JPH06118460A - Optical phase modulation circuit - Google Patents

Abstract

PURPOSE:To generate a remarkably fast optical phase modulation signal not being controlled by the operating speed of an optical phase modulator by utilizing a mutual phase modulation effect in a nonlinear optical medium. CONSTITUTION:This circuit is equipped with a light circuit which generates a light intensity modulation signal, a light source circuit 2 which emits coherent light, two deflection control circuits 4, 5 which control an optical signal outputted from the light source 2 and the deflected state of the light intensity modulation signal outputted from a light amplifier 3, respectively, a light coupling circuit 6 which synthesizes the output signals of the deflection control circuits 4, 5, a transmission line made of a nonlinear optical medium to input the output optical signal of the light coupling circuit 6, and a polarizer 8 to which the optical signal transmitting the medium is inputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical phase modulation circuit used for coherent optical communication or the like.

[0002]

2. Description of the Related Art An optical phase modulation / demodulation system is a modulation / demodulation system used in coherent optical communication and has been actively researched in recent years as a modulation / demodulation system which gives the highest reception sensitivity.
As an optical phase modulator used in a high-speed optical phase modulation circuit with a modulation speed of 1 Gb / s or more, a ferroelectric crystal such as lithium niobate has been used as a material, and when an electric field is applied, a primary electro-optical effect is produced. A phase modulator that utilizes the resulting change in the refractive index is used. The upper limit of the modulation speed of the optical phase modulation signal is given by the modulation band of the phase modulation circuit and the operating speed of the drive circuit that drives the modulator. In general, the output signal amplitude of the drive electric circuit tends to decrease as the operation speed of the modulator increases, so that it is desirable that the optical phase modulator operates at a low drive voltage. The phase modulator of the above type is usually provided with a traveling wave type electrode structure in which the waveguide direction of the optical signal and the propagation direction of the electric signal are aligned with each other for the purpose of lowering the driving voltage. In this case, the modulation band of the modulator is limited by the degree of phase matching between the electrical signal and the optical signal, and the modulation band tends to narrow as the element length increases. On the other hand, since the driving voltage is inversely proportional to the element length, it can be said that the low driving voltage operation and the expansion of the modulation band are in a mutually contradictory relationship. As a lithium niobate phase modulator operating in the wavelength 1.55 μm band, a modulation band of 20 GHz and a π driving voltage of about 5 V have been realized to date.

As a high-speed external modulator that replaces the lithium niobate phase modulator, research using a semiconductor electro-absorption modulator as a phase modulator has been actively conducted in recent years. The semiconductor electro-absorption modulator is an optical modulator which is composed of a PN-junction semiconductor and performs modulation by utilizing a change in absorption coefficient when a reverse bias voltage is applied. The change in the absorption coefficient due to the application of the electric field is called the Franz-Keldeche effect when a bulk crystal is used. When a multiple quantum well structure is used for the active layer in this type of modulator, a peak corresponding to exciton absorption is observed at a wavelength slightly shorter than the absorption edge wavelength even at room temperature, and this peak is observed when an electric field is applied. Also exists, a large absorption change can be obtained as compared with the case of using a bulk crystal. This effect is called the quantum confined Stark effect. With absorption changes,
Since the refractive index also changes due to the Kramers-Kronig relationship, it is possible to use this type of modulator as a phase modulator. The response speed of this type of modulator is extremely high, and the actual modulation band is given by the time constant determined by the structure and mounting of the device. A semiconductor electro-absorption modulator operating in the wavelength band of 1.55 μm has been realized with a modulation band of 20 GHz and a π drive voltage of about 3V.

Although the phase modulation method using the external modulator is called the external phase modulation method, there is a direct phase modulation method in which a signal current is directly applied to the semiconductor laser which is the light source. In this method, a signal current is superimposed on a bias current injected into a semiconductor laser, and a signal amplitude and a signal duty ratio are adjusted so that a frequency change associated with a change in carrier density of an active layer causes a required phase change. The modulation band in this case is limited by the carrier life, and the modulation band when the signal current is injected into the active layer is about 20 GHz, while the modulation band when the signal current is injected into the waveguide region other than the active layer is 1 GHz. Limited to: The same limitation is imposed when performing phase modulation using a semiconductor laser optical amplifier.

