JPH03200923A - Optical modulator - Google Patents

Abstract

PURPOSE:To eliminate an obstacle to the band expansion of an external optical modulator by applying a 1st and a 2nd signal electrode with different electric pulse signals for modulation which are equal in cycle and a half-cycle out of phase with each other, guiding projection light out as a modulated light pulse output, and demodulating it into an electric pulse signal. CONSTITUTION:The 1st signal electrode 3a and 2nd signal electrode 3b formed on branch optical waveguides 2a and 2b are applied with the electric pulse signals A and B for modulation with have the same cycle t1 and are out of phase by a half-cycle with each other. The light beams of both the branch optical waveguides 2a and 2b are multiplexed and the intensity-modulated light pulse output is projected from the light output end of the optical waveguide so that its cycle t0 is equal to a cycle t1/2 a half as long as the cycle t1 of the electric pulse signals A and B for modulation. Consequently, the optical modulation with a double high bit rate is made possible with use of a low-bit- rate electric signal for modulation and the range of the optical modulator 1 for a high-frequency and long-distance optical communication can be expanded.

Description

[Detailed Description of the Invention] [Summary] Regarding an optical modulator, the purpose is to obtain a high bit rate modulated optical signal from a low bit rate modulation electric signal in a high-speed drive external optical modulator, and to improve the electro-optic effect. An optical waveguide having first and second branched optical waveguides is provided on a substrate having a substrate, and a phase difference is generated between light transmitted through the first branched optical waveguide and the second branched optical waveguide. In the Matsuha Tsuender type optical waveguide, a first signal electrode, a first ground electrode, a second signal electrode, and a second ground electrode are respectively disposed in the first and second branch optical waveguides in a symmetrical electrode arrangement. In the modulator, the first signal electrode and the second signal electrode have a period (t,
) and whose phases are shifted by half a cycle are respectively applied, and the light emitted from the light output end of the optical waveguide is controlled by the modulation electric pulse signals (A and B). ) half the period (1+) of period (t0)
Extract it as an intensity-modulated modulated optical pulse output,
The optical modulator is configured to demodulate the signal as it is or as an electrical pulse signal with the original period.

[Industrial application field]

The present invention relates to the configuration and driving method of an optical modulator for performing optical modulation at a high bit rate.

In the optical transmission system of recent optical communication systems, for example,
In optical communication systems up to about 1.6 GHz, a method of directly modulating a laser diode (LD) has been used, but as the modulation frequency becomes higher, small temporal fluctuations in the modulated light wavelength, the so-called chirping phenomenon, occur. This creates a limit to high-speed and long-distance communications.

Furthermore, the development of higher bit rates for drive circuits to obtain electrical signals for modulation has lagged behind optical modulation technology, and technology that uses low bit rate electrical signals to obtain higher bit rate modulated optical signals has been developed. development is required.

[Conventional technology]

As a high-speed optical modulation method, an external modulation method in which semiconductor laser light is modulated externally is well known.

In particular, a Matsuhatsu Enda type optical modulator, which is driven using a traveling wave electrode and has a branched optical waveguide provided on a substrate having an electro-optic effect, is considered to be a promising option.

FIG. 4 is a diagram showing an example of the configuration of a conventional Matsuhatsu Enda type external optical modulator. FIG. B) Y-
It is a Y' sectional view.

In the figure, IO is a substrate having an electro-optic effect, 2 is an optical waveguide, and branch optical waveguides 2a and 2b are formed in the middle.

This guiding wavepath is usually made by selectively diffusing a metal such as Ti on the surface of the substrate only to the leading waveguide part, so that the refractive index of that part is slightly larger than that of the surrounding parts. 3
0 is a signal electrode, for example a traveling wave signal electrode, and 40 is a ground electrode. Reference numeral 7 denotes a buffer layer for reducing the absorption of light into the metal electrode layer on the optical waveguide, and a thin film of SiO□ is usually used.

The signal electrode 30 and the ground electrode 40 are formed on the optical waveguide via the buffer layer 7 by vapor deposition or plating of a metal such as Au.

Now, the DC light from the semiconductor laser 5 is directed to the left optical waveguide 2.
When a high frequency modulated signal voltage is applied to the signal electrode 30 while passing through the branched optical waveguides 2a and 2b, the electricity in the branched optical waveguides 2a and 2b provided on the substrate increases. A phase difference occurs between the two split lights due to optical effects. The two lights are combined again at a combining point 21, a modulated optical signal output is taken out from the light output end of the right optical waveguide 2, and the light is received by a photodetector 6 and converted into an electric signal. There is. Note that RT is a terminating resistor.

FIG. 5 is a diagram showing the operating characteristics of a conventional external optical modulator, in which (a) shows the modulation characteristics, (b) the driving voltage waveform, and (c) the optical output waveform.

That is, if driving voltages (O and Vπ) are applied so that the phase difference between the two lights in the branched optical waveguides 2a and 2b becomes O and π, the optical signal output can be obtained as an ON-OFF pulse signal. .

