CN107490918A - A kind of ultra-low noise amplifier in optical frequency standard transmission - Google Patents

Abstract

An ultra-low-noise optical amplifier in optical frequency standard transmission, including a PID circuit board, a phase detector, a photodetector, an electro-optic modulator, an acousto-optic modulator, a 1×2 optical coupler, a circulator, a slave laser, The voltage-controlled oscillator, low-pass filter, band-pass filter and radio frequency amplifier are based on the injection locking principle of the laser, and a two-stage PID feedback loop is designed. The first feedback loop of the present invention ensures the large-scale real-time tracking and automatic locking of the injected optical frequency and the optical frequency of the slave laser. Ultra-low noise amplification of transmitted optical frequencies. The transmitted optical frequency reference signal can be reproduced with high precision at the remote end, and the whole system simultaneously realizes narrow bandwidth, high gain, and ultra-low noise, and is suitable for optical clock comparison, optical frequency standard transmission, etc. Reduce the complexity of the transmission system and improve the transmission accuracy of the system.

Description

An Ultra-Low Noise Amplifier in Transmission of Optical Frequency Standards

technical field

The invention relates to high-precision optical fiber optical frequency transmission, especially an ultra-low noise amplifier in a long-distance optical fiber optical frequency transmission system. The main purpose is to compensate the loss of signal optical power in long-distance optical fiber transmission, so The high-precision recovery of the frequency reference signal at the local end can be applied to optical clock comparison, optical frequency transmission and other fields.

Background technique

High-precision frequency standards and their transmission and comparison are common basic issues and core key technologies in major infrastructure construction and cutting-edge scientific research such as precise punctuality and timing, precise navigation and positioning, radar networking, deep space exploration, precision metrology and measurement, etc. one. In terms of standards, with the continuous advancement of modern clock technology, the second stability of the ground hydrogen atomic clock used by GPS/Beidou has reached the order of 1×10 ^-13 , and at the same time, the frequency uncertainty of the cesium atomic fountain clock established as the current international time-frequency standard Can be less than 1×10 ^-15 . In recent years, the frequency stability and uncertainty of optical clocks, which are expected to become a new generation of time-frequency standards, have reached the order of 1×10 ^-18 . In order to transmit such a high-precision optical frequency standard, the traditional satellite transmission method is far from meeting the demand, so the high-precision optical frequency transmission technology based on optical fiber has emerged as the times require. It has been proved that the optical fiber-based frequency transmission technology can meet the transmission requirements of optical frequency standards, but this technology needs to compensate for the noise introduced by the optical fiber link during the transmission process It is also extremely important to compensate for signal power loss and improve the signal-to-noise ratio of the detection signal.

In order to compensate the loss of signal power, various solutions have been proposed. Prior art one: Grosche G, Terra O, Predehl K, et al.Optical frequency transfer via 146km fiberlink with 10 ^-19 relative accuracy[J].Optics letters,2009,34(15):2270-2272, by using two-way Erbium-doped fiber amplifier (EDFA) to compensate for the lost signal power. However, due to the large amplification bandwidth of the bidirectional EDFA and any optical isolator cannot be used for bidirectional work, the end face reflection and Rayleigh scattered light in the link will generate unnecessary optical feedback to the EDFA and trigger the stimulated effect of the EDFA. The gain of EDFA is limited (usually 18-20dB), and the stability of optical frequency transmission will be degraded.

Prior art 2: Terra O, Grosche G, Schnatz H. Brillouin amplification inphase coherent transfer of optical frequencies over 480km fiber[J]. Opticsexpress, 2010, 18(15): 16102-16111, proposed the method of optical fiber Brillouin amplification plan. The advantage of the fiber Brillouin amplifier is that the amplification bandwidth of the signal is small (usually less than 30MHz). In a typical optical frequency transmission scenario, the optical frequency signal is transmitted bidirectionally in the optical fiber and the optical frequency in the two directions differs by tens of MHz. In this way, when the fiber Brillouin amplifier is used, the optical frequencies in the two directions can be amplified separately, thus avoiding the influence of the Rayleigh backscattering signal amplification on the amplifier gain and transmission stability. However, due to the small gain bandwidth of the fiber Brillouin amplifier, in order to ensure the amplification effect, the optical frequency of the Brillouin pump laser usually needs to be locked on the optical frequency of the amplified optical signal, which increases the complexity of the system, and due to It still needs to use optical pumping, so it will also degrade the performance of the delivery system to a certain extent.