On the other hand, with respect to the intensity-modulated signal, since it is relatively easy to generate an optical pulse having a short pulse width, this pulse signal train is modulated and multiplexed on the time axis, so that It is possible to obtain an extremely high-speed intensity modulation signal that is not governed by the operating speed. The principle of this method will be described with reference to FIG. A pulse having a repeating period T (FIG. 9A) generated from the light source 29 is branched into n optical paths by the optical branching circuit 31. FIG. 9 shows the case where n = 2. The branched pulses are intensity-modulated by intensity modulators 32 and 33, respectively (FIG. 9b,
c), T / n delay is given and they are multiplexed. In this way, an intensity modulation signal (FIG. 9d) that is n times the operating speed of the modulator is obtained. An intensity modulation signal of 100 Gb / s has already been obtained using such a method. (A. Takada a
nd M. Sarauwari, Electroni
cs Letters vol. 24, p. 1406
(1988))

[0006]

Currently implemented optical phase modulation circuits are roughly classified into an external phase modulation method using an external phase modulator, and a direct current injection method for directly injecting a signal current into a semiconductor laser which is a light source. There is a phase modulation method. In either case, the modulation speed is limited by the operation speed of the phase modulator or the phase modulation element such as a semiconductor laser and the operation speed of the drive circuit that drives them, and operates at the wavelength of 1.55 μm used in the optical communication system. The operation speed of the optical phase modulation circuit is limited to about 20 Gb / s.

The present invention has been made in view of the above-mentioned prior art, and by utilizing the cross-phase modulation effect in the nonlinear optical medium, an extremely high-speed optical phase-modulated signal which is not controlled by the operating speed of the optical phase modulator. An object of the present invention is to provide an optical phase modulation circuit capable of generating

[0008]

In order to solve the above problems in the present invention, an optical phase modulation circuit according to claim 1 comprises an optical circuit for generating a light intensity modulation signal and a light source for emitting coherent light. , Two polarization control circuits for respectively controlling the polarization states of the optical signal output from the light source and the optical intensity modulation signal output from the optical amplifier, and the output optical signal of the polarization control circuit are combined. An optical coupling circuit, a transmission line made of a nonlinear optical medium to which an output optical signal of the optical coupling circuit is to be input, and a polarizer to which an optical signal transmitted through the medium is to be input. To do. The optical phase modulation circuit according to claim 2, wherein an optical circuit that generates a light intensity modulation signal, a light source that emits coherent light having a different wavelength from the light intensity modulation signal, and an optical signal that is output from the light source. An optical coupling circuit for synthesizing a light intensity modulated signal output from the optical amplifier, a transmission line made of a nonlinear optical medium to which an output optical signal of the optical coupling circuit is input, and an optical signal transmitted through the medium. And an optical filter to which is input. The optical phase modulation circuit according to claim 3 is the optical phase modulation circuit according to claim 1.
In the optical phase modulation circuit described in (1), an optical fiber is used as a transmission line made of a nonlinear optical medium. An optical phase modulation circuit according to a fourth aspect is characterized in that, in the optical phase modulation circuit according to the second aspect, an optical fiber is used as a transmission line made of a nonlinear optical medium.

[0009]

