JP2006279882A - Optical frequency modulation method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical frequency modulation apparatus which realizes optical frequency modulation in simple configuration and improves reliability by reducing the number of required components. <P>SOLUTION: The optical frequency modulation apparatus comprises means (8, 9, 10) for temporally modulating an iteration frequency of an optical pulse stream having the peculiar iteration frequency and a carrier frequency, and means (4, 1c, 5) for extracting the modulated iteration frequency component and modulates the intensity of an optical signal. The means for temporally modulating the iteration frequency of an optical pulse stream includes carrier frequency control means (8, 9) for changing the carrier frequency and an optical pulse interval control means (10) which gives a propagation delay time corresponding to the carrier frequency to the optical pulse stream of which the carrier frequency is changed by the carrier frequency control means. The carrier frequency control means comprises a light intensity control means (8) for continuously and periodically changing the light intensity of optical pulses in the optical pulse stream and an optical non-linear medium (9) of which a refractive index is changed in accordance with light intensity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical FM modulation method and an optical FM modulation apparatus. More specifically, the present invention relates to an optical FM modulation method and an optical FM modulation device that modulates the intensity of an optical signal by temporally modulating the repetition frequency using an optical pulse train having a specific repetition frequency and a carrier frequency.

  Conventionally, in video transmission using an optical fiber, an optical signal that combines the low-noise characteristics of an FM modulation method with an existing light intensity modulation method (IM-DD method: Intensity Modulation-Direct Detection) ( FM / AM signal) is being transmitted. This FM / AM modulation signal is generated by temporally changing the modulation frequency for driving the light intensity modulator.

  FIG. 1 shows a configuration example of an existing optical FM modulation device. The optical FM modulation apparatus 100 includes a semiconductor laser (DFB laser: distributed feedback laser) 1a and 1b, a semiconductor laser 1a, Phase modulators 2a and 2b that phase-modulate each of the continuous lights output from 1b at a timing that is 180 degrees out of phase with each other in accordance with the modulation signal supplied from the microwave transmitter 6, and the phase modulator 2a And 2b, the optical coupler 3 that combines the phase-modulated continuous light, and the continuous light that is combined by the optical coupler 3, and receives the beat of the difference frequency of the combined continuous light. An optical receiver 4a for output, and an intensity modulator 5 for intensity-modulating an optical signal input from the semiconductor laser 1c with a beat having a difference frequency output from the optical receiver 4a. .

  The continuous light combined by the optical coupler 3 is phase-modulated with continuous light whose carrier frequencies differ from each other by 3 GHz at a phase that is shifted by 180 degrees. For this reason, the beat of the difference frequency obtained by the optical receiver 4 changes with time.

  FIG. 2 schematically shows changes in beat frequency due to interaction when the phase modulation frequency is 1.5 GHz. In this configuration example, it is possible to apply an FM modulation frequency up to 6 GHz.

J. P. Gordon, “Theory of the soliton self-frequency shift,” Opt. Lett., 11, pp. 662-664, 1986. F. M. Mitschke, and L. F. Mollenauer, “Discovery of the soliton self-frequency shift,” Opt. Lett., 11, pp. 659-661, 1986.

  At present, it is very difficult to realize a microwave transmitter that can change the modulation frequency at high speed. Therefore, in the prior art in order to apply high-speed FM modulation, as shown in FIG. 1, phase modulation is performed for each of the continuous lights output from the two narrow line width DFB lasers, and the beat of the difference frequency due to the interaction is obtained. There was a problem that FM modulation had to be applied by changing it with time. Furthermore, the DFB laser required here is not only a very narrow line width, but also requires that the carrier frequency be strictly different by 3 GHz, which is a problem from the viewpoint of the yield of the DFB laser. It was. Further, as shown in FIG. 1, the number of parts of the optical FM modulator is large, and there is a problem in reliability from a practical viewpoint.

  The present invention has been made in view of such a problem, and an object of the present invention is to realize an optical FM modulation with a simple configuration, and to reduce the number of required parts to provide a highly reliable optical FM modulation apparatus. It is to provide.

  In order to achieve such an object, the present invention uses an optical pulse train having an arbitrary and unique repetition frequency and carrier frequency, and variably controls the pulse interval in each optical pulse to thereby control the repetition frequency. Realize modulation. Thereby, since the repetition frequency extracted by the optical receiver changes with time, a simple optical FM modulated signal with a simple configuration is provided.

