阅读说明：本技术 一种基于dp-qpsk调制器的四倍频三角波发生器 (Quadruple frequency triangular wave generator based on DP-QPSK modulator ) 是由 王创业 李晶 宁提纲 裴丽 郑晶晶 于 2020-04-16 设计创作，主要内容包括：本发明为一种基于DP-QPSK调制器的四倍频三角波发生器。涉及光电子器件,微波光子学领域。该器件包括连续波激光器(1)、偏振控制器(2)、射频信号源(3)、电功分器(4)、三倍频器(5)、电衰减器(6)、90度电桥(7)、90度电桥(8)、DP-QPSK调制器(9)。本发明中生成的三角波的重复率是射频信号频率的四倍。调制器的调制系数不再是固定值并可在一定范围内变化。(The invention relates to a quadruple frequency triangular wave generator based on a DP-QPSK modulator. Relates to the field of photoelectronic devices and microwave photonics. The device comprises a continuous wave laser (1), a polarization controller (2), a radio frequency signal source (3), an electric power divider (4), a frequency tripler (5), an electric attenuator (6), a 90-degree electric bridge (7), a 90-degree electric bridge (8) and a DP-QPSK modulator (9). The repetition rate of the triangular wave generated in the present invention is four times the frequency of the radio frequency signal. The modulation factor of the modulator is no longer a fixed value and can vary over a range.)

1. A quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the device comprises a continuous wave laser (1), a polarization controller (2), a radio frequency signal source (3), an electric power divider (4), a frequency tripler (5), an electric attenuator (6), a 90-degree electric bridge (7), a 90-degree electric bridge (8) and a DP-QPSK modulator (9); the DP-QPSK modulator consists of an optical power splitter (91), double parallel Mach-Zehnder modulators (92), double parallel Mach-Zehnder modulators (93), a 90-degree polarization rotator (94) and a polarization beam combiner (95), wherein the double parallel Mach-Zehnder modulators (92) consist of a single-drive Mach-Zehnder modulator (921) and a single-drive Mach-Zehnder modulator (922), and the double parallel Mach-Zehnder modulators (93) consist of a single-drive Mach-Zehnder modulator (931) and a single-drive Mach-Zehnder modulator (932); the concrete connection mode is as follows:

the output end of the continuous wave laser (1) is connected with the input end of a polarization controller (2), the output end of the polarization controller (2) is connected with the optical input end of a DP-QPSK modulator (9), the output end of a radio frequency signal source (3) is connected with the input end of a power divider (4), the output end of the power divider (4) is respectively connected with the input end of a frequency tripler (5) and the input end of a 90-degree bridge (7), the output end of the frequency tripler (5) is connected with the input end of an attenuator (6), the output end of the electrical attenuator (6) is connected with the input end of the 90-degree bridge (8), the 0-degree output end (71) and the 90-degree output end (72) of the 90-degree bridge (7) are respectively connected with the radio frequency input end of a single-drive Mach-Zehnder modulator (921) and the radio frequency input end of the single-drive Mach modulator (922), and the 0-degree output end (81) and the 90-degree output end (82) of the 90-degree bridge (8) are respectively connected with the radio frequency input end of the single-drive Mach Zehnder modulator (931) and the single-drive Mach Zehnder modulator (922) A radio frequency input of the moving mach-zehnder modulator (932).

2. The DP-QPSK modulator-based quadruple triangular wave generator of claim 1, wherein: the single-drive Mach-Zehnder modulator (921), the single-drive Mach-Zehnder modulator (922), the single-drive Mach-Zehnder modulator (931) and the single-drive Mach-Zehnder modulator (932) are all biased at a maximum transmission point, and the double-parallel Mach-Zehnder modulator (92) and the double-drive Mach-Zehnder modulator (93) are all biased at a minimum transmission point.

3. The DP-QPSK modulator-based quadruple triangular wave generator of claim 1, wherein: the repetition rate of the generated triangular wave is four times the frequency of the radio frequency signal.

