JP2018179662A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow transmission of light to be a highly accurate frequency reference without requiring an accurate RF frequency reference.SOLUTION: A return light generation unit 106 generates a return light source light obtained by shifting the frequency of a stabilization light source light transmitted from a transmission source 101 through an optical fiber 102. The return light generation unit 106 generates a return light source light obtained by shifting the frequency of the stabilization light source light so that the amount of shift from the reference frequency of the stabilization light source light transmitted from the transmission source 101 to a transmission destination 103 and the amount of shift from the reference frequency of the return light source light transmitted from the transmission destination 103 to the transmission source 101 are symmetrical (positive and negative) frequencies with respect to the reference frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission system that transmits an optical frequency reference with high accuracy using an optical fiber.

  Optical fiber transmission technology of ultra high precision optical frequency reference means technology to transmit ultra-high precision stabilized frequency (time) reference to remote place via optical fiber without losing precision ( Non-Patent Document 1). In recent years, research and development of optical clocks based on optical frequency (several hundred THz) has rapidly progressed, and shows performance far exceeding the accuracy of the cesium (Cs) atomic clock which is the current frequency standard. As a result, attention has been focused on ultra-high-precision optical frequency reference fiber transmission technology that enables transmission to a remote place without degrading the accuracy of the optical frequency reference and sharing the ultra-high-precision optical frequency reference.

  In the Cs atomic clock, the reference frequency is in the RF frequency range, whereas the reference frequency of the optical clock is in the light frequency range. For this reason, when the accuracy is compared using the index of the ratio of the frequency uncertainty ΔΔ to the frequency ν (Δν / ν), the optical frequency (100 THz) whose frequency is three to five orders of magnitude higher than the RF frequency ((10 GHz) The accuracy of the optical clock used is improved by three to five digits.

At present, the highest precision in the Cs atomic clock realizes an uncertainty of 1 × 10 ^-16 , while the optical lattice clock, which is an optical clock considered to be the most powerful system, has a mass number of 87. Strontium ( ⁸⁷ Sr) has been used to achieve an uncertainty of 2 × 10 ⁻¹⁸ , and a level of uncertainty of 10 ⁻¹⁹ is just around the corner. Therefore, in the future, it is extremely likely that the frequency reference will replace the RF region of the Cs atomic clock to the light region of the optical lattice clock.

In addition, since an optical lattice clock with such frequency accuracy of ^10-18 level uncertainty is detected as a difference in frequency due to the general relativistic effect, geophysics and geodesy as a gravity sensor Application to science is expected. For this purpose, it is essential not only to simply share the ultra-high-precision optical frequency reference between two points, but also to deploy the ultra-high-precision optical frequency reference fiber network shared between multiple points.

  As shown in FIG. 4, the configuration of a typical ultra-high precision optical frequency reference fiber transmission is composed of a transmitter unit 301, a relay unit 302, and a receiver unit 303. The transmission unit 301 includes an ultra high precision optical frequency reference (for example, an optical lattice clock) 304 and a transmission device 305. The relay unit 302 includes a relay apparatus 306. The receiving unit 303 includes a receiving device 307. The respective devices are connected in series by an optical fiber 308.

  The basic functions of each optical transmission device can be classified into the following two. One function is to reproduce (transfer) the ultra-high-precision optical frequency reference 304 into the transmission frequency-variable narrow line-width laser light of the transmission device 305 without degrading the accuracy.

  For this purpose, a beat signal corresponding to a difference frequency is detected by interfering two laser beams of an ultra-high precision optical frequency reference 304 and a narrow line-width laser for transmission, and this beat signal is detected as a high-precision RF frequency. By synchronizing the phase to the reference, the ultra-high-precision optical frequency reference 304 is regenerated into the transmission laser light (offset synchronization).

  In general, the ultra-high precision optical frequency reference is different from the communication wavelength band suitable for fiber transmission. For example, in the case of the Sr light lattice clock, the wavelength is 698 nm, and in the case of the Yb light lattice clock, the wavelength is 578 nm. Therefore, a beat signal with the transmission laser light is obtained by using the optical frequency comb and the second harmonic generation. The offset synchronization is also used at the relay station, and the ultra-high-precision optical frequency reference sent from the relay station at the former stage is regenerated to the transmission laser at the relay station and transmitted to the latter stage.

