JP2013044991A - Optical subcarrier generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical subcarrier generator, capable of generating the optical subcarrier of which the frequency intervals are equal and the intensity is constant and flat, and also having a simple device constitution and a small device scale.SOLUTION: The optical subcarrier generator includes: a Mach-Zehnder optical modulator 20 equipped with optical phase modulation sections 25, 26 having non-linear phase-voltage characteristics; a CW light source 31; a sine wave electric signal source 32 for applying the sine wave voltage signal of a same amplitude voltage Vto the optical phase modulation sections 25, 26; a DC voltage source 33 for applying a DC bias voltage Vto the optical phase modulation section 25; and a second DC voltage source 34 for applying a DC bias voltages Vto the optical phase modulation section 26. Also, a magnitude relation of these voltages is V>V≥V, and each of the optical phase bias modulation sections 25, 26 is driven by the sine wave voltage signal of the same amplitude voltage Vand mutually different DC bias voltages V, Vand by controlling the optical phase modulation amplitude, optical phase modulation is asymmetrically performed between the optical modulation sections 25, 26.

Description

  The present invention relates to an optical subcarrier generator that generates optical subcarriers (optical subcarriers).

  As a recent demand for optical fiber communication systems, research and development of transmission systems and devices based on wavelength division multiplexing optical transmission technology and capable of a total transmission capacity of several terabits / second to several tens of terabits / second has been actively conducted. It has been broken. In the case of the intensity modulation by binary transmission using only the intensity “0” and “1” as signals as used in the conventional optical fiber communication, and the direct detection method, the speed increases to 40 gigabit / second or 100 gigabit / second. As a result of the spread of the optical spectrum of the transmitted signal, not only the dispersion tolerance deteriorates abruptly, but it is necessary to ensure a wide wavelength interval in order to avoid crosstalk between channels in wavelength division multiplexing. Decrease and network flexibility.

  Therefore, as a method for increasing the bit rate without occupying the frequency band per channel, techniques such as multi-level modulation and multiplexing are important.

  Orthogonal Frequency Division Multiplexing (OFDM) is promising as a technique for improving dispersion tolerance and increasing frequency utilization efficiency. This method is a kind of multi-carrier transmission method, and it is separated at the receiving end against the overlap of modulation spectrum between subcarriers by multiplexing under the condition that each subcarrier is orthogonal. Since it can be demodulated, it has been put to practical use in wireless transmission including mobile radio communication. In orthogonal frequency division multiplexing, the bit rate per subcarrier can be reduced to 1 / N (N: number of optical subcarriers) compared to single carrier transmission. By applying to communication, it is possible to suppress the spectrum spread of the transmission signal of each subcarrier, and to relax the limitation of the transmission capacity due to the chromatic dispersion and polarization mode dispersion of the optical fiber.

  As a means for applying the orthogonal frequency division multiplexing method to an optical fiber communication system, there is a method of driving an optical modulator using an OFDM baseband signal generated by electrical processing as shown in Non-Patent Document 1 below. is there. Also, as shown in the following Patent Documents 1 and 2, etc., a method has been proposed in which optical orthogonal subcarriers are generated in the light region and each subcarrier is intensity modulated or phase modulated. Furthermore, each optical subcarrier is modulated using an OFDM baseband signal generated by electrical processing, so that flexible connection between optical fiber communication and wireless communication can be achieved by using the OFDM method for each optical wavelength band. I can expect.

  As described above, in the method of actively using the optical subcarrier as the carrier wave, it is not only flexible to determine what kind of modulation is performed for each optical subcarrier, but also necessary for transmitting the necessary amount of information. By ensuring the bandwidth (number of optical subcarriers) and performing appropriate modulation, network flexibility can be improved.

  The simplest generation method of optical subcarriers is a method of generating optical subcarriers by controlling the oscillation frequencies (wavelengths) of a plurality of laser light sources with high accuracy. However, with this method, it is difficult to accurately match each frequency (wavelength) for optical OFDM in which the frequency interval (wavelength interval) is important.

  On the other hand, as shown in Non-Patent Documents 2, 3, and 4 below, a method of driving an optical modulator to use a modulation sideband, a method of using forced mode locking of an optical resonator, etc. There is a method of using the frequency spectrum as an optical subcarrier. In these methods, by driving the optical modulator with a sine wave electrical signal, modulation sidebands are generated at the same frequency interval as the modulation frequency, and optical subcarriers can be generated with high accuracy in the frequency interval. Compared with the method using a plurality of laser light sources, it can be said that the method is more suitable for generating optical subcarriers for optical OFDM.

  In addition, the fact that the spectrum intensity of each optical subcarrier is constant is because a transmission signal suitable for the receiving system is generated from the viewpoint of equalization amplification and reception sensitivity of the optical signal when constructing the transmission system. Is essential.

  The optical subcarrier generator of Non-Patent Document 2 uses an optical phase modulator and an optical intensity modulator in combination, drives them with synchronization signals of the same modulation frequency, and generates the modulation generated by the first-stage optical phase modulator. The intensity of the sideband is modulated by a light intensity modulator connected at the subsequent stage, and the spectrum of the optical subcarrier is made constant by causing the frequency spectrum to interfere with each other.

  The optical subcarrier generator of Non-Patent Document 4 uses a single Mach-Zehnder optical phase modulator 10 as shown in FIG. The Mach-Zehnder type optical phase modulator 10 includes a first optical fiber branching section (optical waveguide branching section) 1, an optical coupling section (optical waveguide coupling section) 2, and a first sandwiched between the optical branching section 1 and optical coupling section 2. Formed on the first arm optical waveguide 3, the second arm optical waveguide 4, the first optical phase modulation unit 6 formed on the first arm optical waveguide 3, and the second arm optical waveguide 4. The second optical phase modulation unit 7 is included. The optical subcarrier generator includes this Mach-Zehnder optical phase modulator 10, a CW light source 11 that generates CW light (continuous light), a sine wave electric signal source 12, a first electric amplifier 13, and a second electric amplifier. The electric amplifier 14, the DC voltage source 15, and the polarization controller 16 are provided.

  In the optical subcarrier generator of Non-Patent Document 4, each of the optical phase modulators 5 and 6 formed on the arm optical waveguides 3 and 4 of the Mach-Zehnder optical phase modulator 10 is replaced with a sine wave electric signal source 12. From the CW light source 11 to the polarization controller 16 by driving with sinusoidal voltage signals of different amplitude voltages applied from the AC amplifiers 13 and 14 and the DC voltage applied from the DC voltage source 15. After phase-modulating the light that is input via the optical branching unit 1 and branched at the optical branching unit 1, the respective modulation spectra are combined in a vector manner at the output multiplexing end (optical coupling unit 2) of the Mach-Zehnder optical phase modulator 10. Thus, the spectrum intensity of the optical subcarrier is made constant.

