KR100918459B1 - simultaneous transfer of microwave and optical reference frequencies by selectively amplified optical frequency comb - Google Patents

Abstract

Using a semiconductor amplifier, amplify the optical frequency comb of the desired part of the optical frequency comb that is phase stabilized at the reference frequency while maintaining the phase coherence, and then selectively amplified and transmitted when remotely transmitting through the optical fiber network. The optical comb can be used as an absolute optical frequency measurement or as a microwave reference frequency and optical frequency standard at remote locations. It is located at a remote location, where F = 4 → F ＇= 5 It can be proved experimentally by measuring the absolute optical frequency of the semiconductor laser stabilized on the transition line.

Optical frequency comb, microwave, reference frequency, optical frequency, antireflective coated semiconductor laser, semiconductor amplifier.

Description

Simultaneous transfer of microwave and optical reference frequencies by selectively amplified optical frequency comb}

1 is a device for selectively amplifying a specific portion of the optical frequency comb and then transmits a high stability standard frequency and a microwave reference frequency to an arbitrary position through the optical fiber network.

2 is an experimental device for selectively amplifying a specific portion of an optical frequency comb based on a femtosecond mode synchronous laser, and transmitting the optical frequency standard frequency and the microwave reference frequency of the transmitted optical frequency comb to a long distance through the optical fiber network. The optical frequency of an external resonator diode located remotely and stabilized at the transition line of cesium atoms can be measured remotely.

Figure 3a is a graph showing the amplification gain and power spectrum by the optical frequency comb through the 1.5 nm interference filter (circular) and the 10 nm interference filter (square), the graph increasing from the lower left to the upper right is a graph of the injected light After a fixed power of 1 mW, the amplification gain of AR-coated laser diode was observed while changing the diode current, and the graph decreasing from the top left to the bottom right shows the current of the semiconductor laser. After fixing at 120 mA, the amplification gain was observed while changing the power of the injected light.

Figure 3b shows the output of a semiconductor amplifier, i.e., an AR-coated diode laser, before (square) and after (circular), before an optical frequency comb is injected into an AR-coated diode laser. The figure inserted in the upper left as the power spectrum shows the power spectrum after the optical frequency comb passes through the 10 nm interference filter.

4A is an Allan deviation showing the stability of the repetition rate frequency measured before (square) and after (triangle) transmission of the amplified optical frequency comb through a 100m polarization maintaining optical fiber at a long distance.

FIG. 4b shows a remote measurement of the optical frequency of a semiconductor laser stabilized at a distance F = 4 → F'5 among D2 transition lines of cesium atoms by amplified optical comb, and the average value of the measured result is f _ave = 351 721 960 525.5 ± 2.8 KHz, which shows a difference of about 60 KHz compared to the results measured with cesium atomic beams.

-Explanation of symbols for the main parts of the drawings

1: Cesium atomic clock or hydrogen major or optical clock

2, 12: femtosecond mode synchronous laser 3, 16: polarization maintaining optical fiber

11: hydrogen major 13: Opti Cup isolator

14: anti-reflective coating semiconductor laser 15: controller

17: external resonator semiconductor laser 18: low noise dispensing device

19, 20: frequency counter

21: direct digital synthesizer (DDS)

HP1, HP2: half-wave polarizer IF: interference filter

BS1, BS2: Beam Separator PD1, PD2: High Speed Detector

M: Reflector

The present invention uses a semiconductor amplifier such as an antireflection-coated laser diode to amplify an optical frequency comb of a desired specific portion of the optical frequency combs phase stabilized at a reference frequency while maintaining phase coherence. When transmitting remotely through a fiber network, the selectively amplified and transmitted optical comb is a microwave reference frequency and optical signal using a selectively amplified optical comb that can be used as an absolute optical frequency measurement or a microwave reference frequency and an optical frequency standard at a remote location remotely. The present invention relates to a method of transmitting a frequency standard frequency simultaneously.