4. The DP-QPSK modulator-based quadruple triangular wave generator of claim 1, wherein: the modulation factor is adjustable.

Technical Field

The invention relates to a quadruple frequency triangular wave generator based on a DP-QPSK modulator, and relates to the fields of optoelectronic devices, microwave photonics and the like.

Background

Triangular wave generation is an important research direction in the field of microwave photonics, and has many important applications, such as pulse compression, optical signal conversion, optical signal replication, and the like. The traditional method for generating the triangular wave by electronics has the problems of small bandwidth, low repetition rate of the generated triangular wave and the like. Researchers have proposed many photonics-based approaches in recent years that overcome these shortcomings. One method is to use spectral shaping in conjunction with frequency-time mapping to generate triangular waves (j.ye, l.yan, w.pan, b.luo, x.zuo, a.yi, s.yao, Photonic generation of triangular-shaped pulses based on frequency-time conversion, opt.lett.36(8) (2011) 1458) 1460.), but the duty cycle of the triangular waves generated by this method is less than 1. More common and much studied are methods based on external modulation, which can be implemented in many ways, such as generating a triangle wave by using a modulator in combination with a dispersive fiber (j.li, x.zhang, b.hraimel, t.nin, l.pei, k.wu, Performance Analysis of a Photonic-Assisted Performance Triangular-Shaped lasers Generator, j.lightwave technology.30 (11) (2012) 1617-. Triangular waves (j.yuan, t.nin, j.li, l.pei, h.chen, c.zhang, y.li, inversion on square-shaped lasers generator with a flexible reproduction rate capability, Optical and quantity Electronics 48(3) (2016) 208) can also be generated by using a modulator in combination with a polarizer, but the polarizer is susceptible to environmental disturbances. Triangular waves may also be generated based on a single modulator or multiple modulators, for example: a Dual parallel Mach-Zehnder Modulator (F.Zhang, X.Ge, S.Pan, Triangular pulse generation Using a Dual-parallel Mach-Zehnder Modulator by a single-parallel radio Frequency signal, Opti.Lett.38 (21) (2013) 4493.), a Dual Polarization Modulator (F.Zhang, B.Gao, S.Pan, Time-Domain wave modulation Using a Dual-Polarization Modulator, IEEE Photonic Technology Letters 28(23) (2689), a quadrature phase shift keying Modulator (Z.Zhu, S.Zhang, X.Li, K.Qu, T.Lin, Linear-Modulator by 2016) (20189), a Dual-parallel Mach-Zehnder Modulator by a single-parallel radio Frequency signal, Q.Q.Q.A.Zhang, Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.K.Q.K.Lin, Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.Q.A.Q.Q.Q.Q.A.A.Q.K.K.K.K.K.K.K.K.K.K.K.K.K.K.K.K.K.K.K.Q.Q.Q.Q.Q.Q.Q.2016.Q.Q.Q.Q.2016.Q.Q.Q.Q.Q.Q.2016.Q.21.2016.Q.21.21.21.21.2016.Q.21.21.Q.Q.Q.Q.A.A.A.A.Q.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.B.A.A.A.A.A.A.A.A.A.A.A.A.A.A.A.B.A.A.A.A.B.A.A.A.A.A.A.A.A.A.A., optics Communications 445(2019) 231-. The modulation coefficients in these schemes are fixed values, making the schemes inflexible. Some cannot generate a triangular wave with a high frequency multiplication. The invention provides a quadruple frequency triangular wave generator with a simple structure, and the repetition rate of the generated triangular wave is four times of the frequency of a radio frequency signal. Compared with other quadruple triangular wave schemes, no dispersive optical fiber, no fiber Bragg grating and no complex polarization equipment are used. The modulation factor is not a fixed value and can be varied within a certain range, which increases the flexibility of the system.

Disclosure of Invention

The invention provides a quadruple frequency triangular wave generator based on a DP-QPSK modulator.