JP 2009-17320 A JP 2009-124700 A

SLJansen, I. Morita, N. Takeda, and H. Tanaka, “20-Gb / s OFDM Transmission over 4,160-km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation,” Proc. Of OFC / NFOEC2007, paper PDP15, 2007. Fujiwara, M., J. Kani, et al. (2001). "Flattened optical multicarrier generation of 12.5 GHz spaced 256 channels based on sinusoidal amplitude and phase hybrid modulation." Electronics Letters 37 (15): 967-968. Kourogi, M., K. Nakagawa, et al. (1993). "Wide-span optical frequency comb generator for accurate optical frequency difference measurement." Quantum Electronics, IEEE Journal of 29 (10): 2693-2701. Sakamoto, T., T. Kawanishi, et al. (2007). "Asymptotic formalism for ultraflat optical frequency comb generation using a Mach-Zehnder modulator." Opt. Lett. 32 (11): 1515-1517.

  However, although the optical subcarrier generator of Non-Patent Document 2 can generate optical subcarriers with good flatness, the optical phase modulator and the optical intensity modulator are directly connected, and these synchronous drives are indispensable. Therefore, there are problems that the apparatus scale increases and the apparatus configuration becomes complicated.

  Further, the optical subcarrier generator of Non-Patent Document 4 does not require two directly connected optical modulators for overcoming the problems of Non-Patent Document 2 described above, and their synchronous driving is also unnecessary. Although it is configured, two devices for adjusting the voltages of the electric amplifiers 13 and 14 and the attenuator are required in order to drive each of the phase modulation units 5 and 7 with a sine wave signal having a different amplitude voltage. End up. Therefore, even in the optical subcarrier generator of Non-Patent Document 4, the problem that the device scale increases is not solved.

  Further, as a common problem in Non-Patent Documents 2, 3, and 4, there is a problem that it is necessary to prepare a CW light source that generates CW light outside of the modulator. In this case, not only an external CW light source but also a polarization controller may be required to connect the external CW light source and the optical modulator in an optimal polarization state. By providing the polarization controller, There is a problem that not only the apparatus scale increases but also the entire apparatus configuration becomes complicated.

  Therefore, in view of the above problems, the present invention can generate optical subcarriers having equal frequency intervals, constant intensity, and flatness, and an optical subcarrier with a simpler apparatus configuration and a smaller apparatus scale than conventional ones. It is an object to provide a carrier generator.

An optical subcarrier generator according to a first aspect of the present invention for solving the above problems includes an optical branching unit, an optical coupling unit, a first arm optical waveguide sandwiched between the optical coupling unit and the optical coupling unit, and a second optical waveguide unit. An arm optical waveguide, a first optical phase modulator having a nonlinear phase-voltage characteristic formed on the first arm optical waveguide, and a nonlinear phase formed on the second arm optical waveguide; A Mach-Zehnder optical modulator comprising a second optical phase modulator having voltage characteristics;
A CW light source that outputs CW light to the optical branching unit;
A sine wave electric signal source for applying a sine wave voltage signal of the same amplitude voltage to the first optical phase modulation unit and the second optical phase modulation unit;
A first DC voltage source for applying a first DC bias voltage to the first optical phase modulator;
A second DC voltage source for applying a second DC bias voltage to the second optical phase modulator,
When the amplitude voltage of the sine wave voltage signal is V _m , the first DC bias voltage is V _b1 , and the second DC bias voltage is V _b2 , the magnitude relationship between them is V _b1 > V _b2 ≧ V _m ,
Each of the first optical phase modulation unit and the second optical phase adjustment unit is configured such that the first DC bias voltage and the second DC bias voltage are different from the sine wave voltage signal having the same amplitude voltage. Driving and controlling the optical phase modulation amplitude to perform asymmetric optical phase modulation between the first optical phase modulation unit and the second optical phase modulation unit,
It is characterized by.

  An optical subcarrier generator according to a second aspect of the present invention is the optical subcarrier generator according to the first aspect of the present invention, wherein the first optical phase modulation unit and the second optical phase modulation unit use a semiconductor material. It is characterized by causing a change in refractive index accompanying electric field-light absorption such as a confined Stark effect.

The optical subcarrier generator of the third invention is the optical subcarrier generator of the first or second invention, wherein the CW light source and the Mach-Zehnder optical modulator are monolithically integrated on the same semiconductor substrate, A CW light source monolithic integrated Mach-Zehnder type optical modulator is configured.

  The optical subcarrier generator according to a fourth aspect of the present invention is the optical subcarrier generator according to the third aspect of the present invention, wherein the sine wave voltage of the same amplitude voltage is applied to the first optical phase modulator and the second optical phase modulator. A branch line and an electrode for applying a signal are formed on the semiconductor substrate.

The optical subcarrier generator of the fifth invention is the optical subcarrier generator of any of the first to fourth inventions,
In the adjustment of the first DC bias voltage and the second DC bias voltage, and the phase difference between the first optical phase modulation unit and the second optical phase modulation unit, the phase of the sine wave voltage signal A phase difference between the first optical phase modulation unit and the second optical phase modulation unit with respect to a modulation amplitude component and a phase change amount due to the first DC bias voltage and the second DC bias voltage Are defined as Δφ _m and Δφ _dc , respectively, the first optical phase modulation unit and the second optical phase so as to satisfy the relationship of phase differences Δφ _m and Δφ _dc of a straight line Δφ _m = −Δφ _dc + π An optical subcarrier generator that drives the modulator to make the intensity of the optical subcarrier constant, and an error range err between the initial point Δφ _dc1 and the error bar in the domain of the straight line Δφ _m = −Δφ _dc + π , the phase term phi _nl showing the non-linearity, the following formula (I And characterized in that it is configured to adjust the relationship between so as drive voltage of the phase difference Δφ _m, Δφ _dc to be within the range defined by formula (II).

The optical subcarrier generator of the sixth invention is the optical subcarrier generator of any of the first to fifth inventions,
A first optical phase adjustment unit formed on the first arm optical waveguide;
A second optical phase adjuster formed on the second arm optical waveguide;
A first DC power supply for applying a first DC current or a first DC voltage to the first optical phase adjustment unit;
A second DC power source for applying a second DC current or a second DC voltage to the second optical phase adjustment unit,
By driving each of the first optical phase adjustment unit and the second optical phase adjustment unit with the first and second DC currents or the first and second DC voltages, the Mach-Zehnder light It is characterized in that the initial phase difference at the optical coupling unit, which is the multiplexing output terminal of the modulator, is adjusted to be zero.