Recently, due to the development of a phase stabilized optical frequency comb based on a light source clock with high stability and accuracy, the optical standard frequency and the microwave reference frequency can be transmitted to a remote place while maintaining high stability and accuracy. Necessity is emerging.

In this regard, a technique for transmitting a microwave reference frequency and an ultra-jitter timing signal using a mode-synchronized fiber laser or an amplitude modulated semiconductor laser has been studied, and a mode-synchronized semiconductor laser or a frequency stabilized CW 1.06 The technology of transmitting optical signal through mm Nd: YAG laser through optical fiber network has been studied.

In addition, high resolution spectroscopic experiments using frequency stabilized optical frequency combs or amplified optical frequency combs have been conducted in the laboratory. The long range transmission of microwave reference frequency and optical frequency standard frequency with high stability is proposed a new method in the field of optical frequency measurement and high resolution spectroscopy which rely on accurate frequency measurement in microwave and optical frequency domain.

Until now, research and development have been conducted to transmit microwave reference signals or optical signals using amplitude-modulated semiconductor lasers or frequency stabilized CW lasers in the optical communication domain. There is no report on the research and development of transmission technology.

The present invention for continuing the research and development as described above, by selectively amplifying the optical frequency comb obtained from the femtosecond mode synchronous laser and transmit it over a long distance through the optical fiber network, such as physical constant measurement, high resolution spectroscopy, absolute distance measurement, optical communication field, etc. The aim is to obtain a high stability microwave reference frequency and optical frequency reference frequency that can be widely applied.

A method of simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb according to the present invention for achieving the above object, narrows a part of the optical frequency comb from a phase stabilized femtosecond mode synchronous laser. Selecting by an interference filter; A second step of amplifying a portion of the optical frequency comb selected by the interference filter while maintaining phase formation using a semiconductor amplifier; And a third step of remotely transmitting the optical frequency comb amplified by the semiconductor amplifier through the optical fiber network to use the standard frequency of the optical frequency and the reference frequency of the microwave.

A part of the optical frequency comb selected by the interference filter is an optical frequency comb of a wavelength in which a semiconductor laser exists up to infrared rays including visible light.

The interference filter may be an interference filter having a transmission width of 1.5 nm or an interference filter having a transmission width of 10 nm.

An optical isolator including a polarizer may be used to inject the selected optical frequency comb into the semiconductor amplifier.

A half-wave polarizing plate may be used to match the polarization incident through the optical isolator including the polarizer with the semiconductor amplifier.

As the semiconductor amplifier, an antireflection-coated laser diode may be used.

In addition, according to the present invention, a method of simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb selects a part of the optical frequency comb from a phase stabilized optical fiber femtosecond mode synchronous laser by a narrow optical fiber interference filter. The first step to do; A second step of amplifying a portion of the optical frequency comb selected by the optical fiber interference filter while maintaining phase formation using an optical fiber amplifier; And a third step of remotely transmitting the optical frequency comb amplified by the optical fiber amplifier through the optical fiber network to use the standard frequency of the optical frequency and the reference frequency of the microwave.

A part of the optical frequency comb selected by the optical fiber interference filter is an optical frequency comb of a wavelength in which a semiconductor laser exists up to infrared rays including visible light.

The optical fiber interference filter may be formed of an optical fiber interference filter having a transmission width of 1.5 nm or an optical fiber interference filter having a transmission width of 10 nm.

A Faraday rotatar including a polarizer may be used to cause the selected optical frequency comb to be injected into the optical fiber amplifier.

An optical fiber half-wave plate may be used to match the polarized light incident through the optical fiber Faraday rotator including the polarizer with the optical fiber amplifier.

Hereinafter, the present invention will be described in detail.

