Optical transmitter and optical signal generation method

Abstract

According to the present invention, an optical comb generator uses output light from a first light source and a frequency from a signal source and generates an optical signal in which optical combs are aligned side by side at equal frequency intervals therebetween. An extracting unit extracts a first optical comb and a second optical comb from an optical signal generated by the optical comb generator. A phase modulating unit generates a modulated signal obtained by phase-modulating the first optical comb in response to an input signal. An adjusting unit adjusts the delay amount of the second optical comb. A multiplexing unit multiplexes the modulated signal and the second optical comb the delay amount of which has been adjusted. A light-receiving unit performs square-law detection on the light multiplexed by the multiplexing unit and generates a frequency modulation (FM) signal. An optical intensity modulating unit modulates the intensity of output light of a second light source in response to the FM signal.

Description

[Technical Field]

[0001] The present invention relates to an optical transmitter and an optical signal generating method. [Background technology]

[0002] An optical video distribution system employing an FM (Frequency Modulation) batch conversion method (see, for example, Non-Patent Document 1 and Non-Patent Document 2) has been introduced. FIG. 2 is a diagram showing an outline of the optical video distribution system. The optical video distribution system shown in FIG. 2 includes an optical transmitter and an optical receiver. The optical transmitter and optical receiver are connected via an optical network.

[0003] The optical transmitter has an FM batch converter and an electrical / optical converter. The FM batch converter converts input signals, such as frequency-multiplexed video signals, into wideband FM signals centered around 3 GHz. The electrical / optical converter converts the wideband FM signal into an optical intensity modulated signal and outputs it. The output optical intensity modulated signal is transmitted through an optical network. The optical network enables wide-area optical transmission by connecting optical amplifiers, such as EDFAs (erbium-doped fiber amplifiers), and optical distributors in multiple stages. The optical receiver receives the optical intensity modulated signal transmitted through the optical network.

[0004] The optical receiving device has an optical / electrical converter, a differential detector, and an amplifier. The optical / electrical converter converts the received optical intensity modulated signal into a wideband FM signal. The differential detector demodulates the wideband FM signal back to the original signal. The amplifier amplifies the demodulated signal to a level appropriate for the system and then outputs it.

[0005] 3 is a diagram showing an example of functional blocks of an optical transmission device in an FM batch conversion system (see, for example, Non-Patent Document 3). The optical transmission device shown in FIG. 3 includes a first light source, an optical phase modulation unit, a multiplexing unit, a second light source, a light receiving unit, an optical intensity modulation unit, and a third light source.

[0006] The output light from the first light source is input to the optical phase modulation unit at the subsequent stage. The optical phase modulation unit phase-modulates the output light from the first light source using a first input signal and a second input signal. For example, the first input signal is a frequency-multiplexed signal of 90 to 770 MHz, and the second input signal is a frequency-multiplexed signal of 1.0 to 2.1 GHz. The multiplexer multiplexes the light phase-modulated by the optical phase modulation unit with the output light from the second light source. The optical receiver optically heterodynes the output light multiplexed by the multiplexer. This results in a wideband FM signal centered at a frequency equal to the frequency difference between the light from the first light source and the light from the second light source. The optical intensity modulation unit intensity-modulates the output light from the third light source using this FM signal and outputs it. This method converts all input signals into wideband FM signals at once, resulting in excellent noise resistance in the transmission path. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] "ITU-T J.185 : Transmission equipment for transferring multi-channel television signals over optical access networks by frequency modulation conversion," International Telecommunication Union, June 2012. [Non-patent document 2] Toshiaki Shimoba, Tomoaki Yoshida, Jun Terada, "Optical Video Distribution Technology Using FM Batch Conversion Method", Institute of Electronics, Information and Communication Engineers, IEICE Technical Report CS2019-84, IE2019-64(2019-12), pp.97-101 [Non-patent document 3] R. Miyatake, T. Shitaba, A. Tanabe, Y. Fukada, T. Yoshida, "Optical transmission experiment on FM conversion method with wideband phase modulation," IEICE Communications Express, Vol.10, No.12, p.967-972, 2021. Summary of the Invention [Problem to be solved by the invention]

[0008] The FM batch conversion method described above requires two light sources to generate the FM signal. The oscillation frequencies of these light sources fluctuate randomly over time. This causes a certain amount of phase noise in the FM signal output from the light receiving unit. As a result, the transmission distance is limited.

[0009] In view of the above circumstances, an object of the present invention is to provide an optical transmitting device and an optical signal generating method that can perform FM conversion while suppressing noise. [Means for solving the problem]

[0010] An optical transmission device according to one embodiment of the present invention comprises: an optical comb generator that generates an optical signal in which optical combs are arranged at equal frequency intervals using output light from a first light source and a frequency from a signal source; an extractor that extracts the first optical comb and a second optical comb from the optical signal generated by the optical comb generator; a phase modulator that generates a modulated signal by phase-modulating the first optical comb with an input signal; an adjuster that adjusts the delay of the second optical comb; a combiner that combines the modulated signal with the second optical comb whose delay has been adjusted; a light receiver that performs square-law detection on the light combined by the combiner to generate an FM (Frequency Modulation) signal; and an optical intensity modulator that intensity-modulates the output light of the second light source with the FM signal.

