JP2020027214A - Variable-line width light generation device and variable-line width light generation method - Google Patents

Abstract

To provide a device that varies a line width of continuous light output from a light source without causing a frequency shift.SOLUTION: A variable-line width light generation device is equipped with a light source 10 and a vector modulator 80. the light source generates and sends continuous light to the vector modulator. The vector modulator causes continuous light S1 including a phase component ψ and modulated with a first signal of cosψ and continuous light S1 modulated with a second signal of sinψ to interfere with each other and outputs the resulting light. Here, ψ of the first signal and second signal is a phase noise having a fluctuation of 1/f, where f is a frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a variable line width light generation device and a variable line width light generation method for changing the line width of light output from a light source.

  In recent years, mainly on coherent communication, technologies that focus on light coherence such as a coherent lidar, a laser Doppler sensor, and a coherent optical time-domain reflection measurement (coherent OTDR) have been actively studied. In coherent communication, large-capacity communication is realized by the coherence of light. Further, in lightwave sensing, higher precision of measurement is realized. The performance of these techniques based on light coherence is affected by the linewidth of the continuous light output from the light source. Therefore, in order to evaluate the performance of these techniques, it is necessary to clarify the dependence of the performance on the line width.

  For example, in the case of coherent communication, a semiconductor laser is used as a light source. A semiconductor laser can easily change the wavelength of continuous light within a narrow range by adjusting an injection current or temperature. However, with the change of the wavelength, the line width of the continuous light largely deviates from the nominal value, and the output power also fluctuates. Therefore, when evaluating the performance of the system with a constant line width and a constant output power, it is necessary to use a plurality of light sources having a common line width and a common output power and having different wavelengths. . However, this method cannot only perform verification with fine precision, but also requires a plurality of light sources, which increases costs.

  As a technique for avoiding this, there is a method of changing the line width of continuous light from a light source by external modulation (for example, Non-Patent Documents 1 and 2).

In the technique according to Non-Patent Document 1, continuous light is input to a lithium niobate (LN) phase modulator. In addition to continuous light, a phase noise having a fluctuation of 1 / f ² (hereinafter, also referred to as 1 / f ² noise) is input to the LN phase modulator in addition to continuous light. 1 / f ² noise is generated by integrating white noise output from a white noise source with a complete integration circuit. In the technique disclosed in Non-Patent Document 1, the LN phase modulator to modulate the addition of 1 / f ² noise continuous light. As a result, the line width of the continuous light can be approximated by the phase noise is proportional to 1 / f ^2. For this reason, in the technique according to Non-Patent Document 1, the line width of continuous light can be changed.

  However, in the technology according to Non-Patent Document 1, an active element such as an operational amplifier used as a complete integration circuit cannot in principle avoid an offset voltage. Therefore, there is a problem that the output from the complete integration circuit is saturated when the complete integration circuit continues to integrate the DC offset component superimposed on the white noise.

In order to solve such a problem, the technology according to Non-Patent Document 2 uses a voltage controlled oscillator (VCO) instead of a complete integration circuit. The VCO is an oscillator that outputs a frequency corresponding to the input voltage. In the technique according to Non-Patent Document 2, white noise is input to the VCO, so that the oscillation frequency ω (V) of the VCO fluctuates by 1 / f ² . In the technique disclosed in Non-Patent Document 2, on the basis of the oscillation frequency omega (V), in LN phase modulator by frequency modulating a continuous light having a center angular frequency omega _0, to change the line width of the continuous light it can. In the technique according to Non-Patent Document 2, the use of the VCO does not cause a problem that the output is saturated unlike the technique according to Non-Patent Document 1 in which a complete integration circuit using active elements is used.

