TWI393363B - Directly modulated optical fiber optical catv transport systems without optical amplification - Google Patents

Description

Direct-modulated fiber-optic cable television transmission system constructed by remote injection technology

The invention relates to an optical cable TV transmission system (Optical CATV Transport Systems), in particular, a repeater is not required to use a half-band (Half-Split Band) remote external light source injection (Remote Light Injection) technology. Modulated fiber-optic cable transmission.

Achieving long-distance and high-quality transmission performance is a goal that fiber-optic cable TV has been pursuing. Currently, erbium-doped fiber amplifiers (EDFAs) can operate in long-distance transmission communication systems in the 1.55 micron band (C-band). Erbium-doped fiber amplifiers provide high power output but high power creates many nonlinear effects. In addition, the transmission of the fiber in the 1.55 micron band has a dispersion effect, and the combination of the two effects will seriously affect the quality of the fiber-optic cable TV signal. When two high power analog optical signals interact with the core material, a nonlinear effect is introduced, that is, a quadratic synthesis beat ratio (CSO) and a carrier three synthetic beat ratio (CTB). Recently, the direct modulation optical transmitter has gradually become an attractive modulation method. Because it has a simple design and cost advantage compared with the external modulation, the direct modulation optical transmitter has a Chirping Effect. , so the transmission distance will be limited by the dispersion factor. In order to improve this problem, some techniques must be applied to suppress dispersion and reduce nonlinear distortion.

In the past, external light source injection techniques were often used to improve transmission system performance, such as "digital optical fiber communication systems" and "microwave and fiber hybrid systems." The machine is mainly used to increase the frequency response of the laser diode and to increase the resonance frequency of the laser diode. When the distortion effect of the system can be effectively suppressed, the direct effect of the system is that the transmission quality of the system is better. Therefore, the external light source The direct modulation optical transmitter constructed by the injection technology can indeed increase the wide range of lightning RF and transmit more signals, reduce the influence of nonlinearity and reduce the scrambling effect of direct modulation, and increase the product of bandwidth and transmission distance. And increasing the dynamic range, the laser diode of the transmitting unit has the function of clamping.

The main object of the present invention is to utilize an external light source injection technique for a fiber-optic cable television transmission system that does not require an optical amplifier, and to improve some of the problems of the direct-modulation optical transmitter and for long-distance transmission. The main device of the invention comprises: two sets of direct modulation optical transmitter units, a 50 km single mode optical fiber transmission unit, an external injection light source unit, and a measurement parameter unit. We appropriately select two distributed feedback (DFB) laser diodes to generate an external injection source at the resonant frequency point. After 50 km of single-mode fiber, the external injection unit is used as a relay station to replace the optical amplifier. In this way, not only can the power be improved, but also the effective suppression of nonlinear effects. Finally, we improved the signal quality of the measurement unit, and used the cable TV parameter analyzer to observe the carrier-to-noise ratio (CNR) secondary synthesis beat ratio (CSO) and the carrier three-combination beat ratio (CTB).

The technical content, features and effects of the present invention will be apparent from the following drawings.

Figure 1 shows a direct-modulated fiber-optic cable TV transmission system constructed by half-band multiplex and remote source injection technology. The head end uses the multi-signal generator 101 to generate 77 random phase continuous optical carrier signals to simulate multi-channel signal injection of analog cable television channels (CH2-78; 6MHz/CH) and CH2~CH40 (40 channels of low frequency). The first DFB laser diode 102, and the channels CH41~CH78 (40 channels of high frequency), another set of multiple channels are directly injected into the second DFB laser diode 103. The center wavelengths of the DFB laser diodes of the two direct-modulation optical transmitters are 1532.64 nm and 1535.82 nm, respectively, and the optical modulation index of each channel is about ~3.5%, relative to the noise intensity (RIN; Relative Intensity Noise). It is approximately -170 dB/Hz. In the part injected at the far end, the external injection source wavelength (λ ₁ ') 303 is 1532.76 nm and (λ ₂ ') 304 is 1535.94 nm, and the injection power level is 8 dBm, by the individual optical circulators 303, 304. The 埠1 to 埠2 are coupled to the direct modulation signal transmitting unit 1 at the head end via the coupler 301, and the optical signal sent by the transmitting unit passes through the optical coupler 104 and the single mode fiber and then the optical circulators 303 and 304. 3 is output to the 2 x 1 optical coupler 302. The 3-turn optical circulators 303 and 304 have an insertion loss of ~0.8 dB and an isolation of >40 dB. The high isolation characteristics can effectively isolate the reflected light during injection. We use the spectrum analyzer 305 to measure the spectrum at point A. After the optical signal passes through the 1×2 splitter 401, it enters the optical bandpass filter 402, selects the most appropriate wavelength and filters out unnecessary noise, and finally the 40 channels of the low frequency and the high frequency are combined again. Two sets of optical receivers 403 convert the signals into electrical signals. Finally, the cable TV system (HP-8591C) 404 is used to measure the CNR/CSO/CTB parameter values of the cable television system.