[0011] An optical signal generation method according to one embodiment of the present invention includes an optical comb generation step of generating an optical signal in which optical combs are arranged at equal frequency intervals using output light from a first light source and a frequency from a signal source; an extraction step of extracting a first optical comb and a second optical comb from the optical signal generated in the optical comb generation step; a phase modulation step of generating a modulated signal by phase-modulating the first optical comb with an input signal; an adjustment step of adjusting the delay of the second optical comb; a combining step of combining the modulated signal with the second optical comb whose delay has been adjusted; a signal generation step of generating an FM (Frequency Modulation) signal by performing square-law detection on the light combined in the combining step; and an optical intensity modulation step of intensity-modulating the output light of a second light source with the FM signal. [Effects of the Invention]

[0012] The present invention makes it possible to perform FM conversion with low noise. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a block diagram showing a configuration of an optical transmitting device according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram illustrating a conventional optical video distribution system. [Figure 3] FIG. 1 is a block diagram showing a configuration of a conventional optical transmitting device. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. This embodiment relates to an optical transmitter for an optical transmission system that employs a batch FM conversion method. The batch FM conversion method batch converts input signals into wideband FM signals, and then intensity-modulates the optical signals using the wideband FM signals before outputting them. The optical transmitter of this embodiment requires only one light source to generate the FM signals, and therefore can suppress the phase noise of the FM signals to a level lower than conventional methods. This makes it possible to realize a low-noise optical transmitter.

[0015] 1 is a functional block diagram of an optical transmission device 1 according to an embodiment of the present invention. The optical transmission device 1 includes a first light source 11, an optical comb generator 12, a signal source 13, a distributor 14, a first band-limiting unit 15, a phase modulator 16, a second band-limiting unit 17, an optical path length adjuster 18, a combiner 19, a light-receiving unit 20, an optical intensity modulator 21, and a second light source 22. Of the two optical paths between the distributor 14 and the combiner 19, the optical path including the first band-limiting unit 15 and the phase modulator 16 will be referred to as the first optical path, and the optical path including the second band-limiting unit 17 and the optical path length adjuster 18 will be referred to as the second optical path.

[0016] The primary light source 11 outputs light of a single frequency. The output light from the primary light source 11 is input to the optical comb generator 12. The optical comb generator 12 converts the light input from the primary light source 11 into an optical signal having an optical spectrum spaced at equal intervals at the frequency output from the signal source 13. The optical comb is this optical spectrum of each equally spaced frequency. Here, the optical comb generator 12 is generally a device having a structure in which an optical modulator is inserted into an optical resonator, which generates multiple sidebands spaced at a frequency equal to the frequency of the modulated signal input to the optical modulator. The modulated signal input to the optical modulator corresponds to the signal output by the signal source 13.

[0017] The splitter 14 receives the optical signal converted by the optical comb generator 12. The splitter 14 splits the input optical signal into two, and outputs one optical signal to the first band-limiting unit 15 of the first optical path and the other optical signal to the second band-limiting unit 17 of the second optical path.

[0018] The first band-limiting unit 15 receives the optical signal from the splitter 14 and extracts a specific optical comb from the multiple optical combs included in the received optical signal. That is, the first band-limiting unit 15 extracts one optical comb of a predetermined frequency. The first band-limiting unit 15 outputs the extracted optical comb to the phase modulator 16 and does not output the other optical combs.

[0019] The phase modulation unit 16 phase-modulates the optical comb input from the first band-limiting unit 15 using an input electrical signal. For example, the input signal may be a signal of a single frequency, or may include multiple signals of different frequencies. For example, the input signal may be a signal obtained by multiplexing a frequency multiplexed signal of 90 to 770 Hz and a frequency multiplexed signal of 1.0 to 2.1 GHz.

[0020] On the other hand, the second band-limiting unit 17 receives an optical signal from the splitter 14 and extracts a specific optical comb from the multiple optical combs included in the received optical signal, which is different from that of the first band-limiting unit 15. That is, the second band-limiting unit 17 extracts one optical comb of a predetermined frequency different from the frequency of the optical comb extracted by the first band-limiting unit 15. The second band-limiting unit 17 outputs the extracted optical comb to the optical path length adjuster 18 and does not output other optical combs.

[0021] The optical path length adjustment unit 18 has a predetermined optical path length and adjusts the delay amount of the optical comb output by the second band-limiting unit 17 so that the delay amount of the optical signal in the second optical path is equal to the delay amount of the optical signal in the first optical path. This is because delay occurs mainly due to the optical path length of the phase modulation unit 16. A value measured in advance can be used as the delay amount to be adjusted. The optical path length adjustment unit 18 outputs the optical comb with the adjusted delay amount to the multiplexing unit 19.