Design Theory of Electrically Frequency-Controlled Narrow-Linewidth Semiconductor Lasers for Coherent Optical Communication Systems, Kazuro Kikuchi, IEEE Report on experiments on residual phase error reduction in optical phase locked loop, Akihiro Fujii, Proc. Of the IEICE General Conference 2017, p306

Here, in the technique according to Non-Patent Document 2, since continuous light is modulated based on the oscillation frequency of the VCO, the LN phase modulator outputs ω on the high frequency side and the low frequency side with respect to the center angular frequency ω ₀ . (V) A plurality of angular frequency components frequency-shifted at intervals are output. Then, the line width of the output of these frequency-shifted angular frequency components of ω ₀ ± kω (V) is changed (k is an integer of 1 or more). Therefore, in order to extract one of the plurality of angular frequency components excluding the central angular frequency ω ₀ , in the technique according to Non-Patent Document 2, it is necessary to provide an optical band-pass filter downstream of the LN phase modulator. There is. As a result, the technique according to Non-Patent Document 2 has a problem in that the cost for providing the optical bandpass filter is high, and a loss of output occurs in the optical bandpass filter.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an apparatus and a method for changing the line width of continuous light output from a light source without causing a frequency shift. It is in.

In order to achieve the above object, a variable linewidth light generating device according to the present invention includes a light source and a vector modulator. The light source produces a continuous light and sends it to the vector modulator. The vector modulator causes continuous light modulated by the first signal of cosψ and continuous light modulated by the second signal of sinψ to interfere with each other and output when the phase component is ψ. Ψ in the first signal and the second signal is phase noise having a fluctuation of 1 / f ^{2 where} f is the frequency.

Further, a variable linewidth light generating method according to the present invention includes the following steps. That is, continuous light is generated. Then, the continuous light modulated by the first signal of cosψ and the continuous light modulated by the second signal of sinψ are made to interfere with each other and output when the phase component is ψ. Ψ in the first signal and the second signal is phase noise having a fluctuation of 1 / f ^{2 where} f is the frequency.

According to the variable line width light generating apparatus and the variable line width light generating method of the present invention, continuous light from a light source can be changed to a line width corresponding to 1 / f ² without frequency shift. For this reason, there is no need to provide an optical bandpass filter at a stage subsequent to the vector modulator. Therefore, it is possible to solve the above-described problems of cost and output loss due to the provision of the optical bandpass filter.

It is a typical block diagram of a variable line width light generator. It is a figure showing an angular frequency spectrum. FIG. 3 is a schematic plan view of a vector modulator.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangements of the components are only schematically shown to the extent that the present invention can be understood. Hereinafter, a preferred configuration example of the present invention will be described. However, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the present invention.

(Variable line width light generation device and variable line width light generation method)
With reference to FIGS. 1 to 3, a variable line width light generating apparatus and a variable line width light generating method according to the present invention will be described. FIG. 1 is a schematic block diagram of a variable line width light generator. FIG. 2 is a diagram showing an angular frequency spectrum. FIG. 3 is a schematic plan view of the vector modulator.

  The variable linewidth light generator 100 includes a light source 10, a white noise source 20, a VCO 30, a frequency oscillator 40, an RF (Radio Frequency) mixer 50, an RF hybrid 60, a first low-pass filter (LPF) 71, , A second low-pass filter (LPF) 72, and a vector modulator 80.

The semiconductor laser 10 as a light source generates continuous light S1 of the center angular frequency omega ₀ (FIG. 2 (A)). The continuous light S1 is sent to the vector modulator 80.

  The white noise source 20 generates white noise and sends it to the VCO 30.

  The VCO 30 generates an oscillation signal represented by the following equation (1) using white noise, with the time being t, the angular frequency being ω, and the phase component being ψ.

cos (ωt + ψ) (1)
The phase component に お け る in the above equation (1) is phase noise due to white noise. Here, ψ is set as a phase noise having a fluctuation of 1 / f ² . The oscillation signal is sent to the RF mixer 50.

  The frequency oscillator 40 generates and outputs an RF signal that oscillates at a center frequency ω common to the oscillation frequency of the VCO 30. This RF signal is expressed by the following equation (2). The RF signal is sent to the RF mixer 50.

cosωt (2)
The RF mixer 50 multiplies the oscillation signal sent from the VCO 30 and the RF signal sent from the frequency oscillator 40 to generate a signal represented by the following equation (3).

