JP2015075614A - Optical frequency comb generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a broadband optical frequency comb with a high signal-to-noise power ratio.SOLUTION: An optical frequency comb generator includes: an optical modulator 21 that modulates continuous light from a light source; an optical filter 22 that removes at least one frequency component of frequency components included in a modulation signal; an optical amplifier 23 that amplifies the modulation signal; a pulse compression section 24 that compresses a pulse width of each pulse of the modulation signal by using a dispersion medium; and a bandwidth expansion section 25 that expands a bandwidth of the modulation signal on a frequency axis by using a nonlinear medium.

Description

  The present invention relates to an optical frequency comb generator that generates an optical frequency comb having a wide band and a high signal-to-noise power ratio.

  In order to realize large-capacity wireless transmission, research and development of wireless transmission systems using millimeter waves and terahertz waves higher than conventional frequencies are underway (see, for example, Non-Patent Document 1). Since the carrier frequency in these wireless transmission systems ranges from 100 to 1000 GHz, it is difficult to efficiently generate a carrier wave with a semiconductor oscillator. Therefore, as shown in FIG. 1, an optical frequency comb generator 91 generates an optical frequency comb, extracts signals separated by a necessary frequency interval, and generates a carrier frequency signal by optical heterodyne detection. (See, for example, Non-Patent Document 1).

In FIG. 1, the optical frequency comb is divided into _a plurality of frequency carriers (f _a , f _b , f _c , and f _d ) by the duplexer 92. Then, two of them, f _a and f _b , are input to the optical detector 933, _a high frequency modulation signal having a center frequency of the frequency difference f ₁ = | fa −f _b | is generated by optical heterodyne detection, and wireless Send as a signal. On the other hand, two f _c and f _d are input to the optical detector 942, a high frequency local oscillation signal having a frequency difference f ₂ = | f _c −f _d | is generated, and the received radio signal is down-converted.

  As an optical frequency comb generation method, a plurality of optical modulators are cascaded as shown in FIG. 2 to generate an optical frequency comb by multistage modulation (see, for example, Non-Patent Document 2), and FIG. As described above, there is an apparatus that generates an optical frequency comb using the nonlinearity of an optical fiber (see, for example, Non-Patent Document 3). In the former, by using a plurality of modulators, an equivalently high modulation index is realized to generate a broadband signal, and harmonics are efficiently generated by phase control. In the latter, a narrow-band optical frequency comb generated by a single modulator is converted into a broadband optical frequency comb mainly by third-order nonlinear optical effects (self-phase modulation, cross-phase modulation, four-wave mixing). By these methods, it is possible to generate an optical frequency comb in a band of THz or more at a frequency interval of several GHz or ten and several GHz.

H. -J. Song and T.M. Nagatsuma, “Present and Future of Terahertz Communications”, IEEE Trans. Terahertz Sci. and Technol. , Vol. 1, pp. 256-263, 2011. M.M. Fujiwara, et al. , “Optical carrier supplied module using flattened optical multi-carrier generation based on sinusoidal amplification and phase hybridEd. , Vol. 21, no. 11, pp. 2705-2714, Nov. 2003. R. P. Scott, et al. "3.5-THZ Wide, 175 Mode Optical Comb Source", OFC 2007, paper OWJ3, Mar. 2007.

  As shown in FIG. 1, there are two optical carriers necessary for generating a wireless carrier wave and two local optical signals for generating a reception local oscillation signal, and it is sufficient if there are four optical carriers per system. Also, considering the mounting of a plurality of systems, it is sufficient to have about 10 to 20 optical carriers. On the other hand, since the conventional optical frequency comb generates 100 or more optical carriers, many optical carriers are wasted. Also, in order to efficiently generate a radio carrier frequency by optical heterodyne detection, it is necessary to input two optical carriers to the optical detector with high power, and a high signal-to-noise power ratio is required for the optical carrier. Required.

  In order to realize a high signal-to-noise power ratio, an optical amplifier is indispensable. However, in an optical amplifier, spontaneous emission light is added as noise, and the noise power increases as the input power to the optical amplifier decreases. For this reason, the noise level is lower when the optical carrier wave is amplified before being extracted than when the optical carrier wave is extracted and then amplified. However, since the number of optical carriers is large, it is necessary to increase the output power of the optical amplifier to increase the optical power per optical carrier, and an expensive optical amplifier is required.

  Although there is a method for increasing the reference frequency for generating the optical frequency comb, a high-frequency oscillator, a modulator driver, and a broadband modulator are required, which increases the cost.

  An object of the present invention is to generate an optical frequency comb having a wide bandwidth and a high signal-to-noise power ratio.

