JP2675033B2 - Distributed interferometric optical modulator - Google Patents

Description

[Detailed Description of the Invention] Outline A distributed interferometric optical modulator that utilizes the change in the refractive index of a waveguide due to the electro-optic effect. Then, in the interferometric optical modulator in which the intensity modulation of the combined light is performed by phase-modulating one of the guided lights and synthesizing again, the input-side waveguide and the phase modulation branched from the input-side waveguide For use, an output side waveguide where the phase modulating waveguide merges, and a plurality of modulation signal applying electrodes provided in the longitudinal direction of the phase modulating waveguide. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed interference optical modulator that utilizes a change in the refractive index of a waveguide due to an electro-optic effect. In a general optical communication system, signal light is intensity-modulated on the transmitting side, and received light is directly detected on the receiving side to reproduce transmission information. The modulation on the transmitter side is LD
This can be done by changing the drive voltage of a light source such as a (semiconductor laser) based on a modulation signal. However, when a high modulation speed is required, an optical modulator independent of the light source is used. External modulation schemes are said to be advantageous. A distributed interference (Mach-Zehnder interferometer) optical modulator is one of the optical modulators of this type, and optimization of its configuration is being sought for practical application of an ultrahigh-speed optical communication system. What is required of the distributed interferometric optical modulator is (a) low power consumption, that is, low drive voltage, and (b) broadband modulation is possible. 2. Description of the Related Art FIG. 4 is a block diagram of a conventional general distributed interference optical modulator. For example, the waveguide 32 is formed by diffusing Ti into the waveguide substrate 31 made of LiNbO _3, and the waveguide 32 is formed.
Electrodes 33 for applying a drive voltage to the branch parts 32a, 32b of
34 is mounted. According to the above configuration, the refractive indexes of the branched portions 32a and 32b of the waveguide change according to the strength of the applied electric field, so that the branched lights branched in the same phase undergo different phase changes.
On the other hand, since the waveguide 32 is a single-mode waveguide that propagates only the fundamental mode light including the branch portion, the intensity of the interference light output is maximum when the phase difference of the branched light is 0. Therefore, when the phase difference of the branched light is π, the intensity of the interference light becomes the minimum. Further, when the phase difference is between 0 and π, the intensity of the interference light depends on the phase difference. As described above, the distributed interference optical modulator controls the output light intensity by adjusting the drive voltage with the modulation signal. Problems to be Solved by the Invention Generally, when the operating voltage is V and the length of the refractive index changing portion of the waveguide is L in the distributed interferometric optical modulator shown in FIG. 4, V · L = a ( a is a constant) ... (1) Further, the modulation band Δf is expressed as Δf = 1.4c / π (n _m −n ₀ ) L (2). Here, c is the speed of light in vacuum, n _m is the refractive index of the waveguide for the modulated signal microwave, and n ₀ is the refractive index of the waveguide for the guided light. Therefore, according to the above equations (1) and (2), the modulation band Δf
Is proportional to the drive voltage V, and the drive voltage becomes high when the band is widened. Therefore, there is a problem that it is difficult to simultaneously satisfy the requirements (a) and (b). The present invention has been created in view of such circumstances,
It is an object of the present invention to provide a distributed interference optical modulator capable of reducing the driving power and widening the band. Means for Solving the Problems By transforming the equation (2), Δf = 1.4 / πL (n _m / c−n ₀ /c)=1.4/πL(1/v _m −1 / v ₀ ) ... (3 Therefore, when the driving voltage is constant, the band Δf becomes wider as the difference between the velocity v _m of the modulation signal microwave and the velocity v _{0 of the} guided light in the waveguide is smaller. Therefore, in the present invention, the electrodes for applying the driving voltage are divided so that the speed difference is substantially reduced, and the band is widened. According to the present invention, there is provided an input side waveguide, a phase modulation waveguide branched from the input side waveguide, and an output side optical waveguide where the phase modulation waveguide joins. In a distributed interference optical modulator adapted to modulate light by an electric field applied to a plurality of electrodes, a plurality of electrodes are provided in the longitudinal direction of the phase modulation waveguide, and the plurality of electrodes are provided for modulating light. Provided is a distributed interference optical modulator, which is characterized in that modulated signals are respectively inputted. Desirably, the timings of the modulation signals input to the plurality of electrodes are adjusted so as to cancel out the difference between the propagation speed of the modulation signal and the propagation speed of light in the phase modulation waveguide. Further, desirably, the modulation signals respectively input to the plurality of electrodes are obtained by shifting the phase of one modulation signal. The guided light of the fundamental mode guided through the action input side waveguide 1 is
When divided and guided in the phase modulation waveguides 2 and 3, the phase is changed according to the strength of the electric field applied from the electrode 5. These guided lights are merged into the output side waveguide 4,
Since the output-side waveguide 4 can guide only the light of the fundamental mode, the intensity of the guided light depends on the phase-modulating waveguides 2, 3