[Problem to be solved by the invention]

However, in the conventional optical modulator described above, the bit rate of the modulated signal light corresponds to the bit rate of the electrical signal for modulation, so in order to widen the band of the modulated light, it is necessary to increase the frequency of the electrical signal for modulation. It becomes necessary. Currently, it is being put into practical use in driver circuits for modulators in optical communications. For example, 5
The V-level driver IC still has a limit of about 2.4 Gb/s, which is a serious problem that hinders the expansion of the band of an external optical modulator, and a solution has been sought.

[Means to solve the problem]

The above problem is solved by providing an optical waveguide 2 having first and second branched optical waveguides 2a and 2b on a substrate IO having an electro-optic effect, and providing the first branched optical waveguide 2a and the second branched optical waveguide 2b. said first and second branched optical waveguides 2b so as to create a phase difference between the light transmitted through said branched optical waveguides 2b.
In the Matsuhatsu Enda type optical modulator in which a first signal electrode 3a, a first ground electrode 4a, a second signal electrode 3b, and a second ground electrode 4b are arranged in a symmetrical electrode arrangement at the electrodes a and 2b, respectively, The first signal electrode 3a and the second signal electrode 3a
Separate modulating electric pulse signals (A and B) having the same period (t1) and a phase shift of half a period are applied to the signal electrodes 3b of It is extracted as an intensity-modulated modulated optical pulse output with a period (t0) that is half the period (t1) of the modulating electric pulse signals (A and B), and output as is, or
This can be solved by configuring the optical modulator to demodulate the original periodic electrical pulse signal.

[Effect]

According to the configuration of the present invention, the first and second branch optical waveguides 2a of the Matsuhatsu Enda type optical modulator with symmetrical electrode arrangement,
The first signal electrode 3a formed on each of the electrodes 2b
and the second signal electrode 3b, separate modulating electric pulse signals (A and B
) is applied to merge the lights of both branched optical waveguides 2a and 2b, the period (t0) of the intensity-modulated optical pulse emitted from the light output end of the optical waveguide 2 is equal to the modulation electric pulse signal. Since it can be extracted as a modulated optical pulse output with a period (1+/2) that is half the period (t1) of (A and B), 2
Optical modulation with a double high bit rate becomes possible.

〔Example〕

FIG. 1 is a diagram showing an example of a basic device configuration according to the present invention.
In the figure, l is a Matsuhatsu Enda type optical modulator with symmetrical electrode arrangement, 5 is a light source, for example, a semiconductor laser (LD), 9
is an optical fiber, and 6 is a photodetector (DET). 8a is a first modulation drive circuit, and 8b is a second modulation drive circuit, which connects the first signal electrode and second signal electrode formed on each of the first and second branched optical waveguides. , period (t1
) and whose phases are shifted by half a cycle are respectively applied.

From the light output end of the optical modulator l, the period (t0) of the intensity-modulated optical pulse is half the period (tI/2) of the period (t,) of the modulation electric pulse signal (A and B). It is configured to be introduced into the optical fiber 9 as a modulated light pulse output (C0) and to obtain an electrical signal demodulated by the photodetector 6.

FIG. 2 is a top view showing an example device of the present invention, which has a symmetrical electrode arrangement as shown.

Note that the same reference numerals are given to the same parts as those explained in the above drawings, and the explanation of the same parts will be omitted.

As the substrate IO, two LiNb0+ plates having a size of 60 mm x 3 mm and a thickness of 1 mm were mirror polished and used.

On this substrate, Ti was vacuum-deposited to a thickness of about 1100 nm, and treated by a normal photoetching method so that Ti remained in the portion corresponding to the optical waveguide 2 including the branched optical waveguides 2a and 2b. c. Ti was thermally diffused into LiNbO5 by heating in oxygen for IO hours to form an optical waveguide 2 including branched optical waveguides 2a and 2b with a depth of about 5 μm.

The length of the branched optical waveguide portion was adjusted to 40 mm, and the widths of all optical waveguides were adjusted to 7 μm. Branch optical waveguide 2a
The spacing between and 2b was approximately 1 mm.

Next, SiO□ was formed as a buffer layer 7 to a thickness of 500 nm by sputtering.

The first signal electrode 3a and the second signal electrode 3b are Ti-
After depositing the Au alloy film, branch optical waveguides 2a and 2
A pattern is etched onto b into an electrode shape with a width of 9 μm,
Furthermore, Au with a thickness of 8 μm was deposited thereon by plating. The first ground electrode 4a and the second ground electrode 4b were formed at the same time as the signal electrode by the same process as the signal electrode. The distance between the ground electrode and the signal electrode was 20 μm each, and the ground electrode was designed to be as large as possible. Note that RT is a terminating resistor.

Now, the first signal electrode 3a and the second signal electrode 3b are supplied with different modulating electric pulse signals (A, for example, ottool and B,
For example, when 010110) is applied, from the light output end of the optical modulator l, the period of the intensity-modulated optical pulse is half the period of the modulation electric pulse signal (A and B) (tI/2 ), that is, the modulated optical pulse output (C', for example, 10110
10001) is obtained.