The optical frequency comb generator of the present invention is
An optical modulator that modulates continuous light from the light source;
An optical filter for removing at least one frequency component of a plurality of frequency components included in a modulation signal from the optical modulator;
An optical amplifier for amplifying the modulation signal from the optical filter;
A pulse compression unit that compresses the pulse width of each pulse of the modulated signal from the optical amplifier using a dispersion medium;
A band extending unit that expands the band of the modulation signal from the pulse compression unit on the frequency axis using a nonlinear medium;
Is provided.

  In the optical frequency comb generator of the present invention, the optical modulator may be a light intensity modulator.

  In the optical frequency comb generator of the present invention, the optical modulator may be an optical phase modulator.

  In the optical frequency comb generator of the present invention, the optical filter may be a Mach-Zehnder optical filter, a Fabry-Perot optical filter, or an optical interleaver.

  In the optical frequency comb generator of the present invention, the pulse compression unit may be a normal dispersion fiber or a chirped fiber grating.

  In the optical frequency comb generator according to the present invention, the band extension unit is a fiber having an effective core diameter smaller than an effective core diameter of the optical fiber between the optical modulator and the optical filter, or dispersion reduction. It may be a fiber.

  The above inventions can be combined as much as possible.

  According to the present invention, it is possible to generate an optical frequency comb having a wide bandwidth and a high signal-to-noise ratio.

1 shows an example of a configuration of a wireless communication system. The 1st example of the optical frequency comb production | generation apparatus relevant to this invention is shown. The 2nd example of the optical frequency comb production | generation apparatus relevant to this invention is shown. An example of the optical frequency comb production | generation apparatus which concerns on Embodiment 1 is shown. An example of the optical frequency spectrum output from the light source is shown. An example of the optical frequency spectrum output from the optical modulator is shown. An example of the optical frequency spectrum output from the optical filter is shown. An example of the optical frequency spectrum output from the highly nonlinear optical fiber is shown. An example of the transmission characteristic of a Mach-Zehnder filter is shown. An example of the transmission characteristic of a Fabry-Perot filter is shown. An example of a numerical simulation result is shown. An example of the optical frequency comb production | generation apparatus which concerns on Embodiment 2 is shown. An example of pulse compression of an optical phase modulation signal is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 4 shows a first embodiment of the present invention.
The optical frequency comb generator 91 of this embodiment includes a light source 11, a light intensity modulator 21, an oscillator 13, an optical filter 22, an optical amplifier 23, a single mode fiber 24 that functions as a pulse compression unit, and a high function that functions as a band extension unit. A nonlinear fiber 25 is provided.

The light intensity modulator 21 modulates the light intensity of the continuous light from the light source 11 with the signal of the frequency f ₀ from the oscillator 13 to generate a modulation signal. The optical filter 22 suppresses a plurality of periodic components among the frequency components included in the modulation signal from the light intensity modulator 21. The optical amplifier 23 optically amplifies the output signal from the optical filter 22. The single mode fiber 24 performs pulse compression of each pulse of the modulation signal. The highly nonlinear fiber 25 generates a broadband optical frequency comb using nonlinearity.

The conceptual diagrams of the optical frequency spectrum at each point in this case are shown in FIGS. A single optical carrier wave from the light source 11 shown in FIG. 5 is modulated by the light intensity modulator 21 to become a narrow-band optical frequency comb composed of a plurality of carriers at f ₀ intervals shown in FIG. By suppressing the 2f ₀ cycle of the signal by the optical filter 22 from the optical frequency comb, it generates an optical frequency comb of the 2f ₀ interval shown in FIG. Thereafter, a wideband optical frequency comb shown in FIG. 8 can be generated by optical amplification, pulse compression, and optical nonlinear phenomenon.

  As described above, by inputting optical frequency combs having a wide frequency interval to the optical amplifier 23, it is possible to reduce the number of carriers of the generated wide-band optical frequency comb, and to increase the signal-to-noise ratio for each carrier. Can do.

The optical filter 22 in the present embodiment, the transmission characteristic of the 2f ₀ intervals as shown in Figure 9: Fabry with or Mach-Zehnder type optical filter having a (FSR Free Spectral Range), the FSR of the 2f ₀ as shown in FIG. 10 A Perot filter can be used.

Further, when the reference frequency f ₀ is equal to or greater than 50 GHz, the application of light interleaver using multistage Mach-Zehnder interferometer or the like is also possible. Especially in the case of Fabry-Perot type filter, it is easy to adjust the cavity length, making the frequency interval to be generated in the optical frequency comb and a continuously variable by varying the resonator length with the reference frequency f ₀ Can do.

  In the case of a Mach-Zehnder type filter and an optical interleaver, it is difficult to change the period of the transmission frequency, but since the level change of the transmission band is small, the optical frequency is changed by changing the reference frequency f0 in a range where the modulation index of the optical modulator is not so large. The frequency interval of the comb can be made variable.

Note that the transmission characteristics of the filter 22 in the present embodiment are not limited to the case where the FSR is 2f _0. Various filters can be used by using a wider FSR filter or by cascading a plurality of filters having different FSRs. It is possible to generate an optical frequency comb with a frequency arrangement.