[0022] The multiplexer 19 multiplexes the optical signal phase-modulated by the phase modulator 16 with the optical signal input from the optical path length adjuster 18, and outputs the optical signal obtained by the multiplexing to the optical receiver 20. The optical signal input from the optical path length adjuster 18 is an optical comb with an adjusted delay. The optical receiver 20 performs square-law detection on the optical signal input from the multiplexer 19 and converts it into a wideband FM signal centered on a frequency equal to the frequency difference between the two multiplexed optical signals, i.e., the frequency difference between the first optical comb and the second optical comb. The optical receiver 20 outputs the wideband FM signal to the optical intensity modulator 21.

[0023] The optical intensity modulation unit 21 intensity-modulates the output light from the second light source 22 with the wideband FM signal input from the light receiving unit 20. The optical intensity modulation unit 21 outputs the intensity-modulated optical signal to a transmission line.

[0024] The optical transmitter 1 described above reduces the number of light sources required for conventional FM signal generation from two to one. Because only one light source is used, the degradation of phase noise due to temporal fluctuations in the light source's oscillation frequency is extremely small. Therefore, the phase noise of the FM signal at the light receiving unit output can be kept lower than in the past, realizing an optical transmitter with lower noise than conventional systems. Because the noise is lower, optical signals can be transmitted over longer distances than in conventional systems.

[0025] According to the above-described embodiment, the optical transmission device includes an optical comb generator, an extractor, a phase modulator, an adjuster, a combiner, a light receiver, and an optical intensity modulator. The optical comb generator generates an optical signal in which optical combs are arranged at equal frequency intervals using output light from a first light source and a frequency from a signal source. The extractor extracts the first and second optical combs from the optical signal generated by the optical comb generator. The phase modulator generates a modulated signal by phase-modulating the first optical comb with an input signal. The input signal may be a non-multiplexed signal or a frequency-multiplexed signal. The adjuster adjusts the delay of the second optical comb. The adjuster corresponds, for example, to the optical path length adjuster 18 in the embodiments. The combiner combines the modulated signal with the second optical comb whose delay has been adjusted. The light receiver generates an FM (Frequency Modulation) signal by square-law detection of the light combined by the combiner. The optical intensity modulator intensity-modulates the output light from the second light source with the FM signal.

[0026] The extractor may include a divider, a first extractor, and a second extractor. The divider divides the optical signal generated by the optical comb generator into two. The first extractor extracts a first optical comb from one of the optical signals divided by the divider. The second extractor extracts a second optical comb from the other optical signal divided by the divider. For example, the first extractor corresponds to the first band-limiting unit 15 in the embodiment, and the second extractor corresponds to the second band-limiting unit 17 in the embodiment.

[0027] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention. [Explanation of symbols]

[0028] 1 Optical transmitter 11 First light source 12 Optical comb generator 13 Signal source 14 Distribution section 15 First band limiting section 16 Phase modulation section 17 Second band limiting section 18 Optical path length adjustment section 19 Multiplexing section 20 Light receiving section 21 Light intensity modulation section 22 Second light source

Claims

1. an optical comb generator that generates an optical signal in which optical combs are arranged at equal frequency intervals using the output light from the first light source and the frequency from the signal source; an extractor that extracts a first optical comb and a second optical comb from the optical signal generated by the optical comb generator; a phase modulation unit that generates a modulated signal by phase-modulating the first optical comb with an input signal; an adjusting unit that adjusts the delay amount of the second optical comb so that it is equal to the delay amount of the phase-modulated first optical comb; a multiplexing unit that multiplexes the modulated signal and the second optical comb whose delay amount has been adjusted; a light receiving unit that performs square-law detection on the light multiplexed by the multiplexing unit to generate an FM (Frequency Modulation) signal; a light intensity modulation unit that intensity-modulates the output light of the second light source using the FM signal; An optical transmitting device comprising:

2. The extraction unit a splitter that splits the optical signal generated by the optical comb generator into two; a first extractor that extracts the first optical comb from one of the optical signals divided by the divider; a second extractor that extracts the second optical comb from the other optical signal divided by the divider, 2. The optical transmitter according to claim 1.

3. The input signal is a non-multiplexed signal or a frequency-multiplexed signal.

3. The optical transmitter according to claim 1.

4. an optical comb generating step of generating an optical signal in which optical combs are arranged at equal frequency intervals using output light from the first light source and a frequency from the signal source; an extraction step of extracting a first optical frequency comb and a second optical frequency comb from the optical signal generated in the optical frequency comb generation step; a phase modulation step of generating a modulated signal by phase-modulating the first optical comb with an input signal; an adjusting step of adjusting a delay amount of the second optical comb to be equal to a delay amount of the phase-modulated first optical comb; a combining step of combining the modulated signal with the second optical comb having an adjusted delay amount; a signal generating step of generating an FM (Frequency Modulation) signal by performing square-law detection on the light multiplexed in the multiplexing step; a light intensity modulation step of intensity-modulating the output light of the second light source by the FM signal; 1. A method for generating an optical signal comprising:

Images