(1/2) {cos (2ωt + ψ) + cos} (3)
The RF hybrid 60 branches the signal transmitted from the RF mixer 50 into two. Also, the RF hybrid 60 gives a phase shift of π / 2 to one of the two branched signals. Therefore, the RF hybrid 60 outputs a cos wave represented by the above equation (3) and a sine wave represented by the following equation (4), which is shifted in phase by π / 2 with respect to the cosine wave. Is done.

(1/2) {sin (2ωt + ψ) + sin} (4)
The cosine wave is sent to the first LPF 71, and the sine wave is sent to the second LPF 72.

  The first LPF 71 removes a high-frequency component of the input cos wave and outputs a first signal of cosψ. On the other hand, the second LPF 72 removes a high-frequency component of the input sine wave and outputs a second signal of sinψ. The first signal and the second signal are input to the vector modulator 80, respectively.

  The vector modulator 80 causes the continuous light S1 modulated by the first signal and the continuous light S1 modulated by the second signal to interfere with each other and output.

  The vector modulator 80 includes a main Mach-Zehnder (MZ) interferometer 90 and two sub-MZ interferometers, that is, a first sub-MZ interferometer 91 and a second sub-MZ interferometer, each formed by a planar optical waveguide. 92 (see FIG. 3).

  The main MZ interferometer 90 includes a branch unit 81, a multiplexing unit 82, and a first arm waveguide unit 83 and a second arm waveguide unit 84 that connect them in parallel. The splitting unit 81 splits the input light into two and sends the split light to the first arm waveguide unit 83 and the second arm waveguide unit 84, respectively. The multiplexing section 82 multiplexes and outputs the lights transmitted from the first arm waveguide section 83 and the second arm waveguide section 84.

  The first sub-MZ interferometer 91 is provided in the middle of the first arm waveguide section 83. The second sub-MZ interferometer 92 is provided in the middle of the second arm waveguide section 84. Therefore, the first arm waveguide portion 83 includes a portion (first arm waveguide portion 83 a) disposed before the first sub-MZ interferometer 91 and a portion disposed downstream of the first sub-MZ interferometer 91. (First arm waveguide portion 83b). Further, the second arm waveguide portion 84 includes a portion (second arm waveguide portion 84a) disposed before the second sub-MZ interferometer 92 and a portion disposed downstream of the second sub-MZ interferometer 92. (Second arm waveguide portion 84b).

  The first sub-MZ interferometer 91 includes a first sub-branch portion 181, a first sub-multiplexing portion 182, and a third arm waveguide portion 183 and a fourth arm waveguide portion 184 connecting these components in parallel. I have. The first sub-branching unit 181 branches the light transmitted from the first arm waveguide unit 83a in the preceding stage into two and sends the light to the third arm waveguide unit 183 and the fourth arm waveguide unit 184, respectively. Further, the first sub-multiplexing section 182 multiplexes the light transmitted from the third arm waveguide section 183 and the light transmitted from the fourth arm waveguide section 184, and transmits the multiplexed light to the first arm waveguide section 83b at the subsequent stage.

  Further, the second sub-MZ interferometer 92 includes a second sub-branch unit 281, a second sub-multiplexing unit 282, and a fifth arm waveguide unit 283 and a sixth arm waveguide unit 284 connecting these components in parallel. Contains. The second sub-branch unit 281 divides the light transmitted from the second arm waveguide unit 84a at the previous stage into two and sends the light to the fifth arm waveguide unit 283 and the sixth arm waveguide unit 284, respectively. Further, the second sub-multiplexing unit 282 multiplexes the light transmitted from the fifth arm waveguide unit 283 and the light transmitted from the sixth arm waveguide unit 284, and transmits the multiplexed light to the second arm waveguide unit 84b at the subsequent stage.

  The third arm waveguide section 183 is provided with a first electrode 191 for inputting a modulation signal. Further, the fourth arm waveguide section 184 is provided with a second electrode 192 for inputting a modulation signal. Further, a first DC electrode for giving a phase difference of π between the light propagating through the third arm waveguide portion 183 and the light propagating through the fourth arm waveguide portion 184 is provided on the fourth arm waveguide portion 184. 193 are provided.