The present invention relates to a method for simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency in the near-infrared region. As shown in FIG. 1, a phase stabilized light is provided in a cesium atomic clock or a hydrogen major or optical clock 1 as a reference frequency source. By amplifying the frequency comb and transmitting it directly to a remote location, it is possible to selectively amplify a specific portion of the phase-stabilized optical frequency comb and transmit it to multiple remote locations simultaneously. Therefore, the microwave reference frequency and the optical frequency standard frequency obtained from the highly stable optical frequency comb can be very useful in experiments such as high resolution spectroscopy, multi-wavelength interferometer, reference frequency for optical communication, etc.

In the above method, in order to transmit a microwave reference frequency and an optical frequency standard frequency using an optical fiber network, an optical frequency comb of a narrow region selected in the near infrared region by an interference filter (IF) is applied to a semiconductor amplifier (hereinafter referred to as an antireflection coating). Amplified using an antireflection coated semiconductor laser (LD). For the practical experiment, the 852 nm band optical frequency comb corresponding to the D2 transition line of cesium atom was amplified by an antireflective coated semiconductor laser (LD), and then polarized light was maintained to another laboratory 100 m away in the same building. 3) Transmit through the network and measure the absolute frequency of the frequency stabilized laser at the D2 transition line of cesium atoms in a laboratory 100 m away.

2 is a block diagram for amplifying the selected optical frequency comb in the near infrared and transmitting the amplified optical frequency comb for long distance.

The femtosecond mode synchronous laser 12 used as the dominant (“main”) laser has a repetition rate frequency of 1 GHz, a line width of about 35 fs, a center wavelength of laser output of about 820 nm, and a spectral line width of about 50 nm. to be. The repetition rate f _rep and the carrier-envelope offset frequency f _{ceo of the} femtosecond mode synchronous laser 12 are phase stabilized at the hydrogen major 11, which is a reference frequency.

As can be seen in Figure 1, the repetition rate frequency f _rep and the carrier-envelope offset frequency f _ceo of the optical frequency comb can be phase stabilized in a new optical atomic clock with high stability. The optical frequency comb in the near infrared region selected by the narrow interference filter IF enters the antireflective coated semiconductor laser 14 used as the slave laser. The antireflective coated semiconductor laser 14 used has a gain region of about 810 nm to 880 nm, and when the current is about 120 mA, the self-emitting output power of the antireflective coated semiconductor laser 14 is about 3.5 mW.

The interference filter IF has a center band of about 850 nm and a 3 dB transmission width of about 10 nm. The transmission wavelength of this interference filter IF contains the wavelength of the D2 transition line of cesium atom. The power of the optical frequency comb passing through the interference filter IF is measured at about 1 mW, which corresponds to about 4000 modes with 1 GHz mode spacing. Two cylinder lenses are used to increase coupling efficiency through mode matching between the optical frequency comb and the antireflective coated semiconductor laser 14. Use an optical isolator 13 with a polarizer to ensure that the selected optical frequency comb is injected into the antireflective coated semiconductor laser 14 and match the incident polarization with the antireflective coated semiconductor laser 14. Half-wave polarizing plate (half-wave plate) (HP1) (HP2) is used.

The correlation between the bandwidth of the injected light and the amplification gain of the antireflective coated semiconductor laser 14 can be seen by comparing the amplification factor using an interference filter (IF) having a desired transmission width, i.e. 1.5 nm and 10 nm. .

Experimental results using two interference filters IF are shown in FIG. 3A. The limitation of amplification gain when strong and short pulses are input to the antireflective coated semiconductor laser 14 is due to the fact that the coupling time of holes and holes in the semiconductor is hundreds of picoseconds longer than the input pulse time. The pulse width of the optical frequency comb before passing through the interference filter (IF) is about 35 fs, the spectral width is about 50 nm, and the pulse width of the optical frequency comb passing through the interference filter (IF) at 1.5 nm and 10 nm is About 1 ps and 100 fs. That is, although the pulse width passing through the interference filter IF is shorter than the coupling time of the antireflective coated semiconductor laser 14, it is possible to observe the change in the amplification gain with respect to the pulse width.