Changes according to the phase difference of the guided light. Therefore, the intensity of the input guided light can be modulated according to the voltage applied to the electrode 5. A plurality of electrodes 5 are provided in the longitudinal direction of the phase modulation waveguides 2 and 3 because the propagation speed of the modulation signal (for example, microwave) is smaller than the propagation speed of the guided light of the phase modulation waveguides 2 and 3. This is to prevent the modulation band from being limited due to this. That is, by providing a plurality of electrodes 5, the delay of the modulation signal in each electrode is reduced to achieve a wider band. Embodiments Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a block diagram of a distributed interference optical modulator showing an embodiment of the present invention. Reference numeral 11 denotes a waveguide substrate made of, for example, a ferroelectric crystal such as LiNbO ₃ , and on this waveguide substrate 11, for example, Ti (titanium) is selectively diffused to partially increase the refractive index of the substrate crystal. As a result, the input side waveguide 12, the phase modulation waveguides 13 and 14 and the output side waveguide 15 are formed. The phase modulation waveguide 13 has a first electrode for applying a modulation signal.
The 16 and the second electrode 18 are mounted in this order in the traveling direction of the guided light. Reference numerals 17, 19 are ground electrodes connected to a ground portion (not shown). Reference numeral 21 denotes a strip line for supplying a modulation signal, which is connected to the first electrode 16 via the bonding wire 20. The strip line 21 is also connected to the strip line 23 via a phase shifter 26 for adjusting the phase of the modulation signal as described later,
The strip line 23 is further connected to the bonding wire 22.
It is connected to the second electrode 18 via. 24 and 25 are between the first electrode 16 and the ground electrode 17 and between the second electrode 18 and the ground electrode, respectively.
It is a load resistance or terminating resistance connected between 19. FIG. 3 shows guided light A guided in the phase modulation waveguide 13, guided light B guided in the phase modulation waveguide 14, an electric signal C propagating in the first electrode 16, and a second electrode. The phase states at points a to d (FIG. 2) of the electric signal D propagating through 18 are
The phase of the guided light B is shown as a reference. Here, point a is the branched portion of the input side waveguide 12, and point b is the phase modulation waveguide 13
At the mounting start portion of the first electrode 16, point c is the portion between the phase modulation waveguides 13 and 14, and point d is the mounting end portion of the second electrode 18 at the phase modulation waveguide 13. At the points a and b, the guided lights A and B have the same phase, but at the point c, since the electric field due to the electric signal C has been received,
For example, the phase of the guided light A lags the guided light B by π / 2.
On the other hand, the electrical signal C is delayed time t _d in accordance with the difference in propagation speed between the guided light. If this delay is stored up to the second electrode 18, the modulation band is limited, so in the present embodiment, the second electrode 18 is
The phase shift amount of the phase shifter 26 is adjusted so as to advance the application timing of the electric signal D applied to the signal t _d so as to compensate for the difference in propagation speed between the guided light and the electric signal. At point d, the guided light A is further delayed by π / 2, and the guided light B is added in total.
Since they are delayed by .pi., These guided lights A and B interfere with each other when they join, and as a result, the output light intensity becomes zero. In this way, in this embodiment, the electrode for applying the modulation signal is divided into two (may be three or more), and the phase of the modulation signal applied to each is adjusted.
The guided light and the modulated signal propagate at virtually the same speed,
Therefore, the band of this optical modulator can be widened. EFFECTS OF THE INVENTION As described in detail above, according to the present invention, it is possible to achieve a wide band at the same drive voltage, and thus it is possible to provide a distributed interference optical modulator suitable for low drive power and wide band. Has the effect of becoming.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a principle diagram of the present invention, FIG. 2 is a block diagram of a distributed interferometric optical modulator showing an embodiment of the present invention, and FIG. FIG. 4 is a diagram for explaining the phase states of a wave light and an electric signal, and FIG. 4 is a configuration diagram of a conventional general distributed interference optical modulator. 1,12 ... Input side waveguide, 2,3,13,14 ... Phase modulation waveguide,
4, 15 ... Output side waveguide, 5 ... Electrode, 11 ... Waveguide substrate, 16 ...
First electrode, 17, 19 ... Ground electrode, 18 ... Second electrode, 26 ... Phase shifter.

Continuation of front page    (72) Inventor Yasunari Arai               1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa                 Fujitsu Limited                (56) References JP-A-60-114820 (JP, A)                 JP-A-61-252527 (JP, A)

Claims (1)

(57) [Claims] An electric field applied to the phase-modulating waveguide, which includes an input-side waveguide, a phase-modulating waveguide branched from the input-side waveguide, and an output-side optical waveguide into which the phase-modulating waveguide merges. In a distributed interferometric optical modulator designed to modulate light by means of a plurality of electrodes in the longitudinal direction of the above-mentioned phase modulation waveguide, the phase of the modulation signal is adjusted so as to compensate for the difference in propagation velocity between the light and the electric signal. Is distributed so that the modulation signal is input to each of the plurality of electrodes. 2. The modulation signals respectively input to the plurality of electrodes are
2. The distributed interference optical modulator according to claim 1, wherein the timing is adjusted so as to cancel the difference between the propagation speed of the modulation signal and the propagation speed of light in the phase modulation waveguide. 3. The modulation signals respectively input to the plurality of electrodes are
The distributed interference optical modulator according to claim 1 or 2, which is obtained by shifting the phase of one modulation signal.