According to the optical subcarrier generator of the present invention, each of the first optical phase modulation unit and the first optical phase adjustment unit is connected to the first DC bias voltage different from the sine wave voltage signal having the same amplitude voltage and the first DC bias voltage. The optical phase modulation is asymmetrically performed between the first optical phase modulation unit and the second optical phase modulation unit by driving with a DC bias voltage of 2 and controlling the optical phase modulation amplitude. Therefore, an optical subcarrier having a flattened intensity can be generated only by adjusting the DC bias voltage without using sinusoidal voltage signals having different amplitude voltages. Since it is not necessary to generate sinusoidal voltage signals having different amplitude voltages, an electric amplifier or the like is unnecessary. Although two DC voltage sources are required, a circuit that generates a DC bias voltage and applies it to the optical phase modulator compared to a drive circuit that generates a sine wave voltage signal and applies it to the optical phase modulator. Since the configuration is simple, the optical subcarrier generator of the present invention has a simpler device configuration and a smaller device scale than the conventional optical subcarrier generator.
Further, by using a CW light source monolithically integrated Mach-Zehnder type optical modulator in which a CW light source and the Mach-Zehnder type optical modulator are monolithically integrated on the same semiconductor substrate, an external CW light source can be provided separately from the Mach-Zehnder type optical phase modulator. Since it is not necessary to provide a polarization controller between the CW light source and the Mach-Zehnder optical phase modulator, it is possible to realize an optical subcarrier generator with a simpler device configuration and a smaller device scale.
Furthermore, by forming on the semiconductor substrate a branch line and an electrode for applying a sine wave voltage signal having the same amplitude voltage to the first optical phase modulation unit and the second optical phase modulation unit, an external electric Signal wiring can also be simplified.
Further, by providing the first optical phase adjustment unit and the second optical phase adjustment unit and performing the phase adjustment, the combined output terminal (optical coupling unit) of the Mach-Zehnder optical modulator can be used even if the structure is not completely symmetric. ) At the initial phase difference can be made zero.

It is a block diagram of the optical subcarrier generator which concerns on Example 1 of Embodiment of this invention. It is a figure which shows the direct current bias voltage dependence of the phase change amount by the sine wave voltage signal of the same amplitude voltage. It is a figure which shows the operation | movement principle of planarization of an optical subcarrier. It is a block diagram of the optical subcarrier generator which concerns on Example 2 of this invention. It is a figure which shows the amount of phase changes at the time of the voltage application in a semiconductor. It is a figure which shows the relationship between the phase difference between arm optical waveguides, and flatness. It is a figure which shows the relationship between the phase difference between arm optical waveguides, and flatness. It is a figure which shows the example of calculation of the spectrum of an optical subcarrier. It is a figure which shows the relationship of a straight line (DELTA ₎ phim =-(DELTA) _phidc + (pi) which shows the range which can planarize an optical subcarrier, and an error bar. It is a figure which shows the relationship between the nonlinear phase term (phi) _nl and the range which can planarize an optical subcarrier. It is a block diagram of the optical subcarrier generator which concerns on Example 3 of Embodiment of this invention. It is a block diagram of the conventional optical subcarrier generator.

  Embodiments of the present invention will be described below in detail with reference to the drawings.

<Embodiment 1>
The optical subcarrier generator according to Embodiment 1 of the present invention will be described with reference to FIGS.

  As shown in FIG. 1, the optical subcarrier generator according to Embodiment 1 of the present invention uses a single Mach-Zehnder optical phase modulator 20. The Mach-Zehnder type optical phase modulator 20 includes a first optical fiber branching unit (optical waveguide branching unit) 21, an optical coupling unit (optical waveguide coupling unit) 22, and a first optical unit sandwiched between the optical branching unit 21 and the optical coupling unit 22. Arm optical waveguide 23 and second arm optical waveguide 24, first optical phase modulator 25 formed on first arm optical waveguide 23, and first optical phase modulator 25 formed on second arm optical waveguide 24. 2 optical phase modulators 26.

  Both the first optical phase modulation unit 25 and the second optical phase modulation unit 26 have non-linear phase-voltage characteristics (non-linear characteristics). Such nonlinear characteristics of the optical phase modulation units 25 and 26 can be obtained, for example, by causing the optical phase modulation units 25 and 26 to change the refractive index by combining the Pockels effect and the quantum confined Stark effect (QCSE).

More specifically, if an optical phase modulation unit using a refractive index change by applying an electric field causes a refractive index change using only the Pockels effect, the optical phase modulation unit exhibits linear characteristics.
On the other hand, the electro-optic effect showing nonlinear refractive index changes such as the quantum confined Stark effect, the electro-optic Kerr effect, the Franz-Keldish effect can be used singly or in combination, or the electro-optic effect When a change in refractive index is caused in the optical phase modulators 25 and 26 by using any of them in combination with the Pockels effect, the optical phase modulators 25 and 26 have nonlinear characteristics.
It should be noted that which electro-optic effect appears when an electric field is applied to the optical phase modulators 25 and 26 depends on the material constituting the optical phase modulators 25 and 26, the core layer of the optical phase modulators 25 and 26, It can be determined by the structure of the cladding layer, etc., and a known technique may be used.

  The optical subcarrier generator according to the first embodiment includes a Mach-Zehnder optical phase modulator 20 having a configuration in which the optical phase modulators 25 and 26 having nonlinear characteristics as described above are formed in the arm optical waveguides 23 and 24, A CW light source 31 that generates CW light, a sine wave electric signal source 32 for driving the first optical phase modulation unit 25 and the second optical phase modulation unit 26, and a drive of the first optical phase modulation unit 25 For this purpose, a first DC voltage source 33 and a second DC voltage source 34 for driving the second optical phase modulator 26 are provided.

As the CW light source 31, for example, a semiconductor laser is used. The sine wave electric signal source 32 generates a sine wave voltage signal having the same amplitude voltage V _m for driving the first optical phase modulation unit 25 and the second optical phase modulation unit 26. The first DC voltage source 33 generates a first DC bias voltage V _b1 for driving the first optical phase modulator 25. The second DC voltage source 34 generates a second DC bias voltage V _b2 for driving the second optical phase modulator 26. The first DC bias voltage V _b1 and the second DC bias voltage V _b2 are different from each other. The second DC bias voltage V _b2 is a voltage equal to or higher than the amplitude voltage V _m of the sine wave voltage signal. That is, the magnitude relationship among V _b1 , V _b2 , and V _m is V _b1 > V _b2 ≧ V _m .

  The optical branching unit 21, which is the input end of the Mach-Zehnder optical phase modulator 20, receives CW light output from the CW light source 31 to the optical branching unit 21, and uses the input light as the first branched light and the second branched light. Branch to light. The first branched light propagates through the first arm optical waveguide 23, and the second branched light propagates through the second arm optical waveguide 24.

In the first optical phase modulator 25, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 32 and a first DC bias voltage V _b1 applied from the first DC voltage source 33. To generate a first phase-modulated light by phase-modulating the first branched light. In the second optical phase modulation unit 26, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 32 and a second DC bias voltage V _b2 applied from the second DC voltage source 34. To generate a second phase modulated light by phase modulating the second branched light.

That is, the sine wave voltage signals for driving the optical phase modulation units 25 and 26 are sine wave voltage signals having the same amplitude voltage V _m , while the DC bias voltages for driving the optical phase modulation units 25 and 26 are different from each other. The DC bias voltages V _b1 and V _b2 are used. Even if the sine wave voltage signals have the same amplitude voltage V _m , the optical phase modulation units 25 and 26 have nonlinear characteristics, and the optical phase modulation units 25 and 26 have different DC bias voltages V _b1 and V _b from each other. _{Since b2} is applied, the state is substantially the same as in the case of applying sinusoidal voltage signals having different amplitude voltages as in the prior art.