As shown in Figure 3a showing the measurement results, the lower part on the left is a result of measuring the current of the antireflective coated semiconductor laser 14 at 120 mA and changing the optical frequency comb injection power, and the upper part on the left is After fixing the optical frequency comb injection power to 1 mW, the observation result while changing the current of the anti-reflective coating semiconductor laser 14.

As can be seen in Figure 3a, the amplification gain by the optical frequency comb that passed through the 1.5 nm interference filter IF is about 1 dB greater than the amplification gain by the optical frequency comb that passed through the 10 nm interference filter IF. In both cases, however, the gain is saturated around 10 dB. Similar characteristics can be seen in the amplification of the optical frequency comb by tapered amplifiers and single mode lasers.

3B is an optical frequency comb amplification spectrum by the antireflective coated semiconductor laser 14. At this time, the current of the antireflection-coated semiconductor laser 14 is about 120 mA, and the optical frequency comb power of 1 mW passing through the 10 nm interference filter IF is used as the injection power. When the power of the optical frequency comb injected through the two interference filters IF of 1.5 nm and 10 nm and injected into the antireflective coated semiconductor laser 14 is 1 mW, the laser outputs 9.1 mW and 10.7 mW, respectively. This amplified power corresponds to approximately 2.3 mW and 17.8 mW per optical frequency comb mode, respectively. This amplification factor is relatively low in terms of amplification factor compared to a method capable of amplifying about 60 dB by single mode amplification using a distributed-ragrag-reflection (DBR) laser.

However, the injection lock width problem, which is the biggest problem of the single mode amplification method, is not considered at all in the method using the antireflective coated semiconductor laser 14, which is very advantageous for long-term stability measurement and can be amplified in multiple modes. Therefore, the microwave reference signal of the optical frequency comb and the optical frequency standard frequency can be simultaneously transmitted at a long distance.

In FIG. 3B, the bottom square is the self-emitting output of the anti-reflective coated semiconductor laser 14 itself, and the circle is injected into the anti-reflective coated semiconductor laser 14 by passing the optical frequency comb passing through the 10 nm interference filter IF. The signal is then amplified. The inserted figure shows the spectrum of the optical frequency comb that passed through the 10 nm interference filter (IF). As shown in FIG. 3B, it can be seen that the optical frequency comb spectrum selected by the interference filter IF is amplified by the only antireflective coated semiconductor laser 14.

Since the multi-mode amplified optical frequency comb is all stabilized at the reference frequency, each mode is regarded as one laser, which is about 4000 stable amplified lasers. In addition, since the RF signal between the optical frequency modes maintains the stability of the reference frequency, it is possible to use the reference frequency having the stability of the cesium atomic clock at a remote location for long distance transmission. A 100 m long polarization-maintaining optical fiber 16 is used to remotely transmit the RF signal and the optical signal of the optical frequency comb amplified by the antireflective coated semiconductor laser 14. It is about 100 m away from the amplification system, and is transferred to the stabilized laser at the F = 4 → F '= 5 transition line of the cesium atom's D2 transition line.

At this time, the power incident on the polarization maintaining optical fiber 16 is about 5 mW, and the coupling efficiency is about 50%. And the power at the transmitted place is about 2 mW, and the transmission loss is about 20%. The main cause of this loss is Rayleigh scattering. In order to minimize this loss, when the optical frequency comb in the optical communication area is used, the optical frequency comb stabilized over a longer distance can be transmitted using less optical loss.

In the present invention, the remotely transmitted optical frequency comb is divided into two parts by the thin beam separator BS1. The reflected optical frequency comb is used to measure the repetition rate frequency of the optical frequency comb through the high speed detector PD1. The measured repetition rate frequency is used as the reference microwave signal. The repetition rate frequency of the optical frequency comb measured by the high speed detector PD1 has a resolution of about 300 kHz and a signal-to-noise ratio of 60 dB or more. This signal is divided into about 1/16 by the low noise dividing device 18, and then enters a commercial direct digital synthesizer (DDS) 21 and is converted to 10 MHz.

