Laser frequency offset stabilization method and apparatus

Abstract

The present invention relates to a laser frequency offset stabilization apparatus for generating a frequency discrimination signal, which is a reference signal for laser frequency offset stabilization, such that a slave laser is stabilized to have a constant frequency difference with a master laser, the apparatus comprising: a photodetector for receiving the master laser and the slave laser and generating a beating signal; a power divider for receiving the beating signal from the photodetector and dividing same into two beating signals; two mixers which receive the respective two beating signals divided by the power divider, and which mix same with RF signals generated by respective signal generators so as to generate respective down-converted difference frequency signals; two low-pass filters for receiving the difference frequency signals from the respective two mixers and performing low-pass filtering on each; two signal magnitude detectors for receiving low-pass filter signals from the respective two low-pass filters and acquiring the magnitude of each signal; and a differential signal detector, which receives signal magnitudes from the two signal magnitude detectors so as to generate a difference signal, thereby outputting a frequency discrimination signal.

Description

Laser offset frequency stabilization method and device

[0001] The present invention relates to a laser offset frequency stabilization method and apparatus, and more specifically, to a laser offset frequency stabilization method and apparatus that generates a frequency discrimination signal, which is a reference signal for laser frequency offset stabilization, so that the longitudinal laser is stabilized with a constant frequency difference from the