An optoelectronic oscillation system

Abstract

The present invention provides an optoelectronic oscillation system, which includes an optoelectronic oscillation module, a delay compensation module, and a multiplexing module; the optoelectronic oscillation module includes a first optical signal input terminal and a first electrical signal output terminal; the delay compensation module includes a second optical signal input terminal and a second electrical signal output terminal; the multiplexing module is electrically connected between the first optical signal input terminal and the first electrical signal output terminal, and is electrically connected to the second optical signal input terminal, and the multiplexing module is used to generate a delay; wherein, the optoelectronic oscillation module is used to generate a first electrical signal, the first electrical signal output terminal is electrically connected to the second optical signal input terminal, the first electrical signal is delayed and then transmitted to the delay compensation module, the delay compensation module generates a second electrical signal according to the first electrical signal and the multiplexing module, and the second electrical signal output terminal is electrically connected to the first optical signal input terminal to form a closed loop. Generate high-quality microwave signals and construct delay pre-compensation to eliminate the influence caused by fiber jitter in principle.

Description

Technical Field

[0001] The present invention relates to the technical field of microwave photonics, and particularly relates to an optoelectronic oscillation system. Background Art

[0002] Generating radio frequency signals using microwave photonics technology is a field that has been studied for a long time. There are many drawbacks in the electrical signals generated by traditional electrical methods, such as poor signal quality and difficulty in generating high-frequency signals. With the development of microwave photonics technology, people use long fiber energy storage in optoelectronic oscillators to achieve high Q values, thereby generating high-quality microwave signals. However, due to the influence of environmental temperature and vibration, the stability problem of long fibers has always attracted much attention. There are currently many studies on this, such as injection locking, phase-locked loops, etc. Summary of the Invention

[0003] The main object of the present invention is to propose a frequency-sweeping optoelectronic oscillator, aiming to solve the stability problem of long fibers.

[0004] To achieve the above object, the present invention proposes an optoelectronic oscillation system, including:

[0005] An optoelectronic oscillation module, including a first optical signal input end and a first electrical signal output end;

[0006] A delay compensation module, including a second optical signal input end and a second electrical signal output end; and,

[0007] A multiplexing module, electrically connected between the first optical signal input end and the first electrical signal output end, and electrically connected to the second optical signal input end. The multiplexing module is used to generate a delay;

[0008] Wherein, the optoelectronic oscillation module is used to generate a first electrical signal. The first electrical signal output end is electrically connected to the second optical signal input end. The first electrical signal is delayed and then transmitted to the delay compensation module. The delay compensation module generates a second electrical signal according to the first electrical signal and the multiplexing module. The second electrical signal output end is electrically connected to the first optical signal input end to form a closed loop.

[0009] Optionally, the optoelectronic oscillation module includes a phase modulator, a notch filter, and an isolator that are electrically connected in sequence. The phase modulator, the notch filter, and the isolator together form the first optical signal input end;

[0010] Wherein, the second electrical signal output end is electrically connected to the phase modulator, and the multiplexing module is electrically connected between the isolator and the first electrical signal output end.

[0011] Optionally, the optoelectronic oscillation module further includes an optical filter, a first photodetector, and a first electrical amplifier that are electrically connected in sequence. The optical filter, the first photodetector, and the first electrical amplifier together form the first electrical signal output end;

[0012] Wherein, the first electrical amplifier is electrically connected to the second optical signal input end, and the multiplexing module is electrically connected between the first optical signal input end and the optical filter.

[0013] Optionally, the delay compensation module includes an intensity modulator, a first optical coupler, and a first circulator that are electrically connected in sequence. The intensity modulator, the first optical coupler, and the first circulator together form the second optical signal input end;

[0014] Wherein, the first electrical signal output end is electrically connected to the intensity modulator, and the multiplexing module is electrically connected to the first circulator.

[0015] Optionally, the second electrical signal output end includes:

[0016] A first optoelectronic conversion path, electrically connected to the first optical coupler, for generating a first electrical signal to be mixed;

[0017] A second optoelectronic conversion path, electrically connected to the first circulator, for generating a second electrical signal to be mixed; and,

[0018] A mixing path, electrically connected to the first optoelectronic conversion path and the second optoelectronic conversion path, and electrically connected to the first optical signal input end, for mixing the first electrical signal to be mixed and the second electrical signal to be mixed to form a second electrical signal.