Contents of the invention

The invention proposes an ultra-low-noise amplifier in the transmission of optical frequency standards. On the basis of realizing narrow-bandwidth and high-gain optical amplification from laser injection locking, the amplifier designs a two-stage PID feedback loop. The first-stage feedback loop It ensures the large-scale real-time tracking and automatic locking of the injected optical frequency and the optical frequency of the slave laser; the second-level feedback loop realizes the phase locking of the injected optical frequency and the output optical frequency of the laser, thereby achieving ultra-low noise of the transmitted optical frequency enlarge.

Technical solution of the present invention is as follows:

An ultra-low noise amplifier for optical frequency standard transmission, which is characterized in that it includes a first 1×2 optical coupler, an electro-optical modulator, an acousto-optic modulator, a circulator, a slave laser, and a second 1×2 optical coupler , the third 1×2 optocoupler, the fourth 1×2 optocoupler, radio frequency bandpass filter, first radio frequency amplifier, radio frequency reference signal circuit, first phase detector, first PID circuit board, radio frequency low pass filter, a second radio frequency amplifier, a second phase detector, a second PID circuit board, a first photodetector, a second photodetector and a voltage controlled oscillator;

The radio frequency reference signal circuit sends three frequency reference signals: the first local frequency reference signal enters the reference frequency input port of the first phase detector, and the second frequency reference signal enters the reference frequency input port of the second phase detector , the third frequency reference signal enters the microwave input port of the electro-optic modulator, the optical frequency reference signal is input through the input port of the first 1×2 optical coupler, and passes through the first 1×2 optical coupler The optical frequency standard signal output by an output port is input through the optical input port of the electro-optic modulator, modulated by the electro-optic modulator to form a laser with modulation sidebands, and connected to the optical input end of the acousto-optic modulator through the output port of the electro-optic modulator , the output port of the acousto-optic modulator is connected to the first port of the circulator, and the output light of the second port of the circulator is injected into the optical port of the slave laser, and output from the optical port after being amplified by the slave laser, and returned to the second port of the circulator port, and then output to the second 1×2 optical coupler from the third port of the circulator, the input light is divided into two paths through the second 1×2 optical coupler, and one path passes through the second 1×2 optical coupler One output terminal output, incident to the first photodetector to generate radio frequency signals, then input to the signal frequency input terminal of the first phase detector through the radio frequency bandpass filter and radio frequency amplifier, and compare with the local frequency reference signal to generate an error After the signal is output by the output terminal of the phase detector, the control signal output by the PID circuit is input through the current modulation port of the slave laser, and the current modulation of the slave laser is performed; the other is output through the second output terminal of the second 1×2 optical coupler, The third 1×2 optical coupler is divided into two routes, one of which is output from the first output port of the third 1×2 optical coupler as the amplified optical frequency output and injected into the next optical fiber link or provided to the user In use, another channel is output from the second output port of the third 1×2 optical coupler, and the optical frequency reference signal output through the second output port of the first 1×2 optical coupler is in the fourth 1× 2. After combining the beams on the optical coupler, the output terminal of the fourth 1×2 optical coupler is output to the beat frequency of the second photodetector to generate a radio frequency signal, and the radio frequency signal is passed through the radio frequency low-pass filter and the second radio frequency amplifier in turn. The signal frequency input port of the second phase detector is input into the second phase detector, and after comparing with the second frequency reference signal, the error signal is output to the second PID circuit through the output port of the second phase detector, and the The second PID circuit outputs a control signal to act on the voltage-controlled oscillator, and the voltage-controlled oscillator outputs a radio frequency signal to act on the acousto-optic modulator, thereby driving the acousto-optic modulator to perform frequency modulation, and completing the second-stage PID loop feedback.