The optical frequency comb transmitted from the beam separator BS2 is used for the absolute frequency measurement of the external cavity laser diode 17 which is frequency stabilized at the transition line of cesium atoms. The beat signal between the transmitted optical frequency comb and the external resonator semiconductor laser 17 is detected by the high speed detector PD2, and amplified by a low pass filter and a low noise RF amplifier. This signal is measured by a frequency counter 19 which is phase locked to 10 MHz, which is the reference frequency output by the DDS 21 as shown in FIG. In order to confirm the accuracy of the measurement, the frequency counters 19 and 20 are phase-locked to the hydrogen major 11 as the reference frequency.

4 is a reference frequency of the frequency counter 20 used for the measurement as a result of measuring the stability before and after long-distance transmission through the polarization maintaining optical fiber 16 after amplifying the optical frequency comb passing through the 10 nm interference filter (IF) Uses a hydrogen major (11).

The hydrogen major (11) is time-compared to UTC of the Korea Research Institute of Standards and Science and is connected by an RF coaxial cable. The stability of the reference signal transmitted by the RF coaxial cable in this case is about 5 × 10 at an average time of 1 second ^{- 14} a. In practice, the absolute frequency measurement using the amplified optical frequency comb uses a 10 MHz extracted from the optical frequency comb. The repetition rate traceability of the amplified optical frequency comb is about 2 × 10 ^-15 at an average time of 100 seconds, and the repetition rate tracking capability of the optical frequency comb remotely transmitted by the polarization maintaining optical fiber 16 is 100 seconds. The mean time is about 1.7 × 10 ^-14 . The reason why the stability is impaired is considered to be a factor due to group delay, vibration and temperature change generated in the path of the installed optical fiber.

In the present invention, since the stability of the cesium atom transition line measured in absolute frequency is greater than the stability fluctuation caused by the optical transmission, the operation of removing phase noise generated during the optical signal transmission is not performed.

The external resonator semiconductor laser 17 used for the remote optical frequency measurement is frequency stabilized by a MTS (modulation transfer spectroscopy) method on the F = 4 → F ′ = 5 transition line of the D2 transition line of cesium atoms. The beat signal f _beat between the amplified optical frequency comb and the frequency stabilized external resonator diode laser 17 has a signal-to-noise ratio of about 35 dB or more when measured with a 300 kHz resolution. The absolute frequency of the F = 4 → F ′ = 5 transition line in the D2 transition line of cesium atoms using the measured beat signal has a relationship of f _ECDL = n × f _rep ± f _ceo ± f _beat .

Here, n is an integer, to determine the n value on the basis of the previously measured results.

Each measured data point is the average of 500 measured data, and statistical uncertainty is measured to be less than 2.5 kHz. The total mean variance is about 2.8 kHz, which corresponds to a relative standard uncertainty of 9.3 × 10 ^-12 . Therefore, the total average frequency is measured as 351 721 960 525.5 ± 2.8 KHz, which is about 60 kHz lower than the conventional measurement result.

In the previous experiment, the measurement was performed using a cesium atomic beam, but the present invention was measured using a cesium atomic cell, so the systematic frequency shift caused by Zeman shift, pressure shift, AC stark shift, etc. This experiment is included in the experiments using the cesium atomic beam and the difference of several tens of KHz is seen. In addition, considering the stability before and after transmission of the amplified optical frequency comb used in the present invention, the stability of the reference frequency, the stability of the stabilization system, the frequency instability at a distance is less than 500 Hz.

Furthermore, the present invention is not limited to the above embodiments, and since all processes are performed in free space, the present invention replaces components such as semiconductor amplifiers, femtosecond mode synchronous lasers, optical isolators, interference filters, and half-wave polarizers. It can also consist of parts with an optical fiber attached.