As an example of the operation in this embodiment, a numerical simulation result is shown in FIG. In the simulation, the reference frequency is f ₀ = 25 GHz, driven by a voltage 1.8 times the half-wave voltage Vπ of the light intensity modulator 21, and a Mach-Zehnder type filter having an FSR of 50 GHz is used as the optical filter 22 to amplify the average power to 200 mW The optical frequency comb is generated using the 2 km single mode fiber 24 and the highly nonlinear fiber 25. As shown in the figure, and extracts a frequency comb of the narrowband 2f ₀ interval from the frequency comb of the output from the optical intensity modulator 21, it is clear that the broadband optical frequency comb with amplification and nonlinear optical effects can be produced.

In the case of the present embodiment, since the number of carriers generated in a non-linear manner is smaller than in the case where the frequency interval is f ₀ , it is possible to obtain a high signal power per carrier, and an equivalently high signal-to-noise power ratio. Can be obtained.

  In this embodiment, a single mode fiber 24 (ordinary dispersion fiber) in which the chromatic dispersion in the 1.55 μm band is about 18 ps / nm / km is assumed for optical pulse compression, but it is like a chirped fiber grating. Various dispersion media may be used. In this case, since the optical nonlinearity in the dispersion medium is small, it is possible to place the dispersion medium in front of the optical amplifier.

  The same effect can be obtained by using a dispersion-reducing fiber that reduces the dispersion in the longitudinal direction and obtains an equivalent gain in addition to the highly nonlinear fiber 25 with a reduced core diameter and increased nonlinear optical constant.

In the present embodiment, the optical filter 22 suppresses the original carrier wave component and the frequency component away from it by ± 2 nf ₀ (n = 1, 2, 3,...), And ± (2n−1) f ₀ (n = 1). , 2, 3... May suppress only distant components, but in the case of the former, since the direct current component of the intensity is suppressed, it is easy to generate a wider-band optical frequency comb.

(Embodiment 2)
FIG. 12 shows a second embodiment of the present invention. The difference of the second embodiment from the first embodiment is that an optical phase modulator 31 is used instead of the light intensity modulator 21 as an optical modulator.

The optical phase modulator 31 optically modulates the continuous light from the light source 11 with the signal of the frequency f ₀ from the oscillator 13 to generate a modulation signal. The optical filter 22 suppresses a plurality of periodic components included in the modulation signal from the optical phase modulator 31.

  Here, although the intensity of the optical phase modulated signal by the optical phase modulator 31 is constant, a delay corresponding to the instantaneous frequency is received when passing through the dispersion medium, and the intensity modulated signal is obtained as shown in FIG. Converted. When the optical intensity modulator 21 is used, a signal is deleted to generate a pulse, so that an excessive loss occurs. However, in the case of the optical phase modulator 31, this excessive loss can be reduced. For this reason, the invention according to the present embodiment can generate an optical frequency comb having a higher signal-to-noise ratio.

  The present invention can be applied to the information communication industry.

11: Light source 12: Optical modulator 13: Oscillator 14: Phase adjuster 15: Dispersion medium 16: Optical amplifier 17, 25: Highly nonlinear fiber 91: Optical frequency comb generator 21: Optical intensity modulator 22: Optical filter 23: Optical amplifier 24: Single mode fiber 31: Optical phase modulator 92: Demultiplexer 931: Optical modulator 932: Multiplexer 933: Optical detector 934: Amplifier 935: Antenna 941: Multiplexer 942: Optical detector 943 : Power amplifier 944: Mixer 945: Low noise amplifier 946: Antenna

Claims (6)

An optical modulator that modulates continuous light from the light source;
An optical filter for removing at least one of frequency components included in a modulation signal from the optical modulator;
An optical amplifier for amplifying the modulation signal from the optical filter;
A pulse compression unit that compresses the pulse width of each pulse of the modulated signal from the optical amplifier using a dispersion medium;
A band extending unit that expands the band of the modulation signal from the pulse compression unit on the frequency axis using a nonlinear medium;
An optical frequency comb generator. The optical frequency comb generator according to claim 1, wherein the optical modulator is an optical intensity modulator. The optical frequency comb generator according to claim 1, wherein the optical modulator is an optical phase modulator. The optical frequency comb generator according to any one of claims 1 to 3, wherein the optical filter is a Mach-Zehnder optical filter, a Fabry-Perot optical filter, or an optical interleaver. The optical frequency comb generator according to any one of claims 1 to 4, wherein the pulse compression unit is a normal dispersion fiber or a chirped fiber grating. The band extension unit is a fiber having an effective core diameter smaller than an effective core diameter of an optical fiber between the optical modulator and the optical filter, or a dispersion reducing fiber. Item 6. The optical frequency comb generator according to any one of Items 1 to 5.

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