Accordingly, each of the first optical phase modulation unit 25 and the second optical phase adjustment unit 26 is driven by a sine wave voltage signal having the same amplitude voltage V _m and different DC bias voltages V _b1 and V _b2 . By controlling the optical phase modulation amplitude, the optical phase modulation can be performed asymmetrically between the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26).

  In the optical coupling unit 22, which is the output end of the Mach-Zehnder optical phase modulator 20, the first phase modulated light phase-modulated by the first optical phase modulation unit 25 and the phase by the second optical phase modulation unit 26. The modulated second phase-modulated light is multiplexed. As a result, flat optical subcarriers having the same frequency interval and constant intensity are generated and output from the output end (optical coupling unit 22) of the Mach-Zehnder optical phase modulator 20.

Thus, by driving the single Mach-Zehnder type optical phase modulator 20 with the sine wave voltage signal having the same amplitude voltage Vm and the different DC bias voltages V _b1 and V _b2 , the frequency intervals are equal and the intensity is increased. The principle of generating optical subcarriers with a flattening will be described in detail below.

<Principle of optical subcarrier generation>
In general, the input electric field and the output electric field of the Mach-Zehnder optical modulator 20 are expressed by the following equations.
Here, φ ₁ and φ ₂ indicate phases by the optical phase modulators 25 and 26 formed on the arm optical waveguides 23 and 24 of the Mach-Zehnder type optical phase modulator 20, respectively. φ ₀₁ and φ ₀₂ are initial phases in the arm optical waveguides 23 and 24 in the respective Mach-Zehnder optical phase modulators 20. Adopting a completely symmetric structure or an optical phase adjustment region (see optical phase adjustment units 91 and 92 in FIG. 11) for adjusting the initial phase separately from the optical phase modulation units 25 and 26 as arm light By providing on the waveguides 25, 25, these initial phase differences can be made zero. Thereby, the phase change of only the optical phase modulators 25 and 26 can be considered.

Due to the temporal refractive index changes Δn ₁ and Δn _{2 in the} optical phase modulators 25 and 26, the temporal phase change is shown as follows.
λ is the wavelength of the propagating light wave, and L is the length of the medium in which the light wave undergoes phase modulation in the optical phase modulators 25, 26, that is, the working length that is the length of the phase modulation region in the traveling direction of the light wave.

A refractive index change in a semiconductor has a non-linear response due to a Pockels effect that causes a refractive index change due to application of an electric field and a change in the refractive index accompanying a change in the light absorption coefficient. In general, a nonlinear refractive index change Δn in a semiconductor is expressed as follows.
c ₁ and c ₂ are coefficients with respect to the refractive index change voltage V (t).

The present invention uses sidebands when the optical phase modulators 25 and 26 are driven by sinusoidal voltage signals to modulate the light waves. Therefore, the voltage V (t) is expressed as follows by the bias voltage V _b , the amplitude voltage V _m, and the angular frequency ω _m .

  In optical subcarrier generation using sidebands by modulation, optical subcarriers are generated at intervals that are integer multiples of the modulation frequency, and can be handled by changing the modulation frequency according to the desired optical subcarrier frequency interval. .

By substituting Equations (3) and (4) into Equation (2), the amplitude voltage V _m of the sine wave voltage signal is constant in both arm optical waveguides 23 and 24 (optical phase modulators 25 and 26), and the DC bias is applied. When the voltages are V _b1 and V _{b2 in the} arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26), respectively, the phase changes in the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26) are as follows. It is expressed as follows.
The first term on the right side represents the DC bias voltage, the second term represents the phase modulation amplitude induced by the sinusoidal voltage signal, and the third term represents the phase change due to the component caused by the nonlinear phase response.

In the conventional optical subcarrier generator (FIG. 12), the second term on the right side drives the optical phase modulators 5 and 7 using sinusoidal voltage signals having different amplitude voltages. By using a simple refractive index change, it is possible to control the phase modulation amplitude component only by adjusting the DC bias voltages V _b1 and V _b2 , so that optical subcarrier generation with a simple configuration and easy adjustment is possible. The flattened optical subcarrier can be generated by the device.

  Further, since the nonlinear component does not depend on the DC bias voltage and is handled as a phase modulation term generated in common in both arm optical waveguides 23 and 24, the combined output terminal (optical coupling unit) of the Mach-Zehnder optical phase modulator 20 is used. 22), it can be seen that the output is not directly affected as an in-phase component.

When the coefficients c ₁ and c ₂ of the refractive index change are multiplied by 2πL / λ to be converted into phase change coefficients C ₁ and C ₂ , and each term of the equation (5) is replaced by the DC bias voltages V _b1 and V _b2 . It is shown as follows.
A phase change due to DC bias voltages V _b1 and V _b2 (phase change amount) φ _dc , a phase modulation amplitude component φ _m due to a sinusoidal voltage signal of amplitude voltage V _m , and a phase component generated by nonlinear voltage-phase characteristics ( Phase term indicating nonlinearity) φ _n1 .

Therefore, the phase change in each arm optical waveguide 23, 24 (optical phase modulation unit 25, 26) can be expressed by equation (7).
As apparent from the equation (7), in each of the arm optical waveguides 23 and 24, the phase modulation characteristics can be controlled independently only by adjusting the DC bias voltages V _b1 and V _b2 of the optical phase modulators 25 and 26. I understand that

FIG. 2 shows how the phase modulation amplitudes (φ _m1 , φ _m2 ) are changed by changing the DC bias voltages V _b1 and V _b2 on the nonlinear phase-voltage characteristics. A feature of the present invention is that the phase change amplitude can be controlled by adjusting the bias point even if it is a sinusoidal voltage signal having the same amplitude voltage V _m by using a non-linear phase-voltage response. Compared to the above, it is not necessary to use two two sine wave signals having different amplitude voltages, so that asymmetric phase modulation operation is easily performed between the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26). Is to make it possible.

When Expression (7) is substituted into Expression (1) of the output electric field of the Mach-Zehnder optical phase modulator 20 and the initial phase is set to zero, the following Bessel function is used for rearranging, the following Expression (8) It can be expressed as follows.

Further, when the order of the Bessel function is converted and arranged by the modulation angular frequency ω _m , the following expression (9) is obtained.

This equation (9) is a basic equation showing the output electric field of the Mach-Zehnder optical modulator 20 utilizing the nonlinear refractive index change of the semiconductor. From this equation (9), it can be seen that the k-th optical spectrum intensity is represented by the following equation (10).

Furthermore, by using the relationship between Cauchy's inequality and triangular inequality, the maximum value is
Indicated by φ _nl is a coefficient indicating conversion efficiency, and the Bessel function J ₁ (φ _nl ) by the coefficient φ _nl in the nonlinear phase component is treated as a common term. The k-21 order Bessel functions in parentheses are coefficients resulting from independent phase changes in the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26).