Therefore, according to the present invention, the near-infrared portion of the phase-stabilized 1 GHz femtosecond mode synchronous laser optical frequency comb is selected and amplified using a semiconductor amplifier such as an antireflective coated semiconductor laser, and is transmitted remotely using a fiber optic network for cesium even at a remote location. An optical frequency comb that can be used to measure the absolute frequency of the F = 4 → F '= 5 transition line among the D2 transition lines of the atom can be obtained.

That is, the present invention is a multi-selection and amplification method of the optical frequency comb, when the microwave signal and the optical signal are transmitted at the same time over a long distance, the microwave reference frequency and the optical frequency standard frequency having high stability are measured by physical constant, high resolution spectroscopy, and absolute distance measurement. It can be widely applied to the optical communication field.

Claims (11)

Selecting a portion of the optical frequency comb from the phase stabilized femtosecond mode synchronous laser with a narrow interference filter; A second step of amplifying a portion of the optical frequency comb selected by the interference filter while maintaining phase formation using a semiconductor amplifier; And A third step of remotely transmitting the optical frequency comb amplified by the semiconductor amplifier through the optical fiber network to use the standard frequency of the optical frequency and the reference frequency of the microwave; Method for simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb comprising a. The method of claim 1, A part of the optical frequency comb selected by the interference filter is an optical frequency comb of a wavelength in which a semiconductor laser exists up to infrared rays including visible light, and simultaneously transmits a microwave reference frequency and an optical frequency standard frequency using the selectively amplified optical comb. Way. The method of claim 1, And the interference filter comprises an interference filter having a transmission width of 1.5 nm or an interference filter having a transmission width of 10 nm, and simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using an selectively amplified optical comb. The method of claim 1, A method of simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using an optically amplified optical comb, wherein an optical isolator including a polarizer is used to inject the selected optical frequency comb into a semiconductor amplifier. . The method of claim 4, wherein A microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb, wherein a half-wave plate is used to match the polarization incident through the optical isolator including the polarizer with the semiconductor amplifier. How to send at the same time. The method according to any one of claims 1 to 4 or 5, Wherein the semiconductor amplifier is an antireflection-coated laser diode and simultaneously transmits a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb. Selecting a part of the optical frequency comb from the phase stabilized optical fiber femtosecond mode synchronous laser by a narrow optical fiber interference filter; A second step of amplifying a portion of the optical frequency comb selected by the optical fiber interference filter while maintaining phase formation using an optical fiber amplifier; And A third step of remotely transmitting the optical frequency comb amplified by the optical fiber amplifier through the optical fiber network to use the standard frequency of the optical frequency and the reference frequency of the microwave; Method for simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb comprising a. The method of claim 7, wherein Part of the optical frequency comb selected by the optical fiber interference filter is an optical frequency comb of a wavelength in which a semiconductor laser exists up to infrared rays including visible light, and simultaneously transmits a microwave reference frequency and an optical frequency standard frequency using an optically amplified optical comb. How to. The method of claim 7, wherein The optical fiber interference filter is a method of simultaneously transmitting a microwave reference frequency and an optical frequency standard frequency using a selectively amplified optical comb, characterized in that the optical fiber interference filter having a transmission width of 1.5 nm or optical fiber interference filter having a transmission width of 10 nm. . The method of claim 7, wherein In order to simultaneously transmit the microwave reference frequency and the optical frequency standard frequency using a selectively amplified optical comb, characterized in that the optical fiber Faraday rotatar including a polarizer is used to inject the selected optical frequency comb into the optical fiber amplifier Way. The method of claim 10, An optical fiber half-wave plate is used to match the polarized light incident through the optical fiber Faraday rotator including the polarizer with the optical fiber amplifier. How to transmit standard frequency at the same time.

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