The injection locking process of the slave laser is locked by a two-stage PID feedback loop. The first stage is used for automatic wide-range frequency locking of the injected optical frequency and the optical frequency of the slave laser, and the second stage is used for injecting the optical frequency and the slave laser. Optical Frequency Phase Locking

Compared with prior art, the beneficial effect of the present invention is:

1) Based on the injection locking principle of the slave laser, in the locked state, the output characteristics of the slave laser are consistent with the characteristics of the injected light frequency, and when weak light is injected, the locked bandwidth of the injected light frequency and the slave laser light frequency is small, Therefore, when it is used as a signal regeneration amplification, it can simultaneously meet the two characteristics of narrow bandwidth (less than 100MHz) and high gain (40-50dB). Compared with EDFA, its gain bandwidth is small and its gain is high; compared with fiber Brillouin amplifier, it does not require optical pumping, and its structure is simple and easy to use.

2) A two-stage PID feedback loop is designed. The first-stage feedback loop ensures the large-scale real-time tracking and automatic locking of the injected optical frequency and the optical frequency of the slave laser; the second-stage feedback loop amplifies the optical frequency from the laser The phase of the signal is locked on the phase of the injected optical frequency, which eliminates the additional phase fluctuation introduced by the injection locking process (the additional frequency fluctuation is less than 1mHz). Compared with the above two optical amplification methods, the ultra-low noise of the transmitted optical frequency is realized. enlarge.

Description of drawings

Fig. 1 is the structure schematic diagram of ultra-low noise amplifier in the transmission of optical frequency standard of the present invention;

detailed description

The present invention will be further described below in conjunction with the embodiments and accompanying drawings, but the protection scope of the present invention should not be limited thereby.

Please refer to Fig. 1, Fig. 1 is the structure diagram of the ultra-low noise amplifier in the transmission of the optical frequency standard of the present invention, as can be seen from the figure, a kind of ultra-low noise amplifier in the transmission of the optical frequency standard includes the first 1 * 2 optical coupler 11 , an electro-optic modulator 12, an acousto-optic modulator 13, a circulator 14, a slave laser 15, a second 1×2 optical coupler 16, a third 1×2 optical coupler 17, a fourth 1×2 optical coupler 18, RF bandpass filter 19, first RF amplifier 20, RF reference signal circuit 21, first phase detector 22, first PID circuit board 23, low-pass filter 24, second RF amplifier 25, second phase detector 26 . The second PID circuit board 27 , the first photodetector 28 , the second photodetector 29 and the voltage controlled oscillator 30 .

The radio frequency reference signal circuit 21 described in Fig. 1 sends out three frequency reference signals: the first local frequency reference signal 211 enters the frequency reference input port 221 of the first phase detector 22 in the feedback loop one, and the second frequency reference The signal 212 enters the frequency reference input port 261 of the second phase detector 26 in the second feedback loop, and the third frequency reference signal enters the microwave input port 122 of the electro-optical modulator 12 .

The above radio frequency reference signal circuit has at least three radio frequency signal outputs. In an embodiment of the present invention, ports 211 and 212 output 1GHz radio frequency signals, and port 213 outputs 80MHz radio frequency signals. If the phase detection input frequency range of the phase detector is increased, the RF frequencies output from ports 211 and 212 can be increased. In practice, this RF reference signal circuit can be locked to a standard RF source such as a rubidium clock, thus improving the performance of both feedback loops.

In Fig. 1, the optical frequency reference signal is input through the input terminal 111 of the first 1×2 optical coupler 11, and then output from the output terminal 112 of the first 1×2 optical coupler 11, and then input to the electro-optical modulator 12 The optical input end 121 is modulated to form laser light with modulation sidebands, the optical output end 123 of the electro-optic modulator 12 is connected to the optical input end 131 of the acousto-optic modulator 13, and the optical output port 133 of the acousto-optic modulator 13 Connect the first port 141 of the circulator 14, the output light of the second port 142 of the circulator 14 is injected into the optical port 152 of the slave laser 15, and then the optical signal amplified from the laser 15 returns to the port 152 and enters the circulator 14. The second port 142 is output from the third port 143 of the circulator 14 .