The invention is realized by the following technical scheme:

a quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the triangular wave generator comprises a continuous wave laser 1, a polarization controller 2, a radio frequency signal source 3, an electric power divider 4, a frequency tripler 5, an electric attenuator 6, a 90-degree electric bridge 7, a 90-degree electric bridge 8 and a DP-QPSK modulator 9. The concrete connection mode is as follows:

the output end of the continuous wave laser 1 is connected with the input end of the polarization controller 2. The output end of the polarization controller 2 is connected with the optical input end of a DP-QPSK modulator 9, the output end of the radio frequency signal source 3 is connected with the input end of the power divider 4, the output end of the power divider 4 is respectively connected with the input end of the frequency tripler 5 and the input end of the 90-degree bridge 7, the output end of the frequency tripler 5 is connected with the input end of the electric attenuator 6, the output end of the electric attenuator 6 is connected with the input end of the 90-degree bridge 8, and the 0-degree output end and the 90-degree output end of the 90-degree bridge 7 are respectively connected with the radio frequency input end of the single-drive Mach-Zehnder modulator 921 and the radio frequency input end of the single-drive Mach-Zehnder modulator 922. The 0-degree output terminal and the 90-degree output terminal of the 90-degree bridge 8 are respectively connected to the radio frequency input terminal of the single-drive mach-zehnder modulator 931 and the radio frequency input terminal of the single-drive mach-zehnder modulator 932.

The continuous wave laser 1 is connected with the polarization controller 2, and the polarization controller 2 is connected with the DP-QPSK modulator 9 through optical fibers. The radio frequency signal source 3 is connected with the electric power divider 4, the electric power divider 4 is connected with the frequency tripler 5, the electric power divider 4 is connected with the 90-degree electric bridge 7, the frequency tripler 5 is connected with the electric attenuator 6, the attenuator 6 is connected with the 90-degree electric bridge 8, the 90-degree electric bridge 7 is connected with the single-drive Mach-Zehnder modulator 921, the 90-degree electric bridge 7 is connected with the single-drive Mach-Zehnder modulator 922, the 90-degree electric bridge 8 is connected with the single-drive Mach-Zehnder modulator 931, and the 90-degree electric bridge 8 is connected with the single-drive Mach-Zehnder modulator 932 through radio frequency lines.

The specific working principle of the invention is as follows:

an optical signal emitted by the continuous wave laser 1 enters the DP-QPSK modulator 9 after passing through the polarization controller 2, and the optical field expression of the optical signal emitted by the continuous wave laser 1 is set as follows: e_in(t)＝E_oexp(jω_ot)，E_oAnd ω_oRepresenting the amplitude and angular frequency of the optical signal, respectively. The electric field expression of the radio frequency signal sent by the radio frequency signal source 3 is as follows: v_RF(t)＝V_RFcos(ωt),V_RFAnd ω represents the amplitude and angular frequency of the electrical signal, respectively.

Then the optical field expressions of the two optical signals in mutually orthogonal directions output by the DP-QPSK modulator 9 are respectively:

whereinandRepresenting the modulation factor of the dual parallel mach-zehnder modulator 92 and the modulation factor of the dual parallel mach-zehnder modulator 93, respectively. And V pi represents a half-wave voltage. ρ represents the attenuation coefficient of the electrical attenuator 6. Respectively, a single-drive Mach-Zehnder modulator 921, a single-drive Mach-Zehnder modulator 922, a double-parallel Mach-Zehnder modulator 92, a single-drive Mach-Zehnder modulator 931, and a single-drive motorThe herzehnder modulator 932, the phase shift due to the dc offset of the dual parallel mach zehnder modulator 93.

The single-drive mach-zehnder modulator 921, the single-drive mach-zehnder modulator 922, the single-drive mach-zehnder modulator 931 and the single-drive mach-zehnder modulator 932 are all biased at the maximum transmission point. The double parallel mach-zehnder modulator 92 and the double parallel mach-zehnder modulator 93 are both set biased at the minimum transmission point.