Here, if the k-th optical spectrum intensity can take a constant value without depending on the order k-2l of the Bessel function, it is possible to flatten the optical subcarrier generation using the modulation sideband. In each light intensity spectrum, the order change is made by asymmetrical modulation so that the change in the value of the Bessel function J _k-21 (φ _m1 ) due to the change in the order k is compensated by J _k-21 (φ _m2 ). As a result, the optical subcarrier can be flattened at the multiplexing output end (optical coupling unit 22) of the Mach-Zehnder optical phase modulator 20.

In the phase relationship of equation (6):
Thus, the asymmetry of the phase modulation characteristics between the DC component and the component due to the sinusoidal signal voltage between the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26) can be shown as a phase difference. These phase relationships Δφ _m and Δφ _dc are parameters that determine the flatness of the optical subcarrier. At this time, the magnitude relationship between the DC bias voltages V _b1 and V _b2 is V _b1 > V _b2 , and the amplitude voltage V _m of the sine wave voltage signal is V _b2 ≧ V _m .

Further, the DC bias voltages V _b1 and V _b2 necessary for obtaining the phase relationships Δφ _m and Δφ _dc required for flattening can be obtained by solving simultaneous equations of Equation (11).
And obtained.

FIG. 3 shows an operation image of optical subcarrier flattening based on the above principle. A carrier wave which is a single spectrum of the CW light source 31 is flattened in the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26) of the Mach-Zehnder optical phase modulator 20 based on the equation (12). Asymmetric optical subcarriers are generated between the arm optical waveguides 23 and 24 (optical phase modulation units 25 and 26) in the even-order and odd-order, when driving with a voltage that obtains an appropriate phase relationship Δφ _m and Δφ _dc. Is done. As a result, at the output end (optical coupling unit 22) of the Mach-Zehnder optical phase modulator 20, the respective asymmetric optical subcarrier spectra are combined in a vector manner, so that a flat optical subcarrier can be obtained.

  In the description of the principle of optical subcarrier flattening described above, a semiconductor is described as an example. However, a medium having a nonlinear refractive index change with respect to a voltage as represented by Equation (3) is used as an optical medium. Even if it is used as a phase modulator, flattening is possible in the same manner.

In the optical subcarrier generator according to the first embodiment, the intensity of the optical subcarrier generator is flattened only by adjusting the DC bias voltages V _b1 and V _b2 without using sinusoidal voltage signals having different amplitude voltages. Carriers can be generated. Since it is not necessary to generate sinusoidal voltage signals having different amplitude voltages, an electric amplifier or the like is unnecessary. Although two DC voltage sources 33 and 34 are required, a DC bias voltage is generated and applied to the optical phase modulator compared to a drive circuit that generates a sine wave voltage signal and applies it to the optical phase modulator. The circuit is simple in construction. Therefore, the optical subcarrier generator of Embodiment 1 has a simpler device configuration and a smaller device scale than the conventional optical subcarrier generator.

  Next, optical subcarrier generators according to Embodiments 2 and 3 of the present invention will be described. In the optical subcarrier generator of the first embodiment, when the CW light source 31 is provided outside the Mach-Zehnder optical phase modulator 20, as described above, the CW light source 31 and the Mach-Zehnder optical phase modulator are provided. It is necessary to provide a polarization controller between the CW light source 31 and the Mach-Zehnder optical phase modulator 20 in order to connect the 20 in an optimal polarization state. In this case, a polarization controller is provided. However, the following second and third embodiments solve such problems and the like.

<Embodiment 2>
An optical subcarrier generator according to Embodiment 2 of the present invention will be described with reference to FIGS. Note that the principle of optical subcarrier generation by the optical subcarrier generator of the second embodiment is the same as that of the optical subcarrier generator of the first embodiment.

  As shown in FIG. 4, in the optical subcarrier generator according to Embodiment 2 of the present invention, a semiconductor laser 52 serving as a CW light source and a Mach-Zehnder optical phase modulator 53 are monolithically formed on the same semiconductor substrate 51. A CW light source monolithic integrated Mach-Zehnder type optical phase modulator 60 having an integrated configuration is used.

  The Mach-Zehnder optical phase modulator 53 includes an optical branching unit (optical waveguide branching unit) 71, an optical coupling unit (optical waveguide coupling unit) 72, and a first sandwiched between the optical branching unit 71 and the optical coupling unit 72. The arm optical waveguide 73 and the second arm optical waveguide 74, the first optical phase modulator 75 formed on the first arm optical waveguide 73, and the second formed on the second arm optical waveguide 74. The optical phase modulation unit 76 is provided.

By using a semiconductor quantum well structure or the like, the optical phase modulators 75 and 76 have a refractive index change accompanying an absorption coefficient change due to a quantum confined Stark effect in addition to a linear electro-optic effect due to a Pockels effect due to external electric field application. A nonlinear refractive index change (nonlinear phase-voltage characteristic) can be obtained. As the semiconductor material, InP, InGaAsP, AlGaInAs, or the like can be used when optical subcarriers are generated in the optical fiber communication wavelength band. If the wavelength band is not clearly specified, the direct transition emission wavelength band of each compound semiconductor may be used using GaAs or GaN. The combination of the Pockels effect and the quantum confined Stark effect is ideal for showing the nonlinear refractive index change Δn shown in the equation (3), and is appropriately optical phase modulated with respect to the DC bias voltages V _b1 and V _b2 . Since the phase modulation characteristics of the sections 75 and 76 change, the DC bias voltages V _b1 and V _b2 can be easily set.

  In addition, it is not limited to the combination of the Pockels effect and the quantum confined Stark effect, and as described above, it exhibits nonlinear refractive index changes such as the quantum confined Stark effect, the electro-optic Kerr effect, and the Franz-Keldish effect. By using the electro-optic effect singly or in combination, or by using any of the electro-optic effects in combination with the Pockels effect, a change in refractive index may be caused in the optical phase modulators 75 and 76. it can. Which electro-optic effect is manifested when an electric field is applied to the optical phase modulators 75 and 76 depends on the material constituting the optical phase modulators 75 and 76, or the core layers and cladding layers of the optical phase modulators 75 and 76. The known structure may be used.

  The optical subcarrier generator according to the second embodiment drives the CW light source monolithic integrated Mach-Zehnder optical phase modulator 60, the first optical phase modulation unit 75, and the second optical phase modulation unit 76 as described above. A sine wave electric signal source 61 for driving, a first DC voltage source 62 for driving the first optical phase modulator 75, and a second DC for driving the second optical phase modulator 76. The voltage source 63 is provided.

  Further, on the semiconductor substrate 51, a first electrode (high frequency signal line) 81 for the first optical phase modulation unit 75 and a second electrode (high frequency signal) for the second optical phase modulation unit 76 are provided. Line) 82 and a branch line 83 for the second optical phase modulator 75 and the second optical phase modulator 76 are formed. The sine wave electric signal source 61 is connected to one end side of the branch line 83. The other end side of the branch line 83 is branched into a first branch portion 83a and a second branch portion 83b. The first branch portion 83a is connected to the first electrode 81, and the second branch portion 83b. Is connected to the second electrode 82.

