the thermo-optic effect can be adjusted.
[0038]
For the light intensity modulator manufactured as described above, the second harmonic (SH) light of the light from the Q switch operation Nd ³⁺ : YAG laser is generated by the KTP crystal, and the SH light is output to the port P1. Then, while propagating through the waveguide 32, a voltage of 5 kV was applied for 30 minutes. After 30 minutes, the second harmonic light was cut off, and the voltage was reduced to 0V. The repetition frequency of Qsw was 1 kHz.
[0039]
After this poling process, light from a semiconductor laser having a wavelength of 1.55 μm was input from the port P1 with TE polarization using a polarization maintaining fiber. While detecting the output light intensity from the port P3 with a thermal power meter, a modulation voltage of 1 GHz was applied between the electrodes 34 and 35 to modulate the intensity of the light of the semiconductor laser having a wavelength of 1.55 μm. A part of the arm waveguide was heated by the thin film heater 36 so that the modulation intensity was maximized, and the phase of the MZI was adjusted using the thermo-optic effect.
[0040]
FIG. 7 shows the modulated light intensity characteristics at this time. It is shown that the light of a semiconductor laser having a wavelength of 1.55 μm is modulated to 1 GHz. The loss of the present light intensity modulator was 1 dB, and the extinction ratio was 30 dB.
[0041]
As described above, the present invention is very excellent as a method for realizing a light intensity modulator having a low loss, a high extinction ratio, and a high-speed response. It is also useful to use the present invention in combination with thermal poling to improve the poling efficiency.
[0042]
【The invention's effect】
As described above, according to the present invention, since the polling method of applying an external electric field while propagating ultraviolet light or visible light to the optical waveguide, an electrode for applying an external electric field is provided directly above the optical waveguide. Has the advantage that the degree of freedom in the design of the electrode structure is large, and that a large electro-optical effect can be induced as compared with the case where the conventional "thermal poling" is performed. . Therefore, the present invention makes it possible to realize an optical switch and an optical modulator having high-speed response that are practical in the field of optical communication and the like.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an example of a waveguide type optical device manufactured by the method of the present invention. FIG. 2 is a configuration diagram of an apparatus for performing the method of the present invention. FIG. 3 is a coupling ratio of the directional coupler in FIG. FIG. 4 is a switching characteristic diagram of the optical switch shown in FIG. 1. FIG. 5 is a configuration diagram showing another example of the waveguide type optical device manufactured by the method of the present invention. FIG. 6 is shown in FIG. Of the modulated light intensity of the modulated light intensity modulator [Explanation of symbols]
10 optical switch, 11, 31 Si substrate, 12, 13, 32, 33 waveguide, 14 under cladding, 15 over cladding, 16, 34, 35 thin film electrode, 17, 18, 37, 38 ... Directional coupler, 21: mode-locked Q-switch operation Nd ³⁺ : YAG laser, 22: KTP crystal, 23, 24: mirror, 25, 27: lens, 26: voltage source, 28: thermal power meter, 36: thin film heater.

Claims (1)

An optical waveguide produced by surrounding a core made of silica-based glass on a substrate with a clad made of silica-based glass and having a lower refractive index than the core , and connecting two directional couplers and the two directional couplers. A method of performing a polling process on an optical waveguide composed of a plurality of arm waveguides ,
While propagating ultraviolet light or visible light to the optical waveguide , by applying an external electric field to the arm waveguide by an electrode disposed on an over clad that controls the refractive index of the arm waveguide after the poling process is completed. polling method of the Mach-Zehnder interferometer arm, characterized in that to increase the electro-optic constant of the arm waveguide Te.

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