  Another function is to compensate for noise originating from the environment of the optical fiber 308 connecting the transmitting device 305 and the relay device 306, and between each relay device 306, and between the relay device 306 and the receiving device 307. . The noise derived from the environment is, for example, the noise derived from the installation environment of the optical fiber 308, such as heat and vibration.

  In the function of compensating for the noise of the optical fiber 308, for example, the transmission light before entering the optical fiber is frequency shifted by a frequency shifter such as an acousto-optic element (AOM), is incident on the optical fiber 308, and is folded back by the relay device 306. The beat signal is detected by interference between the return light returned to the transmitter 305 and the transmission light before the frequency shift. A noise component derived from the environment of the optical fiber 308 is superimposed on the beat signal as it propagates in the optical fiber 308. By comparing the phase of the beat signal with a high precision RF frequency reference, the fiber is obtained. Extract the noise.

  The optical frequency reference is transmitted to the repeater without degrading the accuracy by applying frequency fluctuation to the transmission light in advance to compensate for frequency fluctuation derived from fiber noise by the frequency shifter (fiber noise (Fiber noise compensation).

As described above, by stabilizing the light beat signal on the RF frequency reference, the frequency accuracy of the regenerated or noise-compensated light is ultimately limited by the accuracy of the RF frequency used for stabilization. It will be. For example, if the frequency of transmitted light is 200 THz (equivalent to a wavelength of 1500 nm known as a communication wavelength band), in order to maintain the frequency accuracy of 10 ^-18 level uncertainty, it is necessary to keep the frequency uncertainty within 0.2 mHz. There is.

In order to obtain this accuracy, an RF frequency accuracy within 10 ⁻¹¹ uncertainty is required when using a 10 MHz RF frequency reference. To do this, either prepare a hydrogen-masser-class single oscillator, or use a rubidium (Rb) oscillator, etc. while calibrating with a Cs atomic clock using a global positioning system, or use these high precision RF frequencies. Equipment is required to transmit the reference over an optical fiber without degrading its accuracy.

L.-S. Ma et al., "Delivering the same optical frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path", Optical Letters, vol. 19, no. 21, pp. 1777-1779, 1994. N. Chiodo et al., "Cascaded optical fiber link using the internet network for remote clocks comparison", OPTICS EXPRESS, vol. 23, no. 26, pp. 33927-33937, 2015.

  However, such a network configuration has the fundamental problem that the frequency references are overlapping in the sense that two of the optical frequency reference and the RF frequency reference are required. In addition, it is desirable that the system configuration of separately installing or distributing a high precision RF frequency reference is unnecessary because the cost of this part is required. In particular, when considering an ultra-high precision optical frequency reference fiber network connecting multiple points, it is not practical to prepare a hydrogen maser class high precision RF frequency reference at every point.

  In addition, it is also difficult at present to use a GPS calibration oscillator without a Global Positioning System (GPS) antenna generally being installed at a telephone station which is one of the leading candidates as a relay point.

  In consideration of the above-mentioned points, the inventors examined a fiber transmission method of a remote high-accuracy optical frequency reference that can be realized without requiring a high-precision RF frequency reference at various places.

  In the optical transmission system described with reference to FIG. 4, a transmission method which does not require a high precision RF frequency reference has been proposed in the past, focusing only on the fiber transmission unit. For example, there is a fiber noise compensation method shown in Non-Patent Document 1.

  Non-patent document 1 of FIG. As shown in FIG. 1, in the transmission section, the frequency shifter (AOM 2) is driven by a signal obtained by dividing the frequency variable RF oscillator (VCXO: Voltage-Controlled Crystal Oscillator) into 1/2, and thereby the frequency A part of the shifted transmission light is extracted as a reference light and then enters a fiber [FIG. 5, (+80)]. Here, +80 indicates that the light frequency is shifted by +80 MHz by AOM2. The value of +80 MHz is an example.