We can clearly see the spectrum before and after the injection of the external light source using the spectrum analyzer 305 at point A of Figure 1 as shown in Figures 2(a) and (b). After the external light source is injected, the power value rises remarkably, and we choose the optimal injection point. The wavelength of the externally injected light source and the wavelength of the laser light source DFB 102, 103 are about 0.12 nm. The far-end external injection method proposed by the present invention can replace the general optical amplifier as a method of a relay station.

Figure 3 (a) shows the value of the CNR parameter, which can be improved to 50 dB after injection through an external light source of 8 dBm. The following equation represents the total equivalent carrier-to-noise ratio CNR at the output of the optical receiver: CNR _RIN is the carrier-to-noise ratio of relative-intensity noise, CNR _th is the carrier-to-noise ratio of thermal noise, and CNR _shot is the carrier-to-noise ratio of quantum noise. This method covers the noise of the whole system. When the external light is injected, the power value accepted by the receiver is effectively improved. Therefore, the comparison of the noise and carrier power values for CNR _th and CNR _shot can be relative to a higher CNR value.

The values of the CSO and CTB parameters can be shown in Figure 3(b)(c). After the external external light injection, we can get the individual values of CSO/CTB of about 65/63 dB, which is about 5 to 6 dB improvement. The distortion representation of its CSO/CTB can be described by the following mathematical formula: Where m is the optical modulation index, D is the dispersion coefficient, λ _c optical carrier wavelength, L fiber length, f is the frequency of RF, Δ τ (=| D |. L .△ λ ) is the fiber dispersion (△ λ is light The spectral linewidth of the signal), N _CSO and N _CTB are the product terms of the carrier synthesis second beat ratio and the combined three beat ratio, respectively. Using frequency division multiplexing technology, the number of distortions of CSO and CTB generated in each frequency band can be almost completely eliminated, and the technique of using remote source injection can reduce the scrambling effect of the laser diode. It can reduce the influence of fiber dispersion on the system, and can significantly improve the CSO and CTB parameter values. If the system adopts half-frequency multiplex technology, the N _CSO and N _CTB (the number of CSO and CTB distortions) will be greatly reduced, and the CSO and CTB of the system will be greatly improved. Therefore, the relay station constructed by the external light source injection technology of the present invention can achieve the effect of reducing the nonlinearity, reducing the scrambling effect of the direct modulation, and increasing the power value instead of using the general amplifier as a relay station. However, the above is only the preferred embodiment of the present invention, and the scope of the present invention is not limited thereto, that is, the simple and equivalent changes made by the scope of the invention and the description of the invention are Modifications are still within the scope of the invention.