  The fifth arm waveguide section 283 is provided with a third electrode 291 for inputting a modulation signal. Further, the sixth arm waveguide section 284 is provided with a fourth electrode 292 for inputting a modulation signal. Further, a second DC electrode for giving a phase difference of π between the light propagating through the fifth arm waveguide 283 and the light propagating through the sixth arm waveguide 284 is provided on the sixth arm waveguide 284. 293 are provided.

  In the first arm waveguide portion 83b at the subsequent stage with respect to the first sub-MZ interferometer 91, the light propagating through the first arm waveguide portion 83 and the light propagating through the second arm waveguide portion 84 have π. A third DC electrode 85 for providing a phase difference of / 2 is provided.

In the variable linewidth light generator 100, the continuous light S 1 having the central angular frequency ω ₀ sent from the semiconductor laser 10 is input to the branching unit 81. Further, a modulation signal of mcosψ is input to the first electrode 191 by using a first signal of cos 送 sent from the first LPF 71. Here, m indicates the degree of modulation. Further, a modulation signal of −mcosψ is input to the second electrode 192 using the first signal of cosψ sent from the first LPF 71. Further, a modulation signal of msinψ is input to the third electrode 291 by using the second signal of sin ら れ る sent from the second LPF 72. Further, a modulation signal of −msinψ is input to the fourth electrode 292 by using the second signal of sinψ sent from the second LPF 72.

  Further, by applying a voltage to the first DC electrode 193, a phase shift of π is given to light propagating through the fourth arm waveguide 184. Further, by applying a voltage to the second DC electrode 293, a phase shift of π is given to the light propagating through the sixth arm waveguide portion 284. Further, by applying a voltage to the third DC electrode 85, the light propagating through the first arm waveguide 83 is given a phase shift of π / 2.

  When the continuous light S1 is modulated on the basis of such conditions, an optical SSB (single-sideband) modulated wave represented by the following equation (5) is generated, and is output from the multiplexing unit 82 (that is, from the vector modulator 80). (See Takashi Kurokawa, Kyoritsu Shuppan, “Optical Functional Devices”, pp. 192 to 194).

Here, J indicates a Bessel function of the first kind. Further, n is an integer. In the above equation (5), when n = 0, the term of J _{2n + 1} is deleted. Furthermore, in the Bessel function of the first kind, when m <1.84, values other than J-1 (m) are sufficiently small. Therefore, the output light from the vector modulator 80 can be approximately expressed by the following equation (6).

As shown in the equation (6), the angular frequency components of the modulated wave of the continuous light S1 is superimposed on the center angular frequency omega _0. Therefore, the output light S2 from the vector modulator 80 (FIG. 2 (B)), the frequency shift is not generated with respect to the center angular frequency omega ₀ of the continuous light S1. Further, as described above, in the variable line width light generation device 100, the phase component ψ is phase noise having a fluctuation of 1 / f ² . Accordingly, the output light S2 from the vector modulator 80 (FIG. 2 (B)) has a peak at the center angular frequency omega _0, and a line width corresponding to 1 / f ² with respect to the center angular frequency omega ₀ .

As described above, in the variable line width light generation device 100, the continuous light S1 from the semiconductor laser (light source) 10 can be changed to a line width corresponding to 1 / f ² without frequency shifting. Therefore, there is no need to provide an optical bandpass filter after the vector modulator 80.

10: Light source 20: White noise source 30: VCO
40: frequency oscillator 50: RF mixer 60: RF hybrid 71: first low-pass filter 72: second low-pass filter 80: vector modulator 100: variable line width light generator

Claims (5)