  In the reception unit, the frequency of the transmitted light emitted is shifted by another AOM 1 [FIG. 5, (−80)], and a part is returned by the mirror. The returned light is again frequency-shifted by the AOM 1 [FIG. 5, (-80)], input to the fiber, superimposed on the reference light in the transmitter, and input to the detector.

  Since a frequency difference twice as high as the driving frequency (80 MHz) of the AOM 1 in the receiving unit occurs in the light and reference light returned from the receiving unit, this is observed as a beat signal. The frequency shift by the AOM 1 distinguishes the light that has returned to the receiver from the light returned from scattering and reflection in the fiber. The beat signal includes frequency noise that the transmission light receives during propagation back and forth in the fiber and fluctuation of the RF frequency of the receiving unit [FIG. 5, (−80b)].

  Therefore, by phase-locking VCXO to this beat frequency, a half of the fiber noise is added in antiphase to the drive frequency of AOM 2 to compensate for the fiber noise. In this technique, no RF frequency reference is used in the transmission unit, fluctuations in the RF frequency of the reception unit are compensated together with the fiber noise, and in the transmitted light of the mirror of the reception unit, the optical frequency of the transmitted light in the transmission unit is regenerated .

  However, although the technique of Non-Patent Document 1 is simple, it has a drawback that it is susceptible to noise in the transmission unit. First, since the path from the frequency shifter to the interferometer is not included in the interferometer in the transmission unit, the noise received between these is not detected by the interferometer and is not compensated.

  Since it is expected that fiber transmission equipment will be made into all fibers in the future, the frequency shifter should be placed after the interferometers from the viewpoint that it is important to make the distance between the noise compensation interferometers as short as possible. Is desirable. In addition, FIG. The configuration shown in 1 is indistinguishable from the reference light because the return light from the fiber enters the interferometer directly without being frequency shifted. In long-haul fiber transmission, this can be noise, as there is relatively large return light due to light scattering within the fiber and reflections from the splice points.

  In the current mainstream, the frequency shifter of the transmission unit is disposed downstream of the interferometer. In this configuration, a beat signal including a frequency shift twice that of a frequency shifter used for noise compensation is detected by an interferometer and phase comparison with an RF frequency reference is made. By using this signal to feed back to the VCXO, in addition to the noise of the fiber, the noise of the VCXO itself and the noise of the circuit are compensated, so that more accurate fiber transmission can be realized. Also, since there is a frequency shifter between the interferometer and the fiber, the return light of the fiber is frequency shifted by the frequency shifter and can be distinguished from the reference light.

  Furthermore, in recent years, a configuration has been increasing in which a repeater laser is installed in place of a frequency shifter in the reception unit. By offset-synchronizing the repeater laser with the received light, it is possible to provide a frequency shift instead of the frequency shifter, and at the same time, it is possible to amplify and reverse optical power attenuated by fiber transmission.

  In the configuration in which the frequency shifter of the transmission unit is included in the interferometer and the repeater is installed in the reception unit, highly accurate noise compensation can be realized, but the frequency shift of transmission light becomes complicated. In order to avoid this, a multi-step fiber transmission scheme has been proposed in which a high precision RF frequency reference is installed only at one transmitting station, and the high precision RF frequency reference is not required for the relay station (non- Patent Document 2, see FIG. 3).

  In this technology, the frequency shift in the Nth relay station and the frequency shift in the N + 1th relay station are inverted and sent, thereby compensating for the fluctuation of the RF frequency of the relay station on the transmission side (see FIG. 6). As a result, fiber distribution independent of the accuracy of the RF frequency reference of the relay station can be realized, and the installation of the high precision RF frequency reference becomes unnecessary at each station of the relay station and the receiving station excluding the transmitting station.

  However, in the prior art described above, the transmitting station necessarily requires a high precision RF frequency reference, and "the high precision RF frequency reference is also required for fiber distribution of the ultra high precision optical frequency reference" The challenge of dual standards is not overcome. Also, since the actual optical frequency reference is in the ultraviolet to visible light region, fiber transmission light in the communication wavelength band is offset-locked to the optical frequency reference using the optical frequency comb, and ultra-high-precision transmission light is realized The premise is that Even when such transmission light is offset-locked to the optical frequency reference, a highly accurate RF frequency reference is required.