The sine wave electric signal source 61 generates a sine wave voltage signal having the same amplitude voltage V _m for driving the first optical phase modulation unit 75 and the second optical phase modulation unit 76. The first DC voltage source 62 generates a first DC bias voltage V _b1 for driving the first optical phase modulator 75. The second DC voltage source 63 generates a second DC bias voltage V _b2 for driving the second optical phase modulator 76. The first DC bias voltage V _b1 and the second DC bias voltage V _b2 are different from each other. The second DC bias voltage V _b2 is a voltage equal to or higher than the amplitude voltage V _m of the sine wave voltage signal. That is, the magnitude relationship among V _b1 , V _b2 , and V _m is V _b1 > V _b2 ≧ V _m .

  The optical branching unit 71, which is the input end of the Mach-Zehnder optical phase modulator 53, receives CW light (laser light) output from the CW light source (semiconductor laser) 52 to the optical branching unit 71. Is branched into a second branched light and a second branched light. The first branched light propagates through the first arm optical waveguide 73, and the second branched light propagates through the second arm optical waveguide 74.

In the first optical phase modulation unit 75, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 61 via the first branch portion 83a of the branch line 83 and the first electrode 81 When driven by the first DC bias voltage V _b1 applied from the first DC voltage source 62, the first branched light is phase-modulated to generate first phase-modulated light. In the second optical phase modulation unit 76, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 61 via the second branch portion 83 b and the second electrode 82 of the branch line 83 and When driven by the second DC bias voltage V _b2 applied from the second DC voltage source 63, the second branched light is phase-modulated to generate second phase-modulated light.

That is, the sine wave voltage signals for driving the optical phase modulation units 75 and 76 are sine wave voltage signals having the same amplitude voltage V _m , while the DC bias voltages for driving the optical phase modulation units 75 and 76 are different from each other. The DC bias voltages V _b1 and V _b2 are used. Even if the sine wave voltage signals have the same amplitude voltage V _m , the optical phase modulators 75 and 76 have nonlinear characteristics, and the optical phase modulators 75 and 76 have different DC bias voltages V _b1 and V _b from each other. _{Since b2} is applied, the state is substantially the same as in the case of applying sinusoidal voltage signals having different amplitude voltages as in the prior art.

Accordingly, each of the first optical phase modulation unit 75 and the second optical phase adjustment unit 76 is driven by a sine wave voltage signal having the same amplitude voltage V _m and different DC bias voltages V _b1 and V _b2 . By controlling the optical phase modulation amplitude, optical phase modulation can be performed asymmetrically between both arm optical waveguides 73 and 74 (optical phase modulation units 75 and 76).

  In the optical coupling unit 72, which is the output end of the Mach-Zehnder optical phase modulator 53, the first phase modulated light phase-modulated by the first optical phase modulation unit 75 and the phase by the second optical phase modulation unit 26. The modulated second phase-modulated light is multiplexed. As a result, flat optical subcarriers having the same frequency interval and constant intensity are generated and output from the output end (optical coupling unit 72) of the Mach-Zehnder optical phase modulator 53.

  In the optical subcarrier generator of the second embodiment, a CW light source monolithic integrated Mach-Zehnder optical phase in which a CW light source (semiconductor laser) 52 and a Mach-Zehnder optical phase modulator 53 are monolithically integrated on the same semiconductor substrate 51. Since the modulator 60 is used, it is not necessary to provide an external CW light source separately from the Mach-Zehnder optical phase modulator 53, and a polarization controller is provided between the CW light source (semiconductor laser) 52 and the Mach-Zehnder optical phase modulator 53. There is no need to provide it. Therefore, an optical subcarrier generator with a simpler device configuration and a smaller device scale is obtained.

Further, in the optical subcarrier generator of the second embodiment, the branch line formed on the semiconductor substrate 51 when the sine wave voltage signal of the voltage signal V _m is applied to the optical phase modulators on both arms of the Mach-Zehnder. 83 and electrodes (high-frequency signal lines) 81 and 82 are used, so that external electric signal wiring can be simplified.

  Further details will be described with reference to FIGS. FIG. 5 shows an example of the amount of phase change when a voltage is applied to the optical phase modulators 75 and 76 formed on the arm optical waveguides 73 and 74 of the Mach-Zehnder modulator 53, respectively. Since the change in the refractive index exhibits nonlinearity with respect to the voltage, the phase change also has a nonlinear response. In general, it has been experimentally obtained that the change in refractive index is given by a quadratic voltage equation as shown in the above equation (3).

In FIG. 4, a sine wave voltage signal having the same amplitude voltage V _m is applied from the sine wave electric signal source 61 to the optical phase modulators 75 and 76. Further, the DC bias voltages V _b1 and V _b2 are also applied (input) from the DC voltage sources 62 and 63 to the optical phase modulators 75 and 76. At this time, a bias point is set on the non-linear phase-voltage characteristic of FIG. 5, and a voltage magnitude is given so that V _b2 ≧ V _m .

6 and 7 show a total of 11 optical subcarriers including carrier components, with the phase differences Δφ _m and Δφ _dc between the arm optical waveguides 73 and 74 (optical phase modulators 75 and 76) as parameters, respectively. The calculation example of the flatness of the spectrum at the time of production | generation is shown. FIG. 8 shows an example of spectrum calculation of an optical subcarrier. The definition of the flatness is the difference between the minimum intensity and the maximum intensity of the generated optical subcarrier, and shows the result within 1 dB. The modulation frequency was 10 GHz. The amplitude voltage V _m of the sinusoidal voltage signal that drives the optical phase modulators 75 and 76 was calculated by the above equation (6) with the phase term φ _nl indicating nonlinearity being 0.125π to 1.5π. As an example, the phase change coefficients C ₁ and C ₂ are set to C ₁ = −0.48 and C2 = 0.57. This value is that of a typical InP-based pin optical phase modulator. The values of the phase change coefficients C ₁ and C ₂ can be arbitrarily set by appropriately setting the semiconductor quantum well structure, carrier density (electron-hole density), and the like. This change in value changes the magnitude of the DC bias voltage necessary to obtain the desired phase relationship, but the essence of the phase relationship required for flattening does not change.

As is apparent from FIGS. 6 and 7, it is understood that the optical subcarrier can be flattened around the straight line of Δφ _m = −Δφ _dc + π (Δφ _m is regarded as a function of Δφ _dc ). . As shown in FIG. 9, the phase relationship Δφ _m and Δφ _dc that can be flattened has a nonlinearity when the domain from Δφdc1 to π is defined on the x axis on the straight line Δφ _m = −Δφ _dc + π. It can be seen that the domain of definition expands as the phase term φ _nl shown increases. The initial point of the domain is
It is represented by It can be seen that as a result of the domain defined by π-Δφ _dc1 being expanded, the range showing flattening is also expanded.

On the other hand, when the error bar is defined for the straight line Δφ _m = −Δφ _dc + π, the error bar error can be confirmed to indicate that the specified error range increases with an increase in φ _nl. The range err (%) is expressed as follows.