When the modulation factor is small, only the second-order sideband can be considered, and the following can be obtained:

the optical intensity expression of the output optical signal of the DP-QPSK modulator 9 is:

the ideal triangular wave Fourier series expansion is as follows:

comparing the formula (3) and the formula (4), it is necessary to generate the triangular wave:

when the modulation factor m is small, it can be:

as can be seen from equation (6), the modulation factor of the modulator is not a fixed value when the condition for generating the triangular wave is satisfied. This increases the flexibility of the system.

The invention has the beneficial effects that:

the invention has simple structure, and realizes the generation of quadruple frequency triangular waves by utilizing the principle of electro-optical modulation. Compared with other quadruple triangular wave schemes, no dispersive optical fiber, no fiber Bragg grating and no complex polarization equipment are used. The modulation factor is not a fixed value and can be varied within a certain range, which increases the flexibility of the system.

Drawings

Fig. 1 is a schematic structural diagram of a quadruple triangular wave generator based on a DP-QPSK modulator.

FIG. 2 shows a modulation factor m₁And a modulation factor m₂A graph of the relationship (c).

FIG. 3 is a time domain diagram of the output triangular wave of the device in the first embodiment.

FIG. 4 is a time domain diagram of the triangular wave output by the device of the second embodiment.

FIG. 5 is a time domain diagram of the triangular wave output by the device in the third embodiment.

FIG. 6 is a time domain diagram of the triangular wave output by the device in the fourth embodiment.

Detailed Description

The invention is further described below with reference to the figures and examples.

The first embodiment is as follows:

a quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the generator comprises a continuous wave laser 1, a polarization controller 2, a radio frequency signal source 3, an electric power divider 4, a frequency tripler 5, an electric attenuator 6, a 90-degree electric bridge 7, a 90-degree electric bridge 8 and a DP-QPSK modulator 9. The DP-QPSK modulator 9 is composed of an optical power splitter 91, a dual-parallel mach-zehnder modulator 92, a dual-parallel mach-zehnder modulator 93, a 90-degree polarization rotator 94, and a polarization beam combiner 95. The concrete connection mode is as follows:

the output end of the continuous wave laser 1 is connected with the input end of the polarization controller 2, the output end of the polarization controller 2 is connected with the optical input end of the DP-QPSK modulator 9, the output end of the radio frequency signal source 3 is connected with the input end of the power divider 4, the output end of the power divider 4 is respectively connected with the input end of the frequency tripler 5 and the input end of the 90-degree electric bridge 7, the output end of the frequency tripler 5 is connected with the input end of the electric attenuator 6, the output end of the electric attenuator 6 is connected with the input end of the 90-degree electric bridge 8, the 0-degree output end 71 and the 90-degree output end 72 of the 90-degree electric bridge 7 are respectively connected with the radio frequency input end of the single-drive Mach-Zehnder modulator 921 and the radio frequency input end of the single-drive Mach-Zehnder modulator 922, the 0-degree output terminal 81 and the 90-degree output terminal 82 of the 90-degree bridge 8 are respectively connected to the radio frequency input terminal of the single-drive mach-zehnder modulator 931 and the radio frequency input terminal of the single-drive mach-zehnder modulator 932.

The single-drive mach-zehnder modulator 921, the single-drive mach-zehnder modulator 922, the single-drive mach-zehnder modulator 931, and the single-drive mach-zehnder modulator 932 are all biased at the maximum transmission point. The dual parallel mach-zehnder modulator 92 and the dual parallel mach-zehnder modulator 93 are biased at a minimum transmission point.

The insertion loss and half-wave voltage of the modulator were set to 5dB and 4V, respectively, and the extinction ratio of the modulator was set to 30 dB. The power and wavelength of the continuous wave laser 1 were set to 15dBm and 1550.12nm, respectively, and the linewidth of the continuous wave laser 1 was set to 10 MHz. The modulation factor of the dual parallel mach-zehnder modulator 92 is set to 1.5 and the modulation factor of the dual parallel mach-zehnder modulator 93 is set to 0.81. The frequency of the radio frequency signal source 3 is set to 7 GHz. The time domain diagram of the output signal of the DP-QPSK modulator 9 is shown in fig. 3.