  The present invention uses a wavelength conversion technique (see Non-Patent Documents 1 and 2) having light intensity dependency as a method for variably controlling the optical pulse interval, and generates optical pulses having different carrier frequencies. Furthermore, a simple optical FM modulation method with a simple configuration in which the optical pulse interval is controlled by using a dispersion medium whose propagation delay time varies depending on the carrier frequency, and the repetition frequency extracted by the optical receiver varies with time, and An optical FM modulator is provided.

  As described above, the invention described in claim 1 is an optical FM modulation apparatus that FM modulates an optical signal, and temporally modulates the repetition frequency of an optical pulse train having a specific repetition frequency and a carrier frequency. And means for extracting the modulated repetition frequency component and intensity-modulating the optical signal.

  A second aspect of the present invention is the optical FM modulation apparatus according to the first aspect, wherein the means for temporally modulating the repetition frequency of the optical pulse train changes the carrier frequency of the optical pulse train. It is characterized by comprising control means and optical pulse interval control means for giving a propagation delay time according to the carrier frequency to the optical pulse train whose carrier frequency has been changed by the carrier frequency control means.

  The invention according to claim 3 is the optical FM modulation device according to claim 2, wherein the carrier frequency control means is a light that continuously and periodically changes the light intensity of each light pulse in the light pulse train. It is characterized by comprising intensity control means and an optical nonlinear medium (9) whose refractive index changes according to the light intensity.

  According to a fourth aspect of the present invention, there is provided an optical FM modulation method for FM-modulating an optical signal, the step of temporally modulating the repetition frequency of an optical pulse train having a specific repetition frequency and a carrier frequency; Extracting the repetitive frequency component modulated to 1 to intensity-modulate the optical signal.

  The invention according to claim 5 is the optical FM modulation method according to claim 4, wherein the step of temporally modulating the repetition frequency of the optical pulse train includes each time interval of the optical pulse in the optical pulse train. It is a step which changes.

  The invention according to claim 6 is the optical FM modulation method according to claim 5, wherein the step of changing each time interval of the optical pulse in the optical pulse train continuously sets the carrier frequency of the optical pulse train. And a step of changing periodically and periodically.

  A seventh aspect of the present invention is the optical FM modulation method according to the sixth aspect, wherein the step of changing the carrier frequency continuously and periodically is performed by measuring the light intensity of the optical pulse in the optical pulse train. Propagating through the optical nonlinear medium whose refractive index changes according to the light intensity, the step of changing each continuously and periodically, and the optical pulse train in which the light intensity of each light pulse is changed continuously and periodically And a step of causing

  The invention according to claim 8 is the optical FM modulation method according to claim 6 or 7, wherein the step of changing each of the time intervals of the optical pulses in the optical pulse train is such that the carrier frequency is continuous and periodic. The optical pulse train that has been changed to a step further includes a step of propagating a dispersion medium whose propagation time varies depending on a carrier frequency.

  As described above, according to the present invention, a simple optical FM modulation device with a simple configuration that could not be realized with the prior art is realized. Note that the optical FM modulator of the present invention is very stable because it operates using a soliton wave that operates particularly stably in the optical nonlinear phenomenon. As a result, it is extremely practical and has a wide range of applications.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 3 shows the configuration of the optical FM modulation apparatus according to the embodiment of the present invention. The optical FM modulation apparatus 200 adjusts the optical intensity of each optical pulse of the optical pulse train output from the optical pulse generator 7 that generates an optical pulse train having a specific carrier frequency and repetition frequency, and the optical pulse generator 7. A light intensity control unit 8, a carrier frequency control unit 9 that converts a carrier frequency of an optical pulse (sequence) whose light intensity has been adjusted by the light intensity control unit 8, and a carrier frequency converted by the carrier frequency control unit 9. The optical pulse interval control unit 10 that controls the propagation delay time difference of the optical pulse (sequence) and the optical pulse (sequence) output from the optical pulse interval control unit 10 are received and the temporally changing repetition frequency is output. An optical receiver 4b and an intensity modulator 5 that modulates the intensity of an optical signal input from the semiconductor laser 1c with a temporally changing repetition frequency output from the optical receiver 4b are provided.

  In FIG. 3, the optical pulse generator 7 can be constituted by, for example, a semiconductor laser. However, the present invention is not limited to this, and if an optical pulse train having a specific carrier frequency and repetition frequency can be generated, a fiber is used. You may comprise using a laser, an optical comb generator, etc.