[0019] Optionally, the first optoelectronic conversion path includes a second photodetector, a first electrical filter, and a second electrical amplifier that are electrically connected in sequence;

[0020] Wherein, the second photodetector is electrically connected to the first optical coupler, and the second electrical amplifier is electrically connected to the mixing path.

[0021] Optionally, the second optoelectronic conversion path includes a third photodetector, a second electrical filter, and a third electrical amplifier that are electrically connected in sequence;

[0022] Wherein, the third photodetector is electrically connected to the first circulator, and the third electrical amplifier is electrically connected to the mixing path.

[0023] Optionally, the mixing path includes a mixer, a third electrical filter, a frequency divider, and a fourth electrical amplifier that are electrically connected in sequence;

[0024] Wherein, the mixer is electrically connected to the first optoelectronic conversion path and the second optoelectronic conversion path, and the fourth electrical amplifier is electrically connected to the first optical signal input end.

[0025] Optionally, the multiplexing module includes a wavelength division multiplexing / demultiplexing, a dispersion-shifted fiber, a second circulator, an optical amplifier, and a second optical coupler that are electrically connected in sequence;

[0026] Among them, the wavelength division multiplexing demultiplexer is electrically connected to the first optical signal input end and the second optical signal input end, and the second optical coupler is electrically connected to the second circulator and the first optical signal output end.

[0027] Optionally, the optoelectronic oscillation system further includes a polarization controller. The polarization controller is disposed in the closed loop and on the optical path of the closed loop. The polarization controller is used to adjust the polarization direction of the light beam; and / or,

[0028] The optoelectronic oscillation system further includes a first laser and a second laser. The first laser is electrically connected to the first optical signal input end, and the second laser is electrically connected to the second optical signal input end.

[0029] In the technical solution provided by the present invention, the first electrical signal generated by the optoelectronic oscillation module is output to the delay compensation module. After being delay pre-compensated by the delay pre-compensation module, a second electrical signal is formed and output to the optoelectronic oscillation module to realize the closure of the optoelectronic oscillation loop; high-quality microwave signals are generated by optoelectronic oscillation, and delay pre-compensation is constructed to eliminate the influence brought by fiber jitter in principle. Description of the Drawings

[0030] Figure 1 It is a schematic connection structure diagram of an embodiment of the optoelectronic oscillation system provided by the present invention.

[0031] Explanation of the Reference Numerals in the Drawings:

[0032]

[0033] Detailed Embodiments

[0034] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further describes the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0035] It should be noted that if there are directional indications in the embodiments of the present invention, the directional indications are only used to explain the relative position relationship and movement conditions between components in a specific posture. If the specific posture changes, the directional indications will also change accordingly.

[0036] In addition, if the embodiments of the present invention involve descriptions such as "first", "second", etc., such descriptions of "first", "second", etc. are only for descriptive purposes and should not be construed as indicating or implying their relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In addition, the technical solutions between various embodiments may be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0037] Generating radio frequency signals using microwave photonics technology is a field that has been studied for a long time. There are many drawbacks to the electrical signals generated by traditional electrical methods, such as poor signal quality and difficulty in generating high-frequency signals. With the development of microwave photonics technology, people have achieved high Q values by means of an optoelectronic oscillator and long fiber energy storage, thus generating high-quality microwave signals. However, due to the influence of environmental temperature and vibration, the stability problem of long fibers has always attracted much attention, and there are currently many studies on this, such as injection locking, phase-locked loops, etc.

[0038] The present invention provides an optoelectronic oscillation system, aiming to solve the stability problem of long fibers. Among them, Figure 1 This is an embodiment provided by the present invention.