  The present invention has been made to solve the above problems, and enables transmission of light serving as an ultra-high-precision frequency reference without requiring a high-precision RF frequency reference or the like. With the goal.

  The optical transmission system according to the present invention is a light transmission system that is disposed at a transmission source and generates stabilized light source light stabilized at a desired reference frequency by the stabilization control using the light frequency reference. A light source light generation unit; an optical fiber provided between a transmission source and a transmission destination; a noise compensation unit disposed at the transmission source for frequency shifting the stabilized light source light; And a return light generation unit for generating return light source light in which the frequency of stabilized light source light transmitted from the transmission source is shifted via the optical fiber, and the noise compensation unit is a return light source transmitted from the transmission source via the optical fiber The frequency shift by the noise compensation unit is controlled by comparing the light and the stabilized light source stabilized and generated at the reference frequency by the stabilization control, and the return light generation unit transmits the light from the transmission source to the transmission destination Stabilized light source Of the stabilized light source light so that the shift amount from the reference frequency and the shift amount from the reference frequency of the return light source light transmitted from the transmission destination to the transmission source are symmetrical with respect to the reference frequency. It generates frequency-shifted return light source light.

  As described above, according to the present invention, it is possible to obtain an excellent effect of transmitting light serving as an ultra-high-precision frequency reference without requiring a high-precision RF frequency reference or the like.

FIG. 1 is a block diagram showing the configuration of an optical transmission system according to an embodiment of the present invention. FIG. 2 is a block diagram showing a more detailed configuration example of the optical transmission system in the embodiment of the present invention. FIG. 3 is a frequency transmission diagram in the optical transmission system illustrated in FIG. FIG. 4 is a block diagram showing the configuration of a typical ultra high precision optical frequency reference fiber transmission. FIG. 5 is the same as FIG. 5 is a frequency transmission diagram in the optical transmission system shown in FIG. FIG. 6 is a diagram of FIG. 10 is a frequency transmission diagram in the optical transmission system shown in FIG.

  Hereinafter, an optical transmission system according to an embodiment of the present invention will be described with reference to FIG. This optical transmission system transmits stabilized light source light to the transmission destination 103 from the transmission source 101 via the optical fiber 102.

  In the transmission source 101, the noise compensation unit 105 compensates for the noise in the optical fiber 102 by the stabilized light source light generation unit 104, and transmits the stabilized light source light to the transmission destination 103 by the optical fiber 102.

  The stabilized light source light generation unit 104 generates stabilized light source light stabilized at a desired reference frequency by the stabilization control using the light frequency reference for the light source light emitted from the light source. Further, the noise compensation unit 105 shifts the frequency of the stabilized light source light generated by the stabilized light source light generation unit 104 in order to compensate for noise in the optical fiber 102.

  At the transmission destination 103, the return light generation unit 106 generates return light source light in which the frequency of the stabilized light source light transmitted from the transmission source 101 via the optical fiber 102 is shifted. The return light is used for noise compensation by the noise compensation unit 105. The noise compensation unit 105 performs noise compensation by comparing the return light source light transmitted from the transmission source 101 via the optical fiber 102 with the stabilized light source light that is stabilized to the reference frequency by the stabilization control. The frequency shift by the unit 105 is controlled. This control is similar to the well-known optical fiber 102 noise compensation.

  As well known, in the function of compensating the noise of the optical fiber, for example, the stabilized light source light before entering the optical fiber is frequency shifted by a frequency shifter such as an acousto-optic element (AOM), and the light is incident on the optical fiber The beat signal is detected by interference between the return light source light that is folded back at the transmission destination and returned to the transmission source and the stabilized light source light before the frequency shift. This beat signal is multiplied by the noise component of the optical fiber as it propagates through the optical fiber. This beat signal is phase-compared with the signal from the RF oscillator that is the RF frequency reference to extract fiber noise.

  By applying frequency fluctuation to the stabilized light source light in advance so as to compensate for frequency fluctuation derived from fiber noise by the frequency shifter, it is possible to transmit the stabilized light source light to the relay device without degrading the accuracy It becomes.