FIG. 3 is a diagram explaining the operating state of the embodiment of the present invention, in which (a) shows the modulation characteristics, and (b) to (h) show an example of the operation of the present invention for easy understanding. be.

In other words, from the modulation characteristics shown in Figure (A), when DC light as shown in Figure (B) enters from the light input end of the leading waveguide 2, the signal applied to the pulse signal input ends of both branch waveguides. If the level is different from Vπ such as 0, the phase difference φ2m−
2b=Fπ, so the optical output becomes 0, and on the other hand, the signal levels applied to the pulse signal input ends of both branch waveguides become O and 0. Alternatively, when Vπ and Vπ are the same, the phase difference is φ2m-2b-0, and therefore the optical output is l.

Therefore, if we apply two random pulse signals A and B, both of which have a period of h and whose phases are shifted by, for example, t1/2, as shown in (c) and (d) of the same figure, the above-mentioned result will occur. From the relationship, a logical formula as shown in the table below is established.

Therefore, when the lights phase-modulated in each of the branched optical waveguides 2a and 2b are combined at the combining point 21 and taken out to the outside, the period is t as shown in FIG.

A modulated light pulse output C° with intensity modulation of =t,/2 is obtained.

If this modulated light pulse output C' is received by the photodetector 6 and demodulated directly into an electrical signal, the period t shown in FIG. = 1.72, that is, a demodulated electrical pulse signal C having a bit rate twice that of the original modulating electrical pulse signals A and B is obtained.

On the other hand, if the modulated optical pulse output C° is reversely processed using the logical formula in Table 1 above, it will be exactly the same as the original modulating electrical pulse signal, as shown in (G) and (H) of the same figure. demodulated electrical pulse signals A and B are obtained separately.

The embodiments described above are merely examples, and it goes without saying that preferred materials and process configurations can be used, or combinations thereof, as long as they comply with the spirit of the present invention.

〔Effect of the invention〕

As explained above, according to the configuration of the present invention, the first and second optical modulators of the Matsuhatsu Enda type optical modulator have symmetrical electrode arrangement.
The first signal electrode 3a and second signal electrode 3b formed on each of the branched optical waveguides 2a and 2b have a period (
When separate electric pulse signals (A and B) for modulation with equal phase t1) and a half-cycle shift are applied respectively, and the lights of both dazzling optical waveguides 2a and 2b are combined, the light output end of the optical waveguide 2 The period of the intensity-modulated light pulse emitted from (
Since t0) can be extracted as a modulated optical pulse output with a period (1+/2) that is half the period (t1) of the modulating electrical pulse signals (A and B), 2. This makes it possible to perform optical modulation at twice the bit rate, which greatly contributes to expanding the bandwidth and improving the functionality of optical modulators for high-frequency, long-distance optical communications.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a basic device configuration example according to the present invention, FIG. 2 is a top view showing an example element of the present invention, FIG. 3 is a diagram explaining the operating state of the example of the present invention, and FIG. The figure shows a conventional Matsuhatsu Enda external optical modulator. In the figure, l is an optical modulator, 2 is an optical waveguide, 2a is a first branch optical waveguide, 2b is a second branch optical waveguide, 3a is a first signal electrode, 3b is a second signal electrode, and 4a is a matrix. 1st ground electrode, 4b is the 2nd ground electrode, 5 is the light source, 6
7 is a photodetector, 7 is a buffer layer, 8a is a first modulation drive circuit, 8b and 2nd modulation drive circuit 9 are optical fibers, and IO is a substrate. Constant output yc 'hL Voltage / Invented by Hakama Yu 1's operating state rainbow explanation Figure 3 Hemoto Taakiko base tree vJrl bush placement structure array Σ tree T area 1st
Figure Y-Y' cross-sectional diagram Diagram 5 showing the movement of the conventional Tatobe-Yanawa rope tool) Raw 1)
B

Claims (3)

[Claims] (1) An optical waveguide (2) having first and second branched optical waveguides (2a, 2b) is provided on a substrate (10) having an electro-optic effect, and the first branched optical waveguide (2a) and A first signal electrode ( 3a
), the first ground electrode (4a) and the second signal electrode (3
b) and a second ground electrode (4b) arranged in a symmetrical electrode arrangement, the first signal electrode (3a) and the second signal electrode (3b)
Separate modulating electric pulse signals (A and B) having the same period (t_1) and a phase shift of half a period are applied to each of them, so that the light emitted from the light output end of the optical waveguide (2) is
The period (t_
1) An optical modulator that outputs an intensity-modulated modulated optical pulse output with a period (t_0) that is half of the period (t_0). (2) The optical modulator according to claim 1, wherein the modulated optical pulse output is extracted as a demodulated electric pulse signal (C) with the same period (t_0). (3) In the optical modulator according to claim (1), the modulated optical pulse output is converted into the original modulating electric pulse signal (A and B).
). An optical modulator that demodulates and extracts the same electrical pulse signals (A and B).