[0039] Please refer to Figure 1 , the present invention provides an optoelectronic oscillation system 100, including an optoelectronic oscillation module, a delay compensation module, and a multiplexing module; the optoelectronic oscillation module includes a first optical signal input end and a first electrical signal output end; the delay compensation module includes a second optical signal input end and a second electrical signal output end; the multiplexing module is electrically connected between the first optical signal input end and the first electrical signal output end, and is electrically connected to the second optical signal input end. The multiplexing module is used to generate a delay; among them, the optoelectronic oscillation module is used to generate a first electrical signal, the first electrical signal output end is electrically connected to the second optical signal input end, and the first electrical signal is transmitted to the delay compensation module after being delayed. The delay compensation module generates a second electrical signal according to the first electrical signal and the multiplexing module, and the second electrical signal output end is electrically connected to the first optical signal input end to form a closed loop.

[0040] In the technical solution provided by the present invention, the first electrical signal generated by the optoelectronic oscillation module is output to the delay compensation module. After the delay compensation module performs delay pre-compensation on it, a second electrical signal is formed and output to the optoelectronic oscillation module to realize the closure of the optoelectronic oscillation loop; high-quality microwave signals are generated using optoelectronic oscillation, and delay pre-compensation is constructed to eliminate the influence brought by fiber jitter in principle.

[0041] Specifically, the optoelectronic oscillation module includes a phase modulator 11, a notch filter 12, and an isolator 13 that are electrically connected in sequence. The phase modulator 11, the notch filter 12, and the isolator 13 together form a first optical signal input terminal. Among them, the second electrical signal output terminal is electrically connected to the phase modulator 11, and the multiplexing module is electrically connected between the isolator 13 and the first electrical signal output terminal. In this embodiment, the isolator 13 is used to achieve unidirectional transmission of optical signals in the optical path where it is located, preventing signals from being incorrectly output from the multiplexing module to the first optical signal input terminal.

[0042] It should be noted that there are various ways to set the notch filter 12. The notch filter 12 includes any one of a phase-shifted fiber Bragg grating, a Fabry-Perot resonator, a gas absorption cell, or a micro-ring, and no specific limitation is made here.

[0043] In addition, the optoelectronic oscillation module further includes an optical filter 14, a first photodetector 15, and a first electrical amplifier 16 that are electrically connected in sequence. The optical filter 14, the first photodetector 15, and the first electrical amplifier 16 together form a first electrical signal output terminal. Among them, the first electrical amplifier 16 is electrically connected to the second optical signal input terminal, and the multiplexing module is electrically connected between the first optical signal input terminal and the optical filter 14. The first electrical amplifier 16 is used to enhance the power of the electrical signal in the optoelectronic oscillation module to obtain a first electrical signal and achieve stable oscillation of the signal.

[0044] It should be noted that in this embodiment, multiple first electrical amplifiers 16 can be provided.

[0045] It should be noted that in the embodiments provided in this application, the two related technical features of the setting of the first optical signal input terminal and the setting of the first optical signal output terminal exist simultaneously, so as to jointly form an optoelectronic oscillation loop with the multiplexing module to generate optoelectronic oscillation and maintain the oscillation starting state of the loop.

[0046] It should be noted that in an embodiment provided in this application, multiple optoelectronic oscillation modules are provided to form a multi-loop system.

[0047] In addition, in this embodiment, the optoelectronic oscillation module further includes an auxiliary optical amplifier, and the auxiliary optical amplifier is arranged on the optical path of the optoelectronic oscillation module to enhance the power of the optical signal in the optoelectronic oscillation module.

[0048] On the other hand, the delay compensation module includes an intensity modulator 21, a first optical coupler 22, and a first circulator 23 that are electrically connected in sequence. The intensity modulator 21, the first optical coupler 22, and the first circulator 23 together form a second optical signal input terminal. Among them, the first electrical signal output terminal is electrically connected to the intensity modulator 21, and the multiplexing module is electrically connected to the first circulator 23.

[0049] It should be noted that there are various ways to set the intensity modulator 21. The intensity modulator 21 includes any one of a Mach-Zehnder modulator (MZM), a dual-parallel Mach-Zehnder modulator (DPMZM), or a dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM), and no specific limitation is made here.