  Here, in the present invention, the return light generation unit 106 is the shift amount from the reference frequency of the stabilized light source light transmitted from the transmission source 101 to the transmission destination 103 and the return amount transmitted from the transmission destination 103 to the transmission source 101 The return light source light is generated in which the frequency of the stabilized light source light is shifted such that the shift amount of the light source light from the reference frequency is symmetrical with respect to the reference frequency. As a result, it becomes possible to transmit light (stabilized light source light) serving as an ultra-high-precision frequency reference from the transmission source to the transmission destination without requiring a high-precision RF frequency reference or the like.

  This will be described in more detail below with reference to FIGS. 2 and 3. FIG. 2 shows a configuration example of the optical transmission system in the present invention. FIG. 3 is a frequency transmission diagram in the optical transmission system shown in FIG.

The optical transmission system shown in FIG. 2 includes a transmission unit (transmission source) 201 and a reception unit (transmission destination) 231. In this optical transmission system, light of the frequency reference of the reference frequency (frequency _{0 0} ) stabilized with ultra high precision is transmitted from the transmission unit 201 to the reception unit 231 through the optical fiber 221.

  The transmission unit 201 includes a light source 202, an optical frequency reference generation unit 203, a second harmonic generation unit 204, a feedback control unit 205, an RF oscillator 206, a frequency shifter 207, a feedback control unit 208, an RF oscillator 209, a beam splitter 210, light A detection unit 211, a semireflecting mirror 212, a beam splitter 213, a light detection unit 214, and a mirror 215 are provided.

  The receiving unit 231 includes a frequency shifter 232, an RF oscillator 233, a beam splitter 234, an optical repeater 235, a light detecting unit 236, a feedback control unit 237, an RF oscillator 238, and a mirror 239.

The light frequency reference generation unit 203 that generates light with a very high precision light frequency reference may use, for example, an Sr light lattice clock (wavelength 698 nm). Generally, the wavelength of the light source light emitted from the light source 202 is 1397 nm of the communication wavelength band, but if the second harmonic is generated from this light source light, the frequency of the light source light can be obtained without using the optical frequency comb. Stabilized light with an optical frequency reference of 2ν ₀ can be generated to produce stabilized source light.

The light source light (a part of the light source light) output from the light source 202 is reflected by the semi-reflecting mirror 212, and the second harmonic generation unit 204 obtains the second harmonic of the light source light. The second harmonic and the light of the optical frequency reference of frequency 2ν ₀ output from the optical frequency reference generator 203 are made to interfere by the beam splitter 210. The light detection unit 211 detects (photoelectrically converts) an optical signal (beat signal) of a frequency difference between the second harmonic of the light source light generated due to the interference and the light of the light frequency reference.

  Thus, based on the beat signal detected by the light detection unit 211, the feedback control unit 205 performs feedback control of the output of the light source 202. In this feedback control, by synchronizing the beat signal with the RF reference signal output from the RF oscillator 206, the frequency of the output light source is stabilized to generate stabilized source light. The light source 202, the light frequency reference generator 203, the second harmonic wave generator 204, the feedback controller 205, the RF oscillator 206, the beam splitter 210, the light detector 211, and the semi-reflecting mirror 212 constitute a stabilized light source light generator. Be done.

In this stabilization, the beat signal of the second harmonic of the light source light and the light of the light frequency reference of frequency 2ν ₀ are observed, and phase comparison with the RF reference signal of the RF oscillator 206 is performed to perform offset synchronization of the light source light. However, in the present invention, the RF reference signal which is the reference signal may not be accurate. Here, when the frequency of the RF reference signal output from the RF oscillator 206 and f _1, the frequency stabilization source light stabilized by the above becomes ν ₀ + f _1.

  Next, the frequency of the stabilized light source light is shifted by the frequency shifter 207 for noise compensation of the optical fiber 221, and is transmitted to the receiving unit 231 by the optical fiber 221. The frequency shifter 207, the feedback control unit 208, the RF oscillator 209, the beam splitter 213, the light detection unit 214, and the mirror 215 form a noise compensation unit.