The optical pulse train having an arbitrary and specific carrier frequency and repetition frequency output from the optical pulse generator 7 is incident on the optical intensity controller 8 to adjust the optical intensity of each optical pulse. The light intensity control unit 8 can be composed of an element such as a ferroelectric crystal such as LiNbO ₃ or an electroabsorption EA modulator (electro-absorption modulator). However, the present invention is not limited to this as long as the transmittance can be controlled. The light intensity control unit 8 applies continuous and periodic light intensity modulation as shown in FIG. At this time, when a light intensity modulator composed of a ferroelectric crystal such as LiNbO ₃ is used as the light intensity controller 8, the drive voltage of the light intensity modulator has the voltage waveform shown in FIG. Further, the control of the light pulse intensity applied by the light intensity control unit 8 may be continuous and periodic, and is not particularly limited to the waveform shown in FIG.

  The carrier frequency control unit 9 adjusts the wavelength of the optical pulse train whose light intensity is adjusted by the light intensity control unit 8 by inducing wavelength conversion having light intensity dependency in a highly nonlinear optical fiber. This phenomenon is known to be that the carrier frequency is converted to the long wavelength side (low frequency side) by propagating the optical pulse through the optical fiber in the anomalous dispersion region. Therefore, the zero dispersion wavelength of the highly nonlinear fiber used in the carrier frequency controller 9 must be shorter than the carrier frequency of the optical pulse output from the optical pulse generator 7 (high frequency side).

  The output pulse from the optical pulse generator 7 maintains the optical pulse waveform even after inducing optical pulses with different carrier frequencies. Therefore, as shown in FIG. 7, the modulation of the repetition frequency (optical FM modulation frequency) is realized by modulating the pulse interval between the output pulse from the optical pulse generator 7 and the newly generated optical pulse having the carrier frequency. To do.

  FIG. 8 shows an embodiment in which wavelength conversion having light intensity dependency is performed. As apparent from FIG. 8, it can be seen that the carrier frequency of the light pulse generated in proportion to the light intensity of the light pulse incident on the highly nonlinear fiber changes. In this example, the wavelength variable range was about 40 nm. Further, since the propagation delay time difference due to the carrier frequency occurs due to the chromatic dispersion of the highly nonlinear fiber used in the carrier frequency control unit 9, the optical pulse interval is modulated to some extent.

  If the light output from the light intensity control unit 8 does not exceed the threshold value for wavelength conversion having the light intensity dependency induced in the subsequent carrier frequency control unit 9, the preceding stage of the carrier frequency control unit 9 It is necessary to insert an element having an optical amplifying function and to increase the light intensity of the entire light pulse as schematically shown in FIG. At this time, it is essential to maintain the light intensity difference added by the light intensity controller 8.

  The optical pulse interval control unit 10 controls the propagation delay time difference of the optical pulse having the carrier frequency newly generated (converted) by the carrier frequency control unit 9. Thereby, a desired repetition frequency (optical FM modulation frequency) can be realized. For example, the optical pulse interval control unit 10 can use a dispersion-shifted fiber (DSF) for communication as a propagation delay time control fiber. FIG. 10 shows input pulses and output pulses for the propagation delay time control fiber. Optical pulses of wavelengths λ1... Λn are input to the propagation delay time control fiber at substantially equal intervals Tn (time intervals equal to the pulse intervals of the optical pulse train generated by the optical pulse generator 7). The propagation delay time control fiber provides a propagation delay time corresponding to each wavelength. FIG. 10 shows the time interval (propagation delay time difference) between the optical pulse with wavelength λn and the optical pulse with wavelength λ1 output from the propagation delay time control fiber as T1, and the time between the optical pulse with wavelength λn and the optical pulse with wavelength λ2. The interval is represented as T2.

In the present embodiment, the optical pulse interval control unit 10 uses a dispersion shift fiber (DSF) for communication having a dispersion of −2 ps / nm / km and a dispersion slope of +0.1 ps / nm ² / km at a carrier frequency of 1550 nm. : Dispersion-shifted fiber). A semiconductor laser having a carrier frequency of 1550 nm and a repetition frequency of 100 MHz (0.1 GHz) was used for the optical pulse generator 7. Therefore, the interval between the light pulses generated from the light pulse generator 7 is 10,000 ps. FIG. 9 shows the relationship between the optical pulse interval and the repetition frequency.

  The wavelength difference that can be generated in the carrier frequency control unit 9 is about 40 nm from FIG. Therefore, in order to obtain 10 GHz as the optical FM modulation frequency with a wavelength difference of 40 nm, it is required to set the pulse interval to about 100 ps / 40 nm at the maximum. Therefore, the length of the DSF was set to about 25 km and the optical pulse interval was set to 100 ps. Thereby, an optical FM modulation frequency of 0.1 GHz to 10 GHz was realized.