[0050] Furthermore, the second electrical signal output terminal includes a first optoelectronic conversion path, a second optoelectronic conversion path, and a mixing path; the first optoelectronic conversion path is electrically connected to the first optical coupler 22 to generate a first electrical signal to be mixed; the second optoelectronic conversion path is electrically connected to the first circulator 23 to generate a second electrical signal to be mixed; the mixing path is electrically connected to the first optoelectronic conversion path and the second optoelectronic conversion path, and is electrically connected to the first optical signal input terminal to mix the first electrical signal to be mixed and the second electrical signal to be mixed to form a second electrical signal.

[0051] Specifically, in this embodiment, the first optoelectronic conversion path includes a second photodetector 241, a first electrical filter 242, and a second electrical amplifier 243 that are electrically connected in sequence; among them, the second photodetector 241 is electrically connected to the first optical coupler 22, and the second electrical amplifier 243 is electrically connected to the mixing path. In this embodiment, the first electrical filter 242 is used to select an electrical signal with a specific center frequency, and the second electrical amplifier 243 is used to enhance the power of the electrical signal.

[0052] On the other hand, the second optoelectronic conversion path includes a third photodetector 251, a second electrical filter 252, and a third electrical amplifier 253 that are electrically connected in sequence; among them, the third photodetector 251 is electrically connected to the first circulator 23, and the third electrical amplifier 253 is electrically connected to the mixing path. In this embodiment, the second electrical filter 252 is used to select an electrical signal with a specific center frequency, and the third electrical amplifier 253 is used to enhance the power of the electrical signal.

[0053] In this embodiment, the mixing path includes a mixer 261, a third electrical filter 262, a frequency divider 263, and a fourth electrical amplifier 264 that are electrically connected in sequence; among them, the mixer 261 is electrically connected to the first optoelectronic conversion path and the second optoelectronic conversion path, and the fourth electrical amplifier 264 is electrically connected to the first optical signal input terminal. Among them, the third electrical filter 262 is used to select an electrical signal with a specific center frequency, and the fourth electrical amplifier 264 is used to enhance the power of the electrical signal.

[0054] It should be noted that the number of the first electrical filter 242, the second electrical filter 252, and the third electrical filter 262 is not specifically limited, and each can be provided with a single one or multiple ones. In this embodiment, multiple ones are provided for each; similarly, the number of the second electrical amplifier 243, the third electrical amplifier 253, and the fourth electrical amplifier 264 is not specifically limited, and each can be provided with a single one or multiple ones. In this embodiment, multiple ones are provided for each.

[0055] In order to facilitate the multiplexing module to be electrically connected to the optoelectronic oscillation module and the delay compensation module simultaneously, in this embodiment, the multiplexing module includes a wavelength division multiplexing demultiplexer 31, a dispersion - shifted fiber 32, a second circulator 33, an optical amplifier 34, and a second optical coupler 35 that are electrically connected in sequence; among them, the wavelength division multiplexing demultiplexer 31 is electrically connected to the first optical signal input terminal and the second optical signal input terminal, and the second optical coupler 35 is electrically connected to the second circulator 33 and the first optical signal output terminal. In this embodiment, the optical amplifier 34 is used to enhance the power of the optical signal to achieve stable oscillation of the signal, and the dispersion - shifted fiber 32 is a low - loss energy storage element, thereby improving the noise performance of the optoelectronic oscillator.

[0056] It should be noted that the number of the optical amplifier 34 is not specifically limited. In this embodiment, multiple optical amplifiers 34 are provided.

[0057] In this embodiment, the optoelectronic oscillation system 100 further includes a polarization controller. The polarization controller is disposed in the closed loop and on the optical path of the closed loop, and the polarization controller is used to adjust the polarization direction of the light beam.

[0058] In addition, the optoelectronic oscillation system 100 further includes a first laser 5 and a second laser 6. The first laser 5 is electrically connected to the first optical signal input terminal, and the second laser 6 is electrically connected to the second optical signal input terminal.

[0059] It should be noted that in the settings of the two related technical features of the above - mentioned optoelectronic oscillation system 100, either one can exist, or both can exist simultaneously. In this embodiment, the above - mentioned two technical features exist simultaneously.

[0060] The present invention provides a specific embodiment:

[0061] The optoelectronic oscillation system 100 includes a first laser 5 and a second laser 6. The first laser 5 and the second laser 6 generate optical signals with different wavelengths as carriers.