Example two:

a quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the generator comprises a continuous wave laser 1, a polarization controller 2, a radio frequency signal source 3, an electric power divider 4, a frequency tripler 5, an electric attenuator 6, a 90-degree electric bridge 7, a 90-degree electric bridge 8 and a DP-QPSK modulator 9. The DP-QPSK modulator 9 is composed of an optical power splitter 91, a dual-parallel mach-zehnder modulator 92, a dual-parallel mach-zehnder modulator 93, a 90-degree polarization rotator 94, and a polarization beam combiner 95. The concrete connection mode is as follows:

the output end of the continuous wave laser 1 is connected with the input end of the polarization controller 2, the output end of the polarization controller 2 is connected with the optical input end of the DP-QPSK modulator 9, the output end of the radio frequency signal source 3 is connected with the input end of the power divider 4, the output end of the power divider 4 is respectively connected with the input end of the frequency tripler 5 and the input end of the 90-degree electric bridge 7, the output end of the frequency tripler 5 is connected with the input end of the electric attenuator 6, the output end of the electric attenuator 6 is connected with the input end of the 90-degree electric bridge 8, the 0-degree output end 71 and the 90-degree output end 72 of the 90-degree electric bridge 7 are respectively connected with the radio frequency input end of the single-drive Mach-Zehnder modulator 921 and the radio frequency input end of the single-drive Mach-Zehnder modulator 922, the 0-degree output terminal 81 and the 90-degree output terminal 82 of the 90-degree bridge 8 are respectively connected to the radio frequency input terminal of the single-drive mach-zehnder modulator 931 and the radio frequency input terminal of the single-drive mach-zehnder modulator 932.

The single-drive mach-zehnder modulator 921, the single-drive mach-zehnder modulator 922, the single-drive mach-zehnder modulator 931, and the single-drive mach-zehnder modulator 932 are all biased at the maximum transmission point. The dual parallel mach-zehnder modulator 92 and the dual parallel mach-zehnder modulator 93 are biased at a minimum transmission point.

The insertion loss and half-wave voltage of the modulator were set to 5dB and 4V, respectively, and the extinction ratio of the modulator was set to 30 dB. The power and wavelength of the continuous wave laser 1 were set to 15dBm and 1550.12nm, respectively, and the linewidth of the continuous wave laser 1 was set to 10 MHz. The modulation factor of the dual parallel mach-zehnder modulator 92 is set to 1.5 and the modulation factor of the dual parallel mach-zehnder modulator 93 is set to 0.81. The frequency of the radio frequency signal source 3 is set to 8 GHz. The time domain diagram of the output signal of the DP-QPSK modulator 9 is shown in fig. 4.

Example three:

a quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the generator comprises a continuous wave laser 1, a polarization controller 2, a radio frequency signal source 3, an electric power divider 4, a frequency tripler 5, an electric attenuator 6, a 90-degree electric bridge 7, a 90-degree electric bridge 8 and a DP-QPSK modulator 9. The DP-QPSK modulator 9 is composed of an optical power splitter 91, a dual-parallel mach-zehnder modulator 92, a dual-parallel mach-zehnder modulator 93, a 90-degree polarization rotator 94, and a polarization beam combiner 95. The concrete connection mode is as follows:

the output end of the continuous wave laser 1 is connected with the input end of the polarization controller 2, the output end of the polarization controller 2 is connected with the optical input end of the DP-QPSK modulator 9, the output end of the radio frequency signal source 3 is connected with the input end of the power divider 4, the output end of the power divider 4 is respectively connected with the input end of the frequency tripler 5 and the input end of the 90-degree electric bridge 7, the output end of the frequency tripler 5 is connected with the input end of the electric attenuator 6, the output end of the electric attenuator 6 is connected with the input end of the 90-degree electric bridge 8, the 0-degree output end 71 and the 90-degree output end 72 of the 90-degree electric bridge 7 are respectively connected with the radio frequency input end of the single-drive Mach-Zehnder modulator 921 and the radio frequency input end of the single-drive Mach-Zehnder modulator 922, the 0-degree output terminal 81 and the 90-degree output terminal 82 of the 90-degree bridge 8 are respectively connected to the radio frequency input terminal of the single-drive mach-zehnder modulator 931 and the radio frequency input terminal of the single-drive mach-zehnder modulator 932.