1. . . Direct modulation signal transmitting unit

101. . . Cable Multichannel Signal Generator (MATRIX SX-16)

102. . . Distributed Feedback (DFB) Laser Diode - Wavelength 153264 nm

103. . . Distributed Feedback (DFB) Laser Diode - Wavelength 153582 nm

104. . . 2×1 optical coupler (Coupler)

2. . . 50 km single mode fiber transmission unit

3. . . External light source injection unit

301. . . 1×2 splitter

302. . . 2×1 optical coupler (Coupler)

303. . . Optical Circulator-1 (Optical Circulator)

304. . . Optical Circulator-2 (Optical Circulator)

305. . . Optical Spectrum Analyzer

4. . . Acceptance and measurement unit

401. . . 1×2 splitter

402. . . 2 optical bandpass filters

403. . . 2 optical receivers

404. . . Cable TV Parameter Analyzer (HP-8591C)

Figure 1: Architecture diagram of the direct-modulation cable TV system for remote external injection. Figure 2 (a): Output spectrum without injection of light source at point A. Figure 2 (b): Output spectrum at point A via injection source. (a): Carrier-to-noise ratio (CNR) parameter value of cable TV Figure 3 (b): Secondary synthesis beat ratio (CSO) parameter value of cable TV Figure 3 (c): Three synthetic wave difference of cable TV Ratio (CTB) parameter value

1. . . Direct modulation signal transmitting unit

101. . . Cable Multichannel Signal Generator (MATRIX SX-16)

102. . . Distributed Feedback (DFB) Laser Diode - Wavelength 153264 nm

103. . . Distributed Feedback (DFB) Laser Diode - Wavelength 153582 nm

104. . . 2×1 optical coupler (Coupler)

2. . . 50 km single mode fiber transmission unit

3. . . External light source injection unit

301. . . 1×2 splitter

302. . . 2×1 optical coupler (Coupler)

303. . . Optical Circulator-1 (Optical Circulator)

304. . . Optical Circulator-2 (Optical...Circulator)

305. . . Optical Spectrum Analyzer

4. . . Receiving and measuring unit

401. . . 1×2 splitter

402. . . 2 optical bandpass filters

403. . . 2 optical receivers

404. . . Cable TV Parameter Analyzer (HP-8591C)

Claims (4)

A sub-band direct modulation optical fiber cable transmission system constructed by a remote external light source injection technology, comprising: a constant modulation signal transmitting unit, comprising a cable television multi-channel signal generator, two sets of laser diodes, and one light a coupler and an optical spectrum analyzer; and a fifty-kilometer standard single-mode fiber transmission link; and an external light source injection unit including two optical circulators, two couplers and two sets of external injection light sources; and a receiving And a measuring unit, comprising two band pass filters, two light receivers, a two-component optical coupler and a cable TV parameter analyzer; the direct-modulation signal transmitting unit at the head end uses the multi-channel signal generator to take two sets of signals The frequencies are respectively carried on two sets of laser light, transmitted through a 50-kilometer standard single-mode fiber link, and then injected into the two sets of light sources by an external light source injection unit. The external light source injection technology can greatly increase the frequency of the laser diode. Response value, and can effectively reduce relative intensity (Rinative Intensity Noise; RIN), further, this can not only effectively compensate for long distances The loss of optical power after the transmission can also avoid unnecessary spontaneous emission amplifying noise (ASE) caused by the use of the optical amplifier, so that the remote external light source injection system replaces the optical amplifier, so It is applied to the analog fiber-optic cable TV system, which will further improve the signal quality of the system. Finally, it is filtered by the band-pass filter of the receiving and measuring unit, and then the parameters are observed by the cable TV parameter analyzer. As described in claim 1 of the patent application, a remote external laser source injection technology is constructed. A sub-band direct-modulation fiber-optic cable television transmission system, the laser of the direct-modulation signal transmitting unit is a distributed feedback (DFB) laser; the laser of the direct-modulation signal transmitting unit, the laser can be a Fabry-Perot laser diode And vertical cavity surface-emitting laser (VCSEL). The sub-band direct-modulation fiber-optic cable television transmission system constructed by the remote external light source injection technology as described in claim 1, wherein the external light source injection unit and the optical circulator have an insertion loss of ~0.8 dB and isolation > 40 dB, its external source can be distributed feedback (DFB) laser, Fabry-Perot laser diode and vertical cavity surface-emitting laser (VCSEL). The external light source injection unit is located after the transmission fiber, as described in claim 1, in the sub-band direct-modulation fiber-optic cable television transmission system constructed by the remote external light source injection technology.