[0062] The optoelectronic oscillation system 100 includes an optoelectronic oscillation module, a delay compensation module, and a multiplexing module.

[0063] The optoelectronic oscillation module includes a phase modulator 11, a notch filter 12, and an isolator 13 that are electrically connected in sequence; among them, the first laser 5 is electrically connected to the phase modulator 11. Through the phase modulator 11, an optical signal loaded with a second electrical signal is obtained, and then through the notch filter 12 and the isolator 13, it is transmitted to the multiplexing module; the multiplexing module includes a wavelength division multiplexing demultiplexer 31, a dispersion - shifted fiber 32, a second circulator 33, an optical amplifier 34, and a second optical coupler 35 that are electrically connected in sequence. After the optical signal enters the wavelength division multiplexing demultiplexer 31, it enters the dispersion - shifted fiber 32, generating a delay τ, and then passes through the second circulator 33, the optical amplifier 34, and the second optical coupler 35, and finally enters the first electrical signal output terminal. The first electrical signal output terminal includes an optical filter 14, a first photodetector 15, and a first electrical amplifier 16. The optical signal passes through the optical filter 14 to filter out the optical signals other than those from the first optical signal input terminal, and then passes through the first photodetector 15 to convert the optical signal into an electrical signal, and then through the first electrical amplifier 16, and then is sent to the delay compensation module.

[0064] The delay compensation module includes an intensity modulator 21, a first optical coupler 22, and a first circulator 23 that are electrically connected in sequence. The second laser 6 is electrically connected to the intensity modulator 21. The optical signal loaded with an electrical signal enters the first optical coupler 22 from the intensity modulator 2, and branching occurs on the first optical coupler 22.

[0065] One of the branches enters the second photodetector 241 and is converted into an electrical signal with a third - harmonic term, and the first electrical filter 242 is used to filter out the third - harmonic signal;

[0066] The other signal of the first optical coupler 22 enters port 1 of the first circulator 23, then is output from port 2 of the first circulator 23 and enters the wavelength division multiplexing demultiplexer 31, then enters the dispersion - shifted fiber 32 to generate a delay τ, enters port 2 of the second circulator 33, is output from port 3 of the second circulator 33 and enters the optical amplifier 34 for amplification, then enters the second optical coupler 35. One optical signal of the second optical coupler 35 returns to port 1 of the second circulator 33, is output from port 2 of the second circulator 33 and enters the dispersion - shifted fiber 32 to generate a delay τ again, then enters the wavelength division multiplexing demultiplexer 31, is demultiplexed and enters port 2 of the first circulator 23, is output from port 3 of the first circulator 23, enters the third photodetector 251 to convert the optical signal into an electrical signal, and at the same time, the fundamental - frequency signal is filtered out by the second electrical filter 252.

[0067] After the electrical signal power is amplified by the third electrical amplifier 253 and the second electrical amplifier 243 respectively, it enters the mixer 261. The third electrical filter 262 is used to filter out the down - converted signal. The frequency divider 263 performs frequency - division processing on the signal to generate an electrical signal with a - τ delay. After being amplified by the fourth electrical amplifier 264, it enters the phase modulator 11.

[0068] In this way, a closed loop is formed for circulation. The delay variation caused by temperature jitter in the zero-dispersion long fiber 32 is eliminated in principle. Finally, a high-quality electrical signal is output.

[0069] In the specific embodiments described above, the purpose, technical solutions and beneficial effects of the present invention have been further described in detail. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An optoelectronic oscillation system, characterized in that, Comprising: An optoelectronic oscillation module, including a first optical signal input end and a first electrical signal output end; A delay compensation module, including a second optical signal input end and a second electrical signal output end; And, A multiplexing module, electrically connected between the first optical signal input end and the first electrical signal output end, and electrically connected to the second optical signal input end, the multiplexing module being used to generate a delay; Wherein, the optoelectronic oscillation module is used to generate a first electrical signal, the first electrical signal output end is electrically connected to the second optical signal input end, the second electrical signal output end is electrically connected to the first optical signal input end to form a closed loop, the optoelectronic oscillation module is used to generate a first optical signal loaded with a second electrical signal, and after being delayed by the multiplexing module, a first electrical signal is obtained and transmitted to the delay compensation module, the delay compensation module is used to generate a second optical signal loaded with the first electrical signal, input the second optical signal loaded with the first electrical signal into the multiplexing module for delay, and generate a second electrical signal according to the delayed second optical signal loaded with the first electrical signal and the second optical signal loaded with the first electrical signal, and input the second electrical signal into the optoelectronic oscillation module.