The single-drive mach-zehnder modulator 921, the single-drive mach-zehnder modulator 922, the single-drive mach-zehnder modulator 931, and the single-drive mach-zehnder modulator 932 are all biased at the maximum transmission point. The dual parallel mach-zehnder modulator 92 and the dual parallel mach-zehnder modulator 93 are biased at a minimum transmission point.

The insertion loss and half-wave voltage of the modulator were set to 5dB and 4V, respectively, and the extinction ratio of the modulator was set to 30 dB. The power and wavelength of the continuous wave laser 1 were set to 15dBm and 1550.12nm, respectively, and the linewidth of the continuous wave laser 1 was set to 10 MHz. The modulation factor of the dual parallel mach-zehnder modulator 92 is set to 1.5 and the modulation factor of the dual parallel mach-zehnder modulator 93 is set to 0.81. The frequency of the radio frequency signal source 3 is set to 9 GHz. A time domain diagram of the output signal of the DP-QPSK modulator 9 is shown in fig. 5.

Example four:

a quadruple frequency triangular wave generator based on a DP-QPSK modulator is characterized in that: the generator comprises a continuous wave laser 1, a polarization controller 2, a radio frequency signal source 3, an electric power divider 4, a frequency tripler 5, an electric attenuator 6, a 90-degree electric bridge 7, a 90-degree electric bridge 8 and a DP-QPSK modulator 9. The DP-QPSK modulator 9 is composed of an optical power splitter 91, a dual-parallel mach-zehnder modulator 92, a dual-parallel mach-zehnder modulator 93, a 90-degree polarization rotator 94, and a polarization beam combiner 95. The concrete connection mode is as follows:

the output end of the continuous wave laser 1 is connected with the input end of the polarization controller 2, the output end of the polarization controller 2 is connected with the optical input end of the DP-QPSK modulator 9, the output end of the radio frequency signal source 3 is connected with the input end of the power divider 4, the output end of the power divider 4 is respectively connected with the input end of the frequency tripler 5 and the input end of the 90-degree electric bridge 7, the output end of the frequency tripler 5 is connected with the input end of the electric attenuator 6, the output end of the electric attenuator 6 is connected with the input end of the 90-degree electric bridge 8, the 0-degree output end 71 and the 90-degree output end 72 of the 90-degree electric bridge 7 are respectively connected with the radio frequency input end of the single-drive Mach-Zehnder modulator 921 and the radio frequency input end of the single-drive Mach-Zehnder modulator 922, the 0-degree output terminal 81 and the 90-degree output terminal 82 of the 90-degree bridge 8 are respectively connected to the radio frequency input terminal of the single-drive mach-zehnder modulator 931 and the radio frequency input terminal of the single-drive mach-zehnder modulator 932.

The single-drive mach-zehnder modulator 921, the single-drive mach-zehnder modulator 922, the single-drive mach-zehnder modulator 931, and the single-drive mach-zehnder modulator 932 are all biased at the maximum transmission point. The dual parallel mach-zehnder modulator 92 and the dual parallel mach-zehnder modulator 93 are biased at a minimum transmission point.

The insertion loss and half-wave voltage of the modulator were set to 5dB and 4V, respectively, and the extinction ratio of the modulator was set to 30 dB. The power and wavelength of the continuous wave laser 1 were set to 15dBm and 1550.12nm, respectively, and the linewidth of the continuous wave laser 1 was set to 10 MHz. The modulation factor of the dual parallel mach-zehnder modulator 92 is set to 1.5 and the modulation factor of the dual parallel mach-zehnder modulator 93 is set to 0.81. The frequency of the radio frequency signal source 3 is set to 10 GHz. A time domain diagram of the output signal of the DP-QPSK modulator 9 is shown in fig. 6.