  In the noise compensation, first, the stabilized light source light partially reflected by the beam splitter 213 and reflected by the mirror 215 is caused to interfere with the return light source light returned from the receiving unit 231 by the beam splitter 213. The beat signal generated due to the interference is detected by the light detection unit 214, and the feedback control unit 208 detects the fiber noise by phase comparison with the RF reference signal output from the RF oscillator 209. The frequency shifter 207 adds a half of the detected fiber noise to the shift in antiphase to compensate for the fiber noise. The RF reference signal of the RF oscillator 209 is synchronized with the RF reference signal of the RF oscillator 206. The RF reference signal of the RF oscillator 209 used for fiber noise compensation may not be accurate.

Here, the RF reference signal by the RF oscillator 206 used to generate the stabilized light source in the transmission unit 201 has the fluctuation a, and when the frequency f ₁ is set, f ₁ ^(a) It is assumed that a signal having a fluctuation of = (1 + a) f ₁ is output. For example, the fluctuation of the RF reference signal generated using a general crystal oscillator is about 10 ⁻⁸ .

When the light source light is stabilized as described above with the RF reference signal having such fluctuation as a reference signal, the frequency _{A A} ^(a) of the stabilized light source light becomes ν ₀ + f ₁ ^(a) . In the optical transmission system shown in FIG. 2 in the embodiment of the present invention, the stabilized light source light of the frequency _{A A} ^(a) is transmitted by the frequency diagram shown in FIG. ν ₀ is to be reproduced.

The present invention can be used even when the light can not be converted into transmission light of the communication wavelength band only by the second harmonic generation as described above. For example, when observing a virtual beat signal of the optical frequency reference (m ₁ / m ₂ ) _{0 0} and the transmission light of the communication wavelength band without stabilizing the optical frequency comb with high accuracy, this beat signal is used. By stabilizing at the RF frequency f ₁ ^(a) , the frequency of the transmission light can be similarly set to _{A A} ^(a) = ν ₀ + f ₁ ^(a) .

Below, it demonstrates in detail using the frequency diagram of FIG. First, as described above, the frequency of the RF reference signal output from the RF oscillator 206 (RF oscillator 209) in the transmitting unit 201 is f ₁ ^(a) = (1 + a) f ₁ . Further, the RF reference signal output from the RF oscillator 238 (RF oscillator 233) in the reception unit 231 has the fluctuation b, and this frequency is f ₂ ^(b) = (when the frequency of f ₂ is set ⁾ 1 + b) f ₂ . For example, suppose that f ₁ = 5 MHz and f ₂ = 10 MHz. The frequency f _AOM1 shifted by the frequency shifter 207 is 80 MHz, and the frequency shifted by the frequency shifter 232 is ₋₇₅ MHz. These figures are reflected in FIG.

As described above, the transmission unit 201 prepares stabilized light source light of frequency _{A A} ^(a) = ν ₀ + f ₁ ^(a) . The stabilized light source light is frequency-shifted by + f _AOM1 ^{drive voltage} by the frequency shifter 207 driven in voltage by the feedback control unit 208, and then transmitted to the receiving unit 231 by the optical fiber 221.

The receiving unit 231 that receives the stabilized light source frequency-shifted as described above generates return light source light for noise compensation of the optical fiber 221 and returns it. In this case, this optical transmission system, the frequency of the returning light ^{_{beam, -2f 1 (b) -2f AOM1}} (b) is shifted the frequency for stabilizing source light received by the receiving unit 231. Here, f ₁ ^(b) is a frequency that is generated with fluctuation b when the frequency of f ₁ is set in the RF oscillator 238 (RF oscillator 233). Further, f _AOM1 ^(b) is a frequency when the frequency shifter 232 performs the same frequency shift as the frequency shifter 207.

  As described above, the shift amount from the reference frequency of the stabilized light source light received by the reception unit 231 and the shift amount of the return light source light received by the transmission unit 201 are frequencies that are positive and negative symmetric with respect to the reference frequency. Thus, the frequency of the stabilized light source light is shifted to generate return light source light.

  Here, in the reception unit 231, the offset may be synchronized by the RF oscillator 238 and the feedback control unit 237, and the return light source light may be generated by the optical repeater 235 as the shift amount described above. However, in this case, the generated return light enters the beam splitter 234 and the light detection unit 236 that constitute an optical system for offset synchronization, and it is distinguished from the reference light that is reflected by the mirror 239 and interfered by the beam splitter 234. It disappears. Therefore, a frequency shifter 232 is provided to generate the return light having the above-described shift amount.