The output of the third port 143 of the above-mentioned circulator 14 is the amplified optical frequency signal. In one embodiment of the present invention, the power of the optical signal injected into the slave laser 15 is <1uw, and the output of the slave laser 15 is 20mw, so the amplification gain is >40dB. The actual amplification gain is determined by the output power of the slave laser 15 and the injected light power, and the injected light power can be adjusted by adjusting the coupling ratio of the first 1×2 optical coupler 11, in one embodiment of the present invention, 1× 2 The coupling ratio of the optical coupler 11 is 2:8.

The amplified optical signal output by the third port 143 of the circulator 14 is input to the input port 161 of the second 1×2 optical coupler 16, and the second 1×2 optical coupler 16 divides the input light into two, and one channel of light Input to the first photodetector 28 in the feedback loop 1 through the first output port 162 for direct detection, and the detected radio frequency signal is sequentially processed by the radio frequency bandpass filter 19 and the radio frequency amplifier 20 and then input to the first phase detector 22 The signal frequency input port 222 is compared with the aforementioned radio frequency reference signal 221 to generate an error signal, which is output to the input port 231 of the PID circuit 23 by the output port 223 of the first phase detector 22, and passes through the proportional-integral-differential signal After processing, the control signal is output from the port 232 to the current modulation port 151 of the slave laser 15, so as to perform frequency modulation on the slave laser.

The above process realizes the feedback loop one. In one embodiment of the present invention, the modulation sideband of the first photodetector 28 is 1 GHz, when the injection locked state is maintained, no error signal is generated, and once the injected optical frequency and the optical frequency of the slave laser 15 deviate, Then the first phase detector 22 outputs an error signal, and after being processed by the PID circuit 23, a control signal is generated to modulate the driving current of the slave laser 15, so that the optical frequency of the slave laser 15 moves until the injected optical frequency and the optical frequency of the slave laser 15 remain consistent. The error signal is zero during locking, at which point the optical frequency of the slave laser 15 stops moving. This feedback ensures the large-scale real-time tracking and automatic locking of the injected optical frequency and the optical frequency of the slave laser.

The light output by the second output port 163 of the second 1×2 optical coupler 16 in FIG. In one optical fiber link or provided to the user, the output of the other 173 ports and the output of the second output port 113 of the first 1×2 optical coupler 11 are combined on the fourth 1×2 optical coupler 18, the first The output ports 183 of the four 1×2 optical couplers 18 output optical signals to the second photodetector 29 of the feedback loop 2, and the beat frequency signals obtained are input to the first radio frequency low-pass filter 24 and radio frequency amplifier 25 successively. In the signal frequency input port 262 of the second phase detector 26, the error signal after comparison with the aforementioned reference signal 261 is input to the input port 271 of the PID circuit 27 via the output port 263 of the second phase detector 26, and is proportional-integrated - After differential circuit signal processing, the output control signal from port 272 acts on the frequency modulation port 301 of the voltage-controlled oscillator 30, and the output of the voltage-controlled oscillator acts on the radio frequency input port 132 of the acousto-optic modulator 13 to drive the acousto-optic modulator 13 for frequency modulation.

The above process realizes the second feedback loop. When the difference between the injected optical frequency and the optical frequency of the slave laser is within the injection locking bandwidth, the optical frequency of the slave laser is locked on the optical frequency of the injected signal, but their relative phase expressions are as follows:

It can be seen from the above formula that even if the feedback loop 1 is implemented so that the optical frequency of the slave laser is locked to the injected optical frequency, the relative phase between them will still change with the changes of many parameters, so that the amplified signal introduce additional phase noise. In an implementation example of the present invention, an AOM with a center frequency of 80 MHz is used as a phase-locking implementation device to phase-lock the injection optical frequency and the slave laser optical frequency, ensuring that the additional frequency fluctuation during the injection locking process is less than 1 mHz. In this way, ultra-low noise amplification of the transmitted optical frequency is realized.