When the error bar of the straight line Δφ _m = −Δφ _dc + π is defined using the equation (14) as an error range, it can be seen that flattening is possible in the phase relationships Δφ _m and Δφ _dc within the range. (Φ _nl = 0 does not exist on the principle of operation)
Therefore, the phase relationships Δφ _m and Δφ _dc that enable the flattening of the optical subcarriers should be defined within the range given by the equations (13) and (14) with respect to the straight line Δφ _m = −Δφ _dc + π. Can do.

FIG. 10 shows the relationship between the initial point of the domain and the error range of the error bar with respect to φ _nl . In the equation (12), in the region where Δφ _m is near 0 and Δφ _dc is large, as is apparent from the equation (12), the necessary DC bias voltage becomes extremely large, which is not practical. Recognize.

The expression of the DC bias voltage in the expression (12) can provide a necessary voltage when a required phase difference is given, and the magnitude thereof depends on the values of the phase change coefficients C ₁ and C _2. decide. For low-voltage operation, the action length L constituting the optical phase modulators 75 and 76 is increased. Furthermore, nonlinear refractive index changes in semiconductors are caused by coefficients c ₁ and c ₂ (phase change coefficients C ₁ and C ₂ ) indicating the amount of refractive index change by controlling the semiconductor quantum well structure and carrier density (electron-hole density). This also applies to the operating voltage.

<Embodiment 3>
Based on FIG. 11, an optical subcarrier generator according to Embodiment 3 of the present invention will be described. In the optical subcarrier generator (FIG. 11) of the third embodiment, the same parts as those of the optical subcarrier generator (FIG. 4) of the second embodiment are denoted by the same reference numerals. The detailed description which overlaps is abbreviate | omitted. The principle of optical subcarrier generation by the optical subcarrier generator of the third embodiment is the same as that of the optical subcarrier generator of the first and second embodiments.

  As shown in FIG. 11, in the optical subcarrier generator of the third embodiment, the first optical phase adjustment unit 91 and the second optical phase adjustment unit, which are optical phase adjustment regions for adjusting the initial phase. 92 is added. The optical subcarrier generator includes a first DC power supply 93 for driving the first optical phase adjustment unit 91 and a second DC power supply for driving the second optical phase adjustment unit 92. 94 is also added. The first optical phase adjustment unit 91 is formed on the first arm optical waveguide 73 of the Mach-Zehnder optical modulator 53, and the second optical phase adjustment unit 92 is the second arm of the Mach-Zehnder optical modulator 53. It is formed on the optical waveguide 74.

  Other configurations of the optical subcarrier generator of the third embodiment are the same as those of the optical subcarrier generator of the second embodiment.

  As described in the second embodiment, in the optical branching unit 71 that is the input end of the Mach-Zehnder optical phase modulator 53, CW light (laser light) output from the CW light source (semiconductor laser) 52 to the optical branching unit 71. ) And the input light is branched into a first branched light and a second branched light. The first branched light propagates through the first arm optical waveguide 73, and the second branched light propagates through the second arm optical waveguide 74.

In the first optical phase modulation unit 75, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 61 via the first branch portion 83a of the branch line 83 and the first electrode 81 When driven by the first DC bias voltage V _b1 applied from the first DC voltage source 62, the first branched light is phase-modulated to generate first phase-modulated light. In the second optical phase modulation unit 76, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 61 via the second branch portion 83 b and the second electrode 82 of the branch line 83 and When driven by the second DC bias voltage V _b2 applied from the second DC voltage source 63, the second branched light is phase-modulated to generate second phase-modulated light.

  The first DC power supply 93 generates a first DC current or a first DC voltage for driving the first optical phase adjustment unit 91, and the second DC power supply 94 generates the second light A second DC current or a second DC voltage for driving the phase adjustment unit 92 is generated. The first optical phase adjustment unit 91 is phase-modulated by the first optical phase modulation unit 75 by being driven by the first DC current or the first DC voltage applied from the first DC power supply 93. The phase of the first phase modulated light is adjusted. The second optical phase adjustment unit 92 is phase-modulated by the second optical phase modulation unit 76 by being driven by the second DC current or the second DC voltage applied from the second DC power supply 94. The phase of the second phase modulated light is adjusted.

  In the optical coupling unit 72, which is the output end of the Mach-Zehnder optical phase modulator 53, the first phase modulated light whose phase is adjusted by the first optical phase adjustment unit 91 and the phase by the second optical phase adjustment unit 92. The adjusted second phase-modulated light is multiplexed. As a result, flat optical subcarriers having the same frequency interval and constant intensity are generated and output from the output end (optical coupling unit 72) of the Mach-Zehnder optical phase modulator 53.

  In the illustrated example, the optical phase adjusting units 91 and 92 are provided after the optical phase modulating units 75 and 76. However, the present invention is not limited to this. The optical phase adjusting units 91 and 92 may be provided.

  In this case, the first optical phase adjusting unit 91 is driven by the first DC current or the first DC voltage applied from the first DC power supply 93, so that the first optical phase adjusting unit 91 is branched by the optical branching unit 71. The phase of the branched light of 1 is adjusted. The second optical phase adjusting unit 92 is driven by the second DC current or the second DC voltage applied from the second DC power supply 94, and thereby the second branch branched by the optical branching unit 71. Adjust the light phase.

Thereafter, in the first optical phase modulation unit 75, the sine wave voltage of the amplitude voltage V _m applied from the sine wave electric signal source 61 via the first branch portion 83 a of the branch line 83 and the first electrode 81. The first branched light phase-adjusted by the first optical phase adjusting unit 91 is driven by the signal and the first DC bias voltage V _b1 applied from the first DC voltage source 62. Phase modulation is performed to generate first phase modulated light. In the second optical phase modulation unit 76, a sine wave voltage signal having an amplitude voltage V _m applied from the sine wave electric signal source 61 via the second branch portion 83 b and the second electrode 82 of the branch line 83 and The second branched light phase-adjusted by the second optical phase adjusting unit 92 is phase-modulated by being driven by the second DC bias voltage V _b2 applied from the second DC voltage source 63. Thus, second phase modulated light is generated.

  In the optical coupling unit 72, which is the output end of the Mach-Zehnder optical phase modulator 53, the first phase-modulated light phase-modulated by the first optical phase modulation unit 75 and the phase of the second optical phase modulation unit 76 The modulated second phase-modulated light is multiplexed. As a result, flat optical subcarriers having the same frequency interval and constant intensity are generated and output from the output end (optical coupling unit 72) of the Mach-Zehnder optical phase modulator 53.

  In the optical subcarrier generator of the third embodiment, the output end (optical coupling unit) of the Mach-Zehnder optical modulator 53 is adjusted by the optical phase adjustment in the first optical phase adjustment unit 91 and the second optical phase modulation unit 92. 72), it is possible to control the initial phase difference during the light wave synthesis. As a result, it is possible to eliminate the initial phase difference due to a structural error between the two arm optical waveguides 73 and 74 and to reliably obtain the characteristics as described in the first and second embodiments.