2. The optoelectronic oscillation system according to claim 1, characterized in that, The optoelectronic oscillation module includes a phase modulator, a notch filter, and an isolator that are electrically connected in sequence, and the phase modulator, the notch filter, and the isolator together form the first optical signal input end; Wherein, the second electrical signal output end is electrically connected to the phase modulator, and the multiplexing module is electrically connected between the isolator and the first electrical signal output end.

3. The optoelectronic oscillation system according to claim 1, characterized in that, The optoelectronic oscillation module further includes an optical filter, a first photodetector, and a first electrical amplifier that are electrically connected in sequence, and the optical filter, the first photodetector, and the first electrical amplifier together form the first electrical signal output end; Wherein, the first electrical amplifier is electrically connected to the second optical signal input end, and the multiplexing module is electrically connected between the first optical signal input end and the optical filter.

4. The optoelectronic oscillation system according to claim 1, characterized in that, The delay compensation module includes an intensity modulator, a first optical coupler, and a first circulator that are electrically connected in sequence, and the intensity modulator, the first optical coupler, and the first circulator together form the second optical signal input end; Wherein, the first electrical signal output end is electrically connected to the intensity modulator, and the multiplexing module is electrically connected to the first circulator.

5. The optoelectronic oscillation system according to claim 4, characterized in that, The second electrical signal output end includes: A first optoelectronic conversion path, electrically connected to the first optical coupler, for generating a first electrical signal to be mixed; A second optoelectronic conversion path, electrically connected to the first circulator, for generating a second electrical signal to be mixed; and, A mixing path, electrically connected to the first optoelectronic conversion path and the second optoelectronic conversion path, and electrically connected to the first optical signal input end, for mixing the first electrical signal to be mixed and the second electrical signal to be mixed to form the second electrical signal.

6. The optoelectronic oscillation system according to claim 5, characterized in that, The first optoelectronic conversion path includes a second photodetector, a first electrical filter, and a second electrical amplifier that are electrically connected in sequence; Among them, the second photodetector is electrically connected to the first optical coupler, and the second electrical amplifier is electrically connected to the mixing circuit.

7. The optoelectronic oscillation system according to claim 5, characterized in that, The second photoelectric conversion circuit includes a third photodetector, a second electrical filter, and a third electrical amplifier that are electrically connected in sequence; Among them, the third photodetector is electrically connected to the first circulator, and the third electrical amplifier is electrically connected to the mixing circuit.

8. The optoelectronic oscillation system according to claim 5, characterized in that, The mixing circuit includes a mixer, a third electrical filter, a frequency divider, and a fourth electrical amplifier that are electrically connected in sequence; Among them, the mixer is electrically connected to the first photoelectric conversion circuit and the second photoelectric conversion circuit, and the fourth electrical amplifier is electrically connected to the first optical signal input terminal.

9. The optoelectronic oscillation system according to claim 1, characterized in that, The multiplexing module includes a wavelength division multiplexing demultiplexer, a dispersion - shifted fiber, a second circulator, an optical amplifier, and a second optical coupler that are electrically connected in sequence; Among them, the wavelength division multiplexing demultiplexer is electrically connected to the first optical signal input terminal and the second optical signal input terminal, and the second optical coupler is electrically connected to the second circulator and the first optical signal output terminal.

10. The optoelectronic oscillation system according to claim 1, characterized in that, The optoelectronic oscillation system further includes a polarization controller. The polarization controller is disposed in the closed loop and on the optical path of the closed loop. The polarization controller is used to adjust the polarization direction of the light beam; and / or, The optoelectronic oscillation system further includes a first laser and a second laser. The first laser is electrically connected to the first optical signal input terminal, and the second laser is electrically connected to the second optical signal input terminal.

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