Claims (2)

1. the ultra-low noise amplifier in a kind of optical frequency standard transmission, it is characterised in that including the one 1 × 2nd photo-coupler (11), electrooptic modulator (12), acousto-optic modulator (13), circulator (14), from laser (15), the 21 × 2nd photo-coupler (16), the 31 × 2nd photo-coupler (17), the 41 × 2nd photo-coupler (18), radio frequency band filter (19), the first radio frequency are put Big device (20), radio frequency reference signal circuit (21), the first phase discriminator (22), the first PID circuit boards (23), radio frequency low pass filter (24), the second radio frequency amplifier (25), the second phase discriminator (26), the 2nd PID circuit boards (27), the first photodetector (28), Second photodetector (29) and voltage controlled oscillator (30)；

Described radio frequency reference signal circuit (21) sends three road frequency reference signals：First via transmitting local frequency reference signal (211) the reference frequency input port (221) of the first phase discriminator (22) is entered, the second road frequency reference signal (212) enters the The reference frequency input port (261) of two phase discriminators (26), the 3rd road frequency reference signal enter described electrooptic modulator (12) microwave input port (122)；

After the input (111) of the one 1 × 2nd described photo-coupler (11) receives optical frequency reference signal, by the 1st × The first output port (112) output optics frequency standard signal of 2 photo-couplers (11), and through the optics of electrooptic modulator (12) Input port (121) input forms swashing with modulation sideband, to electrooptic modulator (12) by electrooptic modulator (12) modulation After light, the optical input port (131) of output end (123) and acousto-optic modulator (13) through the electrooptic modulator (12) is inputted to sound Optical modulator (13), the first port (141) of delivery outlet (133) the connection circulator (14) of the acousto-optic modulator (13), annular The output light of the second port (142) of device (14) passes through optical port (152) injection from laser (15) from laser (15), passes through Should be after laser (15) amplification through should be exported from the optical ports (152) of laser (15), the through circulator (14) successively The input (161) of Two-port netwerk (142), the 3rd port (143) and the 21 × 2nd photo-coupler (16) inputs the 21 × 2nd optocoupler Input light is divided into two tunnels by clutch (16), the 21 × 2nd photo-coupler (16), all the way through the 21 × 2nd photo-coupler (16) First output end (162) export, incide the first photodetector (28) produce radiofrequency signal after successively through radio frequency bandpass filtering The signal frequency input (222) of device (19), radio frequency amplifier (20) and the first phase discriminator (22) inputs the first phase discriminator (22), And compare and produced after error signal by the output end of the first phase discriminator (22) with described transmitting local frequency reference signal (211) (223) export, pass through from the current-modulation mouth (151) of laser (15) and input through PID circuits (23) output control signal, to from swashing Light device (15) carries out current-modulation；Second output end (163) output of the another way through the 21 × 2nd photo-coupler (16), through the 3rd 1 × 2 photo-coupler (17) is further divided into two-way, wherein defeated from the first output end (172) of the 31 × 2nd photo-coupler (17) all the way Optical frequency output injection after going out as amplification in fiber link or is supplied to user to use all the way under, in addition all the way from the 3rd 1 The second output end (173) output of × 2 photo-couplers (17), and the second output end by the one 1 × 2nd photo-coupler (11) After the optical frequency reference signal of mouth (113) output closes beam on the 41 × 2nd described photo-coupler (18), through the 41 × 2nd optocoupler The output end (183) of clutch (18) is output to the second photodetector (29) beat frequency and produces radiofrequency signal, and the radiofrequency signal is successively After radio frequency low pass filter (24) and the second radio frequency amplifier (25), pass through the signal frequency input of the second phase discriminator (26) Mouth (262) is input in the second phase discriminator (26), and is passed through after being compared with the second road frequency reference signal (212) by the second phase discriminator (26) output port (263) output error signal is to the 2nd PID circuits (27), through the 2nd PID circuits (27) output control Signal function acts on acousto-optic modulator (13) in voltage controlled oscillator (30), the voltage controlled oscillator (30) output radiofrequency signal, from And drive acousto-optic modulator (13) to carry out frequency modulation(PFM), complete second level PID loop feedback.

2. the ultra-low noise amplifier in optical frequency standard transmission according to claim 1, it is characterised in that described Locked from the injection locking process of laser (15) by two-stage PID/feedback loop, the first order be used for inject optical frequency and from swash The automatic wide range of frequencies locking of light device optical frequency, the second level are used to inject optical frequency and the PGC demodulation from laser optical frequency.