  The optical phase adjusting units 91 and 92 that are optical phase adjusting regions have their refractive indexes changed by the first and second DC currents or the first and second DC voltages applied from the DC power sources 93 and 94. Thus, the initial phase difference is adjusted. That is, in the first optical phase adjusting unit 91, the refractive index is changed by the first DC current or the first DC voltage applied from the first DC power supply 93, so that the first phase-modulated light of the first phase-modulated light is changed. Adjust the phase. The second optical phase adjustment unit 92 changes the phase of the second phase-modulated light by changing the refractive index by the second DC current or the second DC voltage applied from the second DC power supply 94. adjust. As a result, the initial phase difference becomes zero. In other words, the values of the first and second DC currents or the first and second DC voltages applied from the DC power supplies 93 and 94 to the optical phase adjusting units 91 and 92 are such that the initial phase difference becomes zero. Is set to

  According to the optical subcarrier generator of the third embodiment, by providing the first optical phase adjustment unit 91 and the second optical phase adjustment unit 92 and performing phase adjustment, the structure is not completely symmetrical. In addition, the initial phase difference at the multiplexing output end (optical coupling unit 72) of the Mach-Zehnder optical modulator 53 can be made zero.

  Similarly to the optical subcarrier generator of the second embodiment, in the optical subcarrier generator of the third embodiment, two sine wave voltage signals are generated by the branch line 83 formed on the semiconductor substrate 51. It is the structure which branches to and inputs. For this reason, as the overall structure of the optical subcarrier generator of the third embodiment, the optical phase adjustment units 91 and 92 that are the optical phase adjustment region and the direct current or direct current for the optical phase adjustment region are generated. DC power supplies 93 and 94 are only added.

  The present invention relates to an optical subcarrier generator. An optical subcarrier generator having a simpler device configuration and a smaller device size than conventional ones generates flat optical subcarriers with equal frequency intervals and constant intensity. It is useful to apply to.

20 Mach-Zehnder optical modulator 21 Optical branching unit 22 Optical coupling unit 23 First arm optical waveguide 24 Second arm optical waveguide 25 First optical phase modulation unit 26 Second optical phase modulation unit 31 CW light source 32 Sine wave Electric signal source 33 First DC voltage source 34 Second DC voltage source 51 Semiconductor substrate 52 CW light source (semiconductor laser)
53 Mach-Zehnder Type Optical Modulator 60 CW Light Source Monolithic Integrated Mach-Zehnder Type Optical Modulator 61 Sine Wave Electric Signal Source 62 First DC Voltage Source 63 Second DC Voltage Source 71 Optical Branching Unit 72 Optical Coupling Unit 73 First Arm Light Waveguide 74 Second arm optical waveguide 75 First optical phase modulation unit 76 Second optical phase modulation unit 81 First electrode 82 Second electrode 83 Branch line 83a First branch unit 83b Second branch unit 91 First optical phase adjustment unit 92 Second optical phase adjustment unit 93 First DC power supply 94 Second DC power supply

Claims (6)

An optical branching unit, an optical coupling unit, a first arm optical waveguide and a second arm optical waveguide sandwiched between the optical coupling unit and the optical coupling unit, and the first arm optical waveguide are formed. A Mach-Zehnder comprising: a first optical phase modulation unit having nonlinear phase-voltage characteristics; and a second optical phase modulation unit having nonlinear phase-voltage characteristics formed on the second arm optical waveguide. Type optical modulator,
A CW light source that outputs CW light to the optical branching unit;
A sine wave electric signal source for applying a sine wave voltage signal of the same amplitude voltage to the first optical phase modulation unit and the second optical phase modulation unit;
A first DC voltage source for applying a first DC bias voltage to the first optical phase modulator;
A second DC voltage source for applying a second DC bias voltage to the second optical phase modulator,
When the amplitude voltage of the sine wave voltage signal is V _m , the first DC bias voltage is V _b1 , and the second DC bias voltage is V _b2 , the magnitude relationship between them is V _b1 > V _b2 ≧ V _m ,
Each of the first optical phase modulation unit and the second optical phase adjustment unit is configured such that the first DC bias voltage and the second DC bias voltage are different from the sine wave voltage signal having the same amplitude voltage. Driving and controlling the optical phase modulation amplitude to perform asymmetric optical phase modulation between the first optical phase modulation unit and the second optical phase modulation unit,
An optical subcarrier generator. The optical subcarrier generator according to claim 1, wherein
The first optical phase modulation unit and the second optical phase modulation unit are configured to cause a change in refractive index due to electric field-light absorption such as a quantum confined Stark effect using a semiconductor material. Optical subcarrier generator. The optical subcarrier generator according to claim 1 or 2,
An optical subcarrier generator, wherein the CW light source and the Mach-Zehnder type optical modulator are monolithically integrated on the same semiconductor substrate to constitute a CW light source monolithic integrated Mach-Zehnder type optical modulator. The optical subcarrier generator according to claim 3, wherein
A branch line and an electrode for applying a sine wave voltage signal having the same amplitude voltage to the first optical phase modulation unit and the second optical phase modulation unit are formed on the semiconductor substrate. An optical subcarrier generator. The optical subcarrier generator according to any one of claims 1 to 4,
In the adjustment of the first DC bias voltage and the second DC bias voltage, and the phase difference between the first optical phase modulation unit and the second optical phase modulation unit, the phase of the sine wave voltage signal A phase difference between the first optical phase modulation unit and the second optical phase modulation unit with respect to a modulation amplitude component and a phase change amount due to the first DC bias voltage and the second DC bias voltage Are defined as Δφ _m and Δφ _dc , respectively, the first optical phase modulation unit and the second optical phase so as to satisfy the relationship of phase differences Δφ _m and Δφ _dc of a straight line Δφ _m = −Δφ _dc + π An optical subcarrier generator that drives the modulator to make the intensity of the optical subcarrier constant, and an error range err between the initial point Δφ _dc1 and the error bar in the domain of the straight line Δφ _m = −Δφ _dc + π , the phase term phi _nl showing the non-linearity, the following formula (I And the phase difference [Delta] [phi _m to be within the range defined by Formula (II), optical subcarrier generator which is a configuration for adjusting the driving voltage so that a relationship of [Delta] [phi _dc.
In the optical subcarrier generator according to any one of claims 1 to 5,
A first optical phase adjustment unit formed on the first arm optical waveguide;
A second optical phase adjuster formed on the second arm optical waveguide;
A first DC power supply for applying a first DC current or a first DC voltage to the first optical phase adjustment unit;
A second DC power source for applying a second DC current or a second DC voltage to the second optical phase adjustment unit,
By driving each of the first optical phase adjustment unit and the second optical phase adjustment unit with the first and second DC currents or the first and second DC voltages, the Mach-Zehnder light An optical subcarrier generator characterized in that an initial phase difference at the optical coupling unit which is a multiplexing output terminal of a modulator is adjusted to be zero.