Including a light source and a vector modulator,
The light source generates and sends continuous light to the vector modulator;
The vector modulator causes the continuous light modulated by the first signal of cosｃｏ with the phase component of ψ to interfere with the continuous light modulated by the second signal of sinψ, and outputs the interference.
Ψ in the first signal and the second signal is a phase noise having a fluctuation of 1 / f ^{2 where} f is a frequency, wherein A white noise source that generates white noise;
A voltage-controlled oscillator that generates an oscillation signal provided with phase noise having a fluctuation of 1 / f ² using the white noise;
A frequency oscillator that generates an RF signal that oscillates at a common center frequency with the oscillation frequency of the voltage-controlled oscillator,
An RF mixer that generates a signal obtained by multiplying the oscillation signal and the RF signal;
An RF hybrid that splits a signal generated by the RF mixer into two and gives a phase shift of π / 2 to one of the two split signals, thereby outputting a cosine wave and a sine wave, respectively;
2. The variable linewidth light according to claim 1, further comprising a low-pass filter that generates the first signal and the second signal by removing high-frequency components from the cosine wave and the sine wave, respectively. 3. Generator. The vector modulator includes a main Mach-Zehnder interferometer, and a first sub-Mach-Zehnder interferometer and a second sub-Mach-Zehnder interferometer,
The main Mach-Zehnder interferometer includes a branch portion, a multiplexing portion, and a first arm waveguide portion and a second arm waveguide portion connecting these in parallel,
The branch unit branches the input light into two and sends the split light to the first arm waveguide unit and the second arm waveguide unit, respectively.
The multiplexing unit multiplexes and outputs light transmitted from the first arm waveguide unit and the second arm waveguide unit,
The first sub-Mach-Zehnder interferometer is provided in the middle of the first arm waveguide unit. The first sub-Mach-Zehnder interferometer includes a first sub-branching unit, a first sub-multiplexing unit, and A third arm waveguide section and a fourth arm waveguide section that connect between them in parallel,
The first sub-branch unit bifurcates the light transmitted from a previous stage to the first sub-Mach-Zehnder interferometer of the first arm waveguide unit, and branches the light into the third arm waveguide unit and the third arm waveguide unit. Each of the four arm waveguides,
The first sub-multiplexing unit multiplexes light transmitted from the third arm waveguide unit and the fourth arm waveguide unit, and forms the first sub-Mach-Zehnder interferometer of the first arm waveguide unit. To the latter part,
The second sub-Mach-Zehnder interferometer is provided in the middle of the second arm waveguide section,
The second sub-Mach-Zehnder interferometer includes a second sub-branch unit, a second sub-multiplexing unit, and a fifth arm waveguide unit and a sixth arm waveguide unit connecting them in parallel,
The second sub-branch unit bifurcates the light transmitted from a previous stage to the second sub-Mach-Zehnder interferometer of the second arm waveguide unit, and branches the light into the fifth arm waveguide unit and the second sub-Mach-Zehnder interferometer. To each 6-arm waveguide,
The second sub-multiplexing unit multiplexes the lights transmitted from the fifth arm waveguide unit and the sixth arm waveguide unit, and forms the second sub-Mach-Zehnder interferometer of the second arm waveguide unit. To the latter part,
A modulation signal of mcosψ based on the first signal is input to the third arm waveguide unit, where m is a modulation factor,
A modulation signal of −mcosψ based on the first signal is input to the fourth arm waveguide unit,
A modulation signal of msinψ based on the second signal is input to the fifth arm waveguide unit,
A modulation signal of −msinψ based on the second signal is input to the sixth arm waveguide unit,
The light propagating through the fourth arm waveguide section is given a phase shift of π,
The light propagating through the sixth arm waveguide section is given a phase shift of π,
3. The light propagating through the first arm waveguide section is given a phase shift of π / 2 in a portion subsequent to the first sub-Mach-Zehnder interferometer. 4. Variable line width light generator. The process of producing continuous light;
A step of causing the continuous light modulated by the first signal of cosψ with the phase component ψ to interfere with the continuous light modulated by the second signal of sinψ, and outputting the interference.
Ψ in the first signal and the second signal is a phase noise having a fluctuation of 1 / f ² with a frequency f, wherein the variable line width light is generated. Generating white noise;
Generating an oscillation signal to which phase noise having a fluctuation of 1 / f ² is given using the white noise;
Generating an RF signal that oscillates at a common center frequency with the oscillation frequency of the oscillation signal;
Generating a signal obtained by multiplying the oscillation signal and the RF signal;
Splitting the signal generated in the process of generating the multiplied signal into two, and giving a phase shift of π / 2 to one of the two split signals, thereby outputting a cos wave and a sin wave, respectively;
The method according to claim 4, further comprising generating the first signal and the second signal by removing high-frequency components from the cosine wave and the sine wave, respectively. Method.

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