  At this time, components having an optical pulse interval of less than 100 ps (repetition frequency of 10 GHz or more) are generated at the same time (in the adjacent optical pulses output from the optical pulse generator 7, newly generated optical pulses having different carrier frequencies are generated. Therefore, it is effective to remove high frequency components using an electrical filter. In order to suppress the optical nonlinear optical effect generated in the optical fiber used in the optical pulse interval control unit 10, it is also effective to insert an optical attenuator before the optical pulse interval control unit 10. In order to realize a desired optical FM modulation frequency, the repetition frequency of the pulse generator used in the optical pulse generator, the wavelength difference that can be generated by the carrier frequency generator, and the propagation delay time difference in the optical pulse interval controller are parameters. Optimization is required.

It is a figure which shows the conventional optical FM modulation apparatus typically. It is a figure which shows typically one Example of an optical FM signal. It is a figure which shows the structure of FM modulation apparatus of embodiment light concerning this invention. It is a figure which shows typically an example of the output waveform of the light intensity control part in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows typically an example of the voltage waveform which drives the optical intensity modulation used by the optical intensity control part in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows typically an example which amplifies the optical intensity of the whole optical signal by hold | maintaining an optical intensity difference in the back | latter stage of the optical intensity control part in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows typically an example of the output waveform of the carrier frequency control part in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows typically an example of the wavelength conversion which has the light intensity dependence in the carrier frequency control part in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows the relationship between the optical pulse interval and repetition frequency in the optical FM modulation apparatus of embodiment which concerns on this invention. It is a figure which shows the input pulse train and output pulse train to the optical pulse space | interval control part in the optical FM modulation apparatus of embodiment which concerns on this invention.

Explanation of symbols

1a, 1b, 1c Semiconductor laser (DFB laser)
2 Phase modulator 3 Optical coupler 4a, 4b Optical receiver 5 Optical intensity modulator 6 Microwave oscillator for driving phase modulator 7 Optical pulse generator 8 Optical intensity controller 9 Carrier frequency controller 10 Optical pulse interval controller 11 Light Amplifying element

Claims (8)

An optical FM modulation device that FM modulates an optical signal,
Means for temporally modulating the repetition frequency of an optical pulse train having a unique repetition frequency and a carrier frequency;
Means for extracting the modulated repetitive frequency component and intensity-modulating the optical signal. 2. The optical FM modulation apparatus according to claim 1, wherein the means for temporally modulating the repetition frequency of the optical pulse train comprises:
Carrier frequency control means for changing the carrier frequency of the optical pulse train;
An optical FM modulation device comprising: an optical pulse interval control unit that gives a propagation delay time corresponding to a carrier frequency to the optical pulse train whose carrier frequency has been changed by a carrier frequency control unit. 3. The optical FM modulation apparatus according to claim 2, wherein the carrier frequency control means is
A light intensity control means for continuously and periodically changing the light intensity of each light pulse in the light pulse train;
An optical FM modulation device comprising: an optical nonlinear medium whose refractive index changes according to light intensity. An optical FM modulation method for FM-modulating an optical signal,
Temporally modulating the repetition frequency of an optical pulse train having a unique repetition frequency and a carrier frequency;
Extracting the time-modulated repetition frequency component and intensity-modulating the optical signal. 5. The optical FM modulation method according to claim 4, wherein the step of temporally modulating the repetition frequency of the optical pulse train comprises:
An optical FM modulation method comprising changing each time interval of an optical pulse in the optical pulse train.   6. The optical FM modulation method according to claim 5, wherein the step of changing each time interval of the optical pulse in the optical pulse train includes a step of changing the carrier frequency of the optical pulse train continuously and periodically. An optical FM modulation method comprising: 7. The optical FM modulation method according to claim 6, wherein the step of changing the carrier frequency continuously and periodically includes:
Continuously and periodically changing each of the light intensities of the light pulses in the light pulse train;
Propagating the optical pulse train in which each of the optical intensities of the optical pulses is changed continuously and periodically through an optical nonlinear medium whose refractive index changes in accordance with the optical intensity. Method. The optical FM modulation method according to claim 6 or 7, wherein each of the time intervals of the optical pulses in the optical pulse train is changed.
An optical FM modulation method, further comprising: propagating the optical pulse train whose carrier frequency is continuously and periodically changed through a dispersion medium whose propagation time varies depending on the carrier frequency.

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