In the system shown in FIG. 2, first, in the optical repeater 235 which performs offset synchronization by the RF oscillator 238 and the feedback control unit 237, the offset synchronization frequency is set to -2f ₂ ^(b) . Further, in the frequency shifter 232, the shift amount is −f ₁ ^(b) −f _{AOM 1} ^(b) + f ₂ ^(b) . The stabilized light source light that has reached the receiving unit 231 first passes through the frequency shifter 232, is reflected by the beam splitter 234, is reflected by the mirror 239, and becomes reference light. Since the light output from the optical repeater 235 is offset-locked based on this reference light, the light output from the optical repeater 235 is shifted by the frequency shifter 232 to the shift of -2f ₂ ^(b) . This output light is again frequency-shifted by the frequency shifter 232, and is sent to the transmission unit 201 as return light source light.

Thus, in the process of generating the return light beam, so passing through the frequency shifter 232 twice, by a shift amount of the frequency shifter 232 _{^{'-f 1 (b) -f AOM1 (}} b) + f 2 (b) " if, on the offset synchronizing frequency -2f ₂ optical repeaters 235 ^(b), the _{^{-2f 1 (b) -2f AOM1 (}} b) + 2f 2 (b) that is applied. As a ^{_{result, -2f 2 (b) + (}} - 2f 1 (b) -2f AOM1 (b) + 2f 2 (b)) = - 2f 1 (b) -2f AOM1 the ^(b).

As described above, in response to shifting of -2f ₁ at the receiving section ^{_{231 (b) -2f AOM1 (b}} ) is generated, the frequency of the transmitted return light beam to the transmitting unit ^{_{201, ν A (a) + f}} AOM1 become ^{the driving voltage _{-2f 1 (b) -2f AOM1 (}} b). Thus, the stabilized light source light of the frequency _{A A} ^(a) + f _AOM1 ^{drive voltage} transmitted from the transmission unit 201 _travels back and forth through the optical fiber 221, and ⁽ _A ^(a) + f _AOM1 ^{drive voltage-} 2 f ₁ ^(b) -2f _AOM1 ^(b) returns to the transmitter 201 as return light source light.

When the return light source light thus frequency-shifted is allowed to pass through the frequency shifter 207 and interfered with the stabilized light source light of frequency _{A A} ^(a) = _{0 0} + f ₁ ^{(a) in} the beam splitter 213, | 2f ₁ ^(b) + 2f _AOM1 ^(b) -2f _AOM1 ^{driving voltage} | interference signal is obtained.

This interference signal is phase-compared to the RF reference signal of 2f ₁ ^(a) obtained from the RF oscillator 209, and 2f ₁ ^(b) + 2f _{AOM 1} ^(b) -2f _{AOM 1} ^{drive voltage} = 2f ₁ ^(a) The feedback control unit 208 controls the drive voltage of the frequency shifter 207 and _applies feedback to the shift frequency f _AOM1 ^{drive voltage} of the frequency shifter 207 for stabilization.

By this stabilization, the shift frequency f _AOM1 ^{drive voltage} of the frequency shifter 207 becomes f ₁ ^(b) + f _AOM1 ^(b) −f ₁ ^(a) .

The frequency of light output from the optical repeater 235 when the stabilized light source shifted for fiber compensation by the frequency shifter 207 stabilized in this way is transmitted to the receiving unit 231 via the optical fiber 221 _{B B} ^(b) is
_{B B} ^(b) = _{A A} ^(a) + f _AOM1 ^{driving voltage-} f ₁ ^(b) -f _{AOM 1} ^(b) + f ₂ ^(b) -2 f ₂ ^(b)
= _{A A} ^(a) -f ₁ ^(a) -f ₂ ^(b)
= _{0 0-} f ₂ ^(b)
It becomes.

In the above formulas, there is no term with a fluctuation a in RF reference signal outputted from the RF oscillator 206 of the transmission unit 201 (RF oscillator 209), the frequency [nu _B of the light output from the optical repeater 235 by the above-described ^{( b)} is not affected by the fluctuation a of the RF frequency in the transmission unit 201.

