JP2730697B2 - Optical pulse detection method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse detection method used in a receiving section of an intensity-modulated high-speed optical transmission system.

[Prior Art] Conventionally, in an intensity-modulated high-speed optical transmission system, an optical amplifier is arranged in front of a light receiving element, and the transmission distance is extended by improving reception sensitivity. In such a configuration, since the spontaneous emission light generated by the optical amplifier becomes a large noise source, a narrow band optical bandpass filter is usually arranged between the optical amplifier and the light receiving element to remove the spontaneous emission light. I have.

[Problems to be solved by the invention]

By the way, it is not easy to create the above-mentioned narrow band optical bandpass filter. Even if the narrowband optical bandpass filter can be created, the center wavelength of the optical signal fluctuates under the influence of temperature or the like. The bandwidth must be designed to be wide considering the fluctuation of the optical signal. Accordingly, it has been extremely difficult to efficiently remove spontaneous emission optical noise with the above configuration.

The present invention has been made in view of the above circumstances,
It is an object of the present invention to provide an optical pulse detection method capable of removing noise due to spontaneous emission light generated in an optical amplifier without using an optical bandpass filter.

[Means for solving the problem]

According to the present invention, in an optical pulse detection method for an intensity-modulated high-speed optical transmission system, an incident optical pulse is optically amplified, the wavelength of the optical pulse is converted by a second-order nonlinear optical effect, and then optical detection is performed. Also, it is necessary to optically amplify an incident light pulse, compress the amplified light pulse using a third-order nonlinear optical effect, further convert the wavelength of the light pulse by a second-order nonlinear optical effect, and then perform light detection. Features.

[Action]

Generally, as shown in FIG. 10, when amplifying the incident light pulse P at the optical amplifier A, the optical pulse P ₁ which is amplified spontaneous emission light N ₁ are superimposed as the background noise. Therefore, in the present invention, it amplified light pulses P ₁ and the wavelength conversion by the wavelength converter C according to second-order nonlinear effect, in this case, generally the wavelength conversion efficiency varies depending on the peak power of the incident light It is possible to make a large difference between the wavelength-converted light pulse P ₂ and the spontaneous emission light N ₂ , thereby efficiently removing noise due to the spontaneous emission light N ₂ and guiding the light to the photodetector B. Can be. After the incident light pulse P is amplified by the optical amplifier A, the light pulse compression unit D by the third-order nonlinear effect is obtained.
Then, the light pulse width is compressed to increase its peak power, and then the compressed light pulse P ₃ and the spontaneous emission light N ₃ are combined.
If the wavelength is converted by the wavelength converter C due to the following nonlinear effect,
Furthermore it is possible to give a large difference between the optical pulse P ₄ and the spontaneous emission light N _4, it can be removed efficiently spontaneous emission noise.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention. In this figure, 1 is the wavelength 530 nm, 807 nm, 82
An excitation light source that generates excitation light having a center wavelength around 0 nm, 980 nm, or 1470 nm, and an optical fiber 2 having an extraordinary dispersion in the 1.5 μm band, to which the element or Erbium (Er) element is added at its tip or all. This corresponds to the optical amplifier A shown in FIG. Reference numeral 3 denotes a dichroic mirror for multiplexing the pumping light output from the pumping light source 1 with an optical signal, which is an optical signal. Reference numerals 4 and 5 denote lenses, and reference numeral 6 denotes an SHG (second harmonic generation).
7 is the second harmonic generated by the SHG crystal 6.
A filter for transmitting only HG light, and 8 is a photodetector. The components 3 to 7 described above correspond to the wavelength conversion unit C based on the second-order nonlinear effect shown in FIG.

In the above configuration, the light pulse arriving from the left side of the drawing passes through the dichroic mirror 3 and is incident on the optical fiber 2, and is amplified by the Er element doped by the excitation light source 1. The amplified optical pulse is pulse-compressed by the Kerr effect and the dispersion effect generated in the optical fiber 2.
The light pulse is guided to the SHG crystal 6 via the lens 4 and wavelength-converted. And, before the photodetector 8,
In order to block a part of the light that has passed through the SHG crystal 6 without wavelength conversion, the filter 7 that transmits only the wavelength-converted light pulse is provided. The light is condensed by the lens 5, reaches the photodetector 8, and is subjected to photo-electric conversion by the photodetector 8, whereby a light pulse is detected. Here, considering the current state of optical communication in which the center wavelength of the optical signal is near infrared, the optical signal wavelength-converted by the SHG crystal 6 having the second-order nonlinear optical effect has a visible wavelength band. Therefore, a high-sensitivity Si-APD (avalanche photodiode) having a small dark current can be used as the photodetector 8. Further, in the above-described embodiment, since the Er-doped optical fiber 2 is used as the optical amplifier, both functions of optical amplification and pulse compression can be realized by this optical fiber 2 at the same time.