According to such an optical phase modulation circuit of the present invention,
The high-speed intensity-modulated optical signal is amplified by an optical amplifier, if necessary, to have a sufficiently high optical intensity, then multiplexed with coherent unmodulated light, and input to a transmission line made of a nonlinear optical medium. Since the refractive index of the nonlinear optical medium changes according to the change of the light intensity, as shown in FIG. 5, the coherent unmodulated light is phase-modulated by the change of the refractive index by the intensity modulation signal due to the cross phase modulation effect. It will be. Since this mutual phase modulation effect reacts in the order of 10 ⁻¹⁵ seconds, the optical phase modulation circuit according to the present invention has an operation band of 1 THz or higher. By multiplexing the intensity modulation signal in the time domain, it is possible to easily generate a high-speed intensity modulation signal which is not limited by the operation speed of the intensity modulator. Therefore, according to the phase modulation circuit of the present invention, by generating the phase modulation signal in the optical stage from such a high speed intensity modulation signal, it is possible to obtain a high speed phase modulation signal which is not controlled by the operation speed of the modulator. The phase-modulated signal and the intensity-modulated signal obtained need to be separated.
In the optical phase modulation circuit described in (1), the polarization states of the intensity modulation signal and the phase modulation signal are input so as to be linearly polarized waves orthogonal to each other before being input to the nonlinear optical medium, and they are arranged in the subsequent stage of the transmission line. The polarizer separates the two. On the other hand, in the optical phase modulation circuit according to claims 2 and 4,
The wavelengths of the intensity-modulated signal and the coherent CW signal are set to be different, and the two are separated by an optical filter arranged in the latter stage of the transmission line. The arrangement of wavelengths is as shown in FIG. As described above, in the optical phase modulation circuit of the present invention, it is possible to generate an extremely high-speed phase modulation signal as compared with the conventional optical phase modulation circuit using the optical phase modulator. The phase modulation circuit of the present invention is characterized in that the phase modulation signal is generated at the optical stage from the light intensity modulation signal by utilizing the mutual phase modulation effect in the nonlinear optical medium.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of the present invention. That is, 1 is a light intensity modulation circuit, which generates a light intensity modulation signal. Reference numeral 2 denotes a light source that generates coherent light, and a distributed feedback (DFB) semiconductor laser, a distributed reflection (DBR) semiconductor laser, a dye laser, a solid-state laser, or the like is preferably used. An optical amplifier 3 is preferably an optical fiber type optical amplifier, a semiconductor optical amplifier, or the like. Reference numerals 4 and 5 denote polarization control circuits, and optical fiber type and bulk type polarization controllers are preferably used. Reference numeral 6 is an optical coupling circuit, and an optical branching circuit, a directional coupler or the like is preferably used. Reference numeral 7 denotes a non-linear optical medium whose refractive index changes according to light intensity. 8 is a polarizer. The high-speed intensity modulation signal output from the light intensity modulation circuit 1 is amplified by the optical amplifier 3 if necessary, and then multiplexed by the optical coupling circuit 6 with the coherent unmodulated light emitted from the light source 2. The multiplexed optical signal is input to the nonlinear optical medium 7. At this time, the polarization states of the intensity-modulated signal and the unmodulated light are adjusted by the polarization control circuits 4 and 5 so as to be linearly polarized waves orthogonal to each other. Since the refractive index changes according to the light intensity change in the nonlinear optical medium, the CW light is phase-modulated by the intensity modulation signal, and the phase modulation signal is obtained. Unwanted intensity-modulated signals are removed by the polarizer 8 arranged after the nonlinear optical medium. Since the refractive index change in the nonlinear optical medium occurs in a time of 1/10 ps or less, it is possible to obtain the phase modulation signal of the same speed from the light intensity modulation signal of 1 THz or more. Since it is easier to generate a pulse with a short optical pulse width, that is, a pulse with a small duty ratio in the optical stage compared to the electric stage, it is possible to multiplex in the optical stage to generate a high-speed intensity modulation signal. The method of doing is extremely effective. Generation of an intensity-modulated signal train of 100 Gb / s has already been realized using such a method. However, since a signal with a small duty ratio cannot be generated for the phase modulation signal, the signal speed of the high speed phase modulation signal is limited to the response speed of the phase modulator. According to the phase modulation circuit of the present invention,
Since the phase modulation is performed by the cross phase modulation effect using the high speed intensity modulation signal generated in the optical stage, the high speed phase modulation which is not limited by the response speed of the phase modulator is realized.

In the second embodiment of the present invention, as shown in FIG. 2, instead of separating the intensity modulated signal light and the phase modulated signal light by a polarizer, the wavelengths of the intensity modulated signal light and the phase modulated signal light are changed. The optical filters 9 are set so as to be different from each other, and are separated by the optical filter 9 arranged in the subsequent stage of the nonlinear medium. When the nonlinear optical medium has a chromatic dispersion, that is, the propagation speed in the element varies depending on the wavelength of the signal, a phase difference occurs between the intensity modulation signal and the CW signal as they propagate, resulting in phase modulation. The signal may be degraded. Therefore, it is preferable to set the wavelength difference between the intensity modulation signal and the phase modulation signal as small as possible. To overcome the propagation speed difference of the signal due to chromatic dispersion, yet wavelength lambda ₁ of the intensity-modulated signal light to sufficiently perform separation by optical filter shown in FIG. 6, the phase nonlinear medium wavelength lambda ₂ of the modulated signal light ( It is effective to set the relative delays of the intensity-modulated signal light and the phase-modulated signal light to be equal across the zero-dispersion wavelength λ ₀ of the optical fiber).

In the third and fourth embodiments of the present invention,
As shown in FIGS. 3 and 4, by using the optical fiber 10 as the nonlinear medium, the transmission line itself can be used as a phase modulation element.

Next, an experiment for confirming the principle in order to show the effectiveness of the phase modulation circuit of the present invention was conducted by an experimental system as shown in FIG. The semiconductor lasers 11 and 12 are DFB (distributed feedback type) lasers having wavelengths of 1.551 μm and 1.553 μm, respectively. A LiNbO ₃ Mach-Zehnder intensity modulator 16 was used as the intensity modulator. Modulation is signal source 1
5 with a 10 GHz sinusoidal wavelength signal. The intensity modulation signal and the CW signal have their polarization states controlled by the polarization control circuits 17 and 18, respectively, and then 1: 1.
It is multiplexed by the directional coupler 19. The combined optical signal is added by the erbium-doped fiber optical amplifier 20.
It is amplified to an intensity of 12 dBm and input to the dispersion-shifted fiber 21 of 80 km. The output optical signal of the dispersion shift fiber 21 is subsequently input to the polarizer 22. It was controlled by the polarization control circuit 17 so that the fiber output end of the intensity-modulated signal was in a linear polarization state orthogonal to the main axis of the polarizer 22. On the other hand, the phase modulation signal was controlled by the polarization control circuit 18 so as to be in a linearly polarized state in which the direction of the principal axis of the polarizer 22 coincided. The obtained phase modulation signal is FSR10.
The optical spectrum was observed by a sweeping Fabry-Perot etalon 24 at GHz.

FIG. 8 (a) is a spectrum before inputting the polarizer,
(B) is the spectrum after input. It can be seen that only the intensity-modulated signal light is reduced by 20 dB or more by the polarizer.

[0015]

As described in detail above, according to the present invention, a modulator is provided by performing cross-phase modulation of signal light in a nonlinear optical medium using intensity-modulated signal pulses multiplexed in an optical stage. It is possible to obtain a high-speed phase modulation signal that is not limited to the operating speed of.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the present invention.

FIG. 3 is a configuration diagram showing a third embodiment of the present invention.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention.

FIG. 5 is a schematic diagram illustrating the operation principle of the phase modulation circuit of the present invention.

FIG. 6 is a schematic diagram illustrating a relationship between wavelengths of two light sources in second and fourth embodiments of the present invention.

FIG. 7 is a schematic diagram showing an experimental system including the phase modulation circuit of the present invention.

FIG. 8 shows a signal spectrum (a) before inputting a polarizer and a spectrum (b) after passing through the polarizer.

FIG. 9 is a schematic diagram showing a conventional intensity modulation circuit.

[Explanation of symbols]

1 Optical Intensity Modulator 2 Light Source 3 Optical Amplifier 4 Polarization Control Circuit 5 Polarization Control Circuit 6 Optical Coupling Circuit 7 Nonlinear Optical Medium 8 Polarizer 9 Optical Filter 10 Optical Fiber 11 Semiconductor Laser 12 Semiconductor Laser 13 Optical Isolator 14 Optical Isolator 15 signal source 16 LiNbO ₃ Mach-Zehnder intensity modulator 17 polarization control circuit 18 polarization control circuit 19 1: 1 directional coupler 20 erbium-doped fiber optical amplifier 21 dispersion shift fiber 22 polarizer 23 1: 1 directional coupler 24 Sweeping type Fabry-Perot etalon 25 PIN photodiode 26 Amplifier 27 Oscilloscope 28 Optical spectrum analyzer 29 Light source 30 Signal source 31 Optical branch circuit 32 Optical intensity modulator 33 Optical intensity modulator 34 Optical coupling circuit

Claims (4)

[Claims] 1. An optical circuit for generating a light intensity modulated signal, a light source for generating coherent light, and two optical signals for controlling the polarization state of the optical signal output from the light source and the light intensity modulated signal, respectively. A polarization control circuit, an optical coupling circuit that combines the output optical signals of the polarization control circuit, a transmission line made of a nonlinear optical medium to which the output optical signal of the optical coupling circuit is to be input, and a transmission through the medium. Optical phase modulation circuit having a polarizer to which the input optical signal is input. 2. An optical circuit for generating a light intensity modulation signal, a light source for generating coherent light having a wavelength different from that of the light intensity modulation signal, an optical signal output from the light source, and the light intensity modulation signal. An optical phase including an optical coupling circuit for synthesizing the optical coupling circuit, a transmission line made of a nonlinear optical medium to which an output optical signal of the optical coupling circuit is to be input, and an optical filter to which an optical signal transmitted through the medium is to be input. Modulation circuit. 3. The optical phase modulation circuit according to claim 1, wherein an optical fiber is used as a transmission line made of the nonlinear optical medium. 4. The optical phase modulation circuit according to claim 2, wherein an optical fiber is used as a transmission line made of the nonlinear optical medium.