Further, by shifting the frequency of the light output from the optical repeater 235 by + f ₂ ^(b) in the receiving unit 231, the influence of the fluctuation b of the RF reference frequency in the receiving unit 231 is also canceled, and the stability is extremely high. A signal (frequency _{0 0} ) of a standardized reference frequency can be reproduced.

Further, in the case where the receiving unit 231 is a relay station and the stabilized light source light is transmitted to another receiving station via an optical fiber, the same process as described above may be performed. An optical fiber is used to transmit light whose frequency has been shifted by -f _AOM2 ^{drive voltage} by the phase shifter driven by the drive voltage of the feedback control unit using the RF reference frequency of the RF oscillator synchronized with the RF oscillator 238 (RF oscillator 233) feed to the receiving station, the receiving station, hit back to shift by the frequency of the light is ^{_{+ 2f 2 (c) + 2f}} AOM2 (c), the frequency of the optical repeaters of the offset synchronous frequency 2f ₃ ^(c) and the receiving station of the receiving station The shift frequency of the shifter may be determined.

At the relay station, the light coming back and forth from the optical fiber interferes with the light output from the optical repeater of the relay station, and the f _AOM2 ^{drive voltage} is stabilized by phase comparison with the RF reference signal of 2f ₂ ^(b). For example, the frequency of the light output from the optical repeater at the receiving station is not affected by the fluctuation b of the RF reference frequency f ₂ ^{(b) at} the relay station.

  As described above, in the present invention, the shift amount from the reference frequency of the stabilized light source light received at the transmission destination and the shift amount of the return light source light received at the transmission source are positive or negative with respect to the reference frequency. The return light source light is generated at the transmission destination with the frequency of the stabilized light source light shifted so as to be a symmetrical frequency. As a result, according to the present invention, it becomes possible to transmit light serving as an ultra-high-precision frequency reference without requiring a high-precision RF frequency reference or the like. According to the present invention, regardless of the value of the RF reference frequency used at the transmission source, the shift frequency for fiber compensation, and the RF reference frequency used at the transmission destination, the light of the ultra-high-precision frequency reference at the transmission destination It can be played back.

  According to the present invention, compared to the prior art, the transmission frequency accuracy is not limited by the RF frequency reference to be used, and the transmission frequency accuracy is determined only by the ultra high precision optical frequency reference. Are better. Moreover, since this is a system that can be realized only by appropriately setting the frequency of phase synchronization used in the prior art, it is a very simple system, and there is no problem in the feasibility. Furthermore, in constructing a new infrastructure called multi-point ultra high precision optical frequency reference fiber network, high precision RF frequency reference can be achieved by using an inexpensive RF frequency oscillator with general precision at each point. There is also a great advantage in terms of practicality, as there is no need to deploy

  The present invention is not limited to the embodiments described above, and many modifications and combinations can be made by those skilled in the art within the technical concept of the present invention. It is clear.

  101: transmission source, 102: optical fiber, 103: transmission destination, 104: stabilized light source light generation unit, 105: noise compensation unit, 106: return light generation unit.

Claims (1)

A stabilized light source light generation unit which is disposed at a transmission source and generates stabilized light source light stabilized at a desired reference frequency by stabilizing control using light frequency reference for light source light emitted from the light source;
An optical fiber provided between the source and the destination;
A noise compensation unit disposed at the transmission source and frequency-shifting the stabilized light source light;
A return light generation unit disposed at the transmission destination and generating return light source light in which the frequency of the stabilized light source light transmitted from the transmission source via the optical fiber is shifted;
The noise compensation unit compares the return light source light transmitted from the transmission source via the optical fiber with the stabilized light source light that has been stabilized to the reference frequency by the stabilization control. Control the frequency shift by the noise compensation unit,
The return light generation unit
The shift amount from the reference frequency of the stabilized light source light transmitted from the transmission source to the transmission destination, and the shift amount from the reference frequency of the return light source light transmitted from the transmission destination to the transmission source An optical transmission system characterized in that it generates return light source light in which the frequency of the stabilized light source light is shifted so that the frequency becomes positive and negative symmetric with respect to the reference frequency.

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