Next, FIG. 2 is a block diagram showing a configuration of a second embodiment of the present invention. In this figure, 9 is a wavelength 530 nm, 80
An excitation light source that outputs excitation light having a center wavelength around 7 nm, 820 nm, 980 nm or 1470 nm, 10 is an optical fiber having a normal dispersion in the 1.5 μm band to which Er element is added at its tip or all, and 11 is an excitation light source 9. Dichroic mirrors for multiplexing the excitation light and the optical pulse that is an optical signal output from the lens, 12, 13, 14 are lenses, and 15, 16 are a pair arranged so as to have anomalous dispersion in the 1.5 μm band. Grating lens,
17 is a crystal for SHG, 18 is a filter for transmitting only SHG light, and 19 is a photodetector. 15 and 16 of the components described above
Corresponds to the optical pulse compression unit D based on the three-dimensional nonlinear effect shown in FIG.

In the above configuration, the light pulse arriving from the left side of the drawing passes through the dichroic mirror 11 and is incident on the optical fiber 10, and is amplified by the Er element doped by the excitation light source 9. The amplified optical pulse is converted to optical fiber
Due to the Kerr effect occurring in 10, an instantaneous wavelength becomes an optical pulse having a blue shift chirp shifted to the short wavelength side, and this optical pulse is guided to a pair of grating lenses 15 and 16 having anomalous dispersion via the lens 12, Pulse compression. This light pulse is transmitted through the lens 13 to the SHG crystal 1
Guided to 7 and wavelength converted. Then, part of the light that has passed through the SHG crystal 17 without wavelength conversion is blocked by the filter 18, so that only the wavelength-converted light pulse passes through the filter 18, is condensed by the lens 14, and The light pulse arrives at the detector 19 and is subjected to optical-electrical conversion by the optical detector 19, whereby an optical pulse is detected.

Next, FIG. 3 is a block diagram showing a configuration of a third embodiment of the present invention. In this figure, 20 is an optical fiber 21
Pump light source for performing Raman amplification inside, 21 is an optical fiber having anomalous dispersion in a wavelength band to be Raman-amplified, 22 is for multiplexing the pump light output from the pump light source 20 and the optical pulse which is an optical signal. , 23 and 24 are lenses, 25 is an SHG crystal, 26 is a filter for transmitting only SHG light, and 27 is a photodetector.

In the above configuration, the optical pulse arriving from the left side of the drawing passes through the dichroic mirror 22, enters the optical fiber 21, and is stimulated Raman-amplified by the pumping light output from the pumping light source 20. The amplified optical pulse is pulse-compressed by the Kerr effect and dispersion generated in the optical fiber 21. This light pulse is guided to the SHG crystal 25 via the lens 23, and wavelength-converted. Then, a part of the light that has passed through the SHG crystal 25 without wavelength conversion is blocked by the filter 26, so that only the wavelength-converted light pulse passes through the filter 26, is collected by the lens 24, and The light pulse arrives at the detector 27 and is subjected to optical-electrical conversion by the photodetector 27, whereby an optical pulse is detected. In the above-described embodiment, since the optical fiber 21 for Raman amplification is used instead of the optical fiber amplifier, both functions of the optical amplification and the pulse compression can be realized by the optical fiber 21 at the same time.

FIG. 4 is a block diagram showing a configuration of a fourth embodiment of the present invention. In this figure, reference numeral 28 denotes an excitation light source for performing Raman amplification in an optical fiber 29, reference numeral 29 denotes an optical fiber having normal dispersion in a wavelength band to be subjected to Raman amplification, and reference numeral 30 denotes an excitation light source 28.
Dichroic mirrors for multiplexing the excitation light output from the optical pulse and the optical pulse that is an optical signal, 31, 32, and 33 are lenses,
34 and 35 are a pair of grating lenses arranged so as to have anomalous dispersion in the 1.5 μm band, 36 is a crystal for SHG, and 37 is SHG
A filter for transmitting only light, and 38 is a photodetector.

In the above configuration, the light pulse arriving from the left side of the drawing passes through the dichroic mirror 30, enters the optical fiber 29, and is stimulated Raman-amplified by the excitation light output from the excitation light source. The amplified optical pulse is applied to the optical fiber 29
Due to the Kerr effect, the light pulse has a blue shift chirp whose instantaneous wavelength shifts to the shorter wavelength side,
The light is guided to a pair of grating lenses 34 and 35 having anomalous dispersion via a lens 31 and pulse-compressed. This light pulse is guided to the SHG crystal 36 via the lens 32, and wavelength-converted. Then, some light that has passed through the SHG crystal 36 without wavelength conversion is blocked by the filter 37,
Only the wavelength-converted light pulse passes through the filter 37, is condensed by the lens 33, reaches the photodetector 38, and this photodetector
The optical pulse is detected by the photoelectric conversion by 38.

Next, FIG. 5 is a block diagram showing a configuration of a fifth embodiment of the present invention. In this figure, 39 is a traveling waveform semiconductor amplifier, 40 is an optical fiber having anomalous dispersion in an optical signal band, 41, 42, 43, 44 are lenses, 45 is a crystal for SHG, and 46 is for transmitting only SHG light. The filter 47 is a photodetector.

In the above configuration, the optical pulse arriving from the left side of the drawing enters the traveling waveform semiconductor amplifier 39 via the lens 41, is amplified there, and is guided to the optical fiber 40 via the lens. The amplified optical pulse is pulse-compressed by the Kerr effect and the dispersion effect generated in the optical fiber 40. The light pulse is guided to the SHG crystal 45 via the lens 43, and is subjected to wavelength conversion. Part of the light that has passed through the SHG crystal 45 without wavelength conversion is blocked by the filter 46, and only the wavelength-converted light pulse passes through the filter 46, and
The light reaches the photodetector 47 and is subjected to photo-electric conversion by the photodetector 47, whereby a light pulse is detected.

FIG. 6 is a block diagram showing the configuration of the sixth embodiment of the present invention. In this figure, 48 is a traveling-wave semiconductor amplifier, 49 is an optical fiber having normal dispersion in the optical signal band, and 50 and 5
1, 52, 53, 54 are lenses, 55, 56 are a pair of grating lenses arranged to have anomalous dispersion in the optical signal band,
57 is a crystal for SHG, 58 is a filter for transmitting only SHG light, and 59 is a photodetector.

In the above configuration, the light pulse arriving from the left side of the drawing enters the traveling waveform semiconductor amplifier 48 via the lens 50, is amplified there, and is guided to the optical fiber 49 via the lens 51. The amplified optical pulse becomes an optical pulse having a blue shift chirp whose instantaneous wavelength is shifted to the shorter wavelength side due to the Kerr effect generated in the optical fiber 49, and
The light is guided to a pair of grating lenses 55 and 56 having anomalous dispersion via 52 and pulse-compressed. This light pulse is guided to the SHG crystal 57 via the lens 53, and is subjected to wavelength conversion. Then, part of the light that has passed through the SHG crystal 57 without wavelength conversion is cut off by the filter 58, and only the wavelength-converted light pulse passes through the filter 58, is collected by the lens 54, and is detected by the photodetector 59. And the light detector 59
The light pulse is detected by the electrical conversion.

Next, FIG. 7 is a block diagram showing a configuration of a seventh embodiment of the present invention. In this figure, 60 is a wavelength of 530 nm, 80
7 nm, 820 nm, 980 nm, or an excitation light source that outputs excitation light having a center wavelength around 1470 nm, 61 is an optical fiber doped with Er element at its tip or all, 62 is an excitation light output from the excitation light source 60. A dichroic mirror for multiplexing an optical pulse which is an optical signal, 63 and 64 are lenses, and 65 is an SHG
Crystal, 66 is a filter for transmitting only SHG light, 6
7 is a photodetector.

In the above configuration, the light pulse arriving from the left side of the drawing enters the optical fiber 1 through the dichroic mirror 62, and is amplified by the Er element doped by the excitation light source 60. This light pulse is applied to the SHG crystal 6 via the lens 63.
Guided to 5 and wavelength converted. Then, part of the light that has passed through the SHG crystal 65 without wavelength conversion is blocked by the filter 66, and only the wavelength-converted light pulse is filtered by the filter 66.
And is collected by the lens 64 and reaches the photodetector 67.
Light-to-electric conversion is performed by the photodetector 67, so that a light pulse is detected.

FIG. 8 is a block diagram showing a configuration of an eighth embodiment of the present invention. In this figure, 68 is an excitation light source for performing Raman amplification in an optical fiber 69, 69 is an optical fiber, 70 is for multiplexing the excitation light output from the excitation light source 68 and an optical pulse as an optical signal. A dichroic mirror, 71 and 72 are lenses, 73 is an SHG crystal, 74 is a filter for transmitting only SHG light, and 75 is a photodetector.

In the above configuration, an optical pulse arriving from the left side of the drawing passes through the dichroic mirror 70, enters the optical fiber 69, and is stimulated Raman-amplified by the excitation light output from the excitation light source 68. This light pulse is transmitted through the lens 71 to the SH
The light is guided to the G crystal 73 and wavelength-converted. Some of the light that has passed through the SHG crystal 73 without wavelength conversion is
Only the light pulse that has been cut off and wavelength-converted by 4 passes through the filter 74, is condensed by the lens 72, reaches the photodetector 75, and is photo-electrically converted by this photo A pulse is detected.

Next, FIG. 9 is a block diagram showing a configuration of a ninth embodiment of the present invention. In this figure, 76 is a traveling waveform semiconductor amplifier, 77, 78, 79 are lenses, 80 is a crystal for SHG, 81 is SHG
A filter for transmitting only light, and 82 is a photodetector.

In the above configuration, the light pulse arriving from the left side of the drawing enters the traveling waveform semiconductor amplifier 76 via the lens 77 and is amplified. This light pulse is transmitted through the lens 78 to the SHG
It is guided to the crystal for use 80 and is wavelength-converted. Some of the light that has passed through the SHG crystal 80 without wavelength conversion is
Only the light pulse that has been cut off and wavelength-converted passes through the filter 81, is condensed by the lens 79, reaches the light detector 82, and is subjected to light-to-electric conversion by the light detector 82. Is detected.

〔The invention's effect〕

As described above, the use of the detection method according to the present invention makes it possible to remove spontaneous emission optical noise generated due to optical amplification of an optical pulse as an optical signal without using a narrow-band optical filter. . Moreover, considering the current state of optical communication where the center wavelength of the optical signal is near infrared, the optical signal wavelength-converted by the SHG crystal having the second-order nonlinear optical effect has a visible wavelength band. There is also an advantage that a high-sensitivity Si-APD with a small dark current can be used as a photodetector, and it is particularly suitable for use in a receiving portion of ultra-high-speed intensity-modulated optical transmission.

[Brief description of the drawings]

FIG. 1 to FIG. 9 are block diagrams showing the respective configurations of the first to ninth embodiments of the present invention, and FIG. 10 is a block diagram showing the overall configuration of the present invention. 1,9,20,28,60,68 …… Excitation light source, 2,10,21,29,40,49,61,69 …… Optical fiber, 3,11,22,30,62,70 …… Dichroic Mirror, 4,5,12-14,23,24,31-33,41-44,50-54,63,64,71,72,
77-79 …… Lens, 6,17,25,36,45,57,65,73,80 …… SHG crystals, 7,18,26,37,46,58,66,74,81 …… Filters , 8,19,27,38,47,59,67,75,82 …… Photodetector, 15,16,34,35,55,56 …… Grating lens, 39,48,76 …… Progressive waveform semiconductor amplifier.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H04B 10/142 10/152

Claims (2)

(57) [Claims] 1. An optical pulse detection method for an intensity-modulated high-speed optical transmission system, comprising: amplifying an incident optical pulse; converting the wavelength of the optical pulse by a second-order nonlinear optical effect; Light pulse detection method. 2. An optical pulse detection method for an intensity-modulated high-speed optical transmission system, wherein an incident optical pulse is optically amplified, the amplified optical pulse is pulse-compressed using a third-order nonlinear optical effect, and further, a second-order nonlinear optical An optical pulse detection method, comprising: converting a wavelength of an optical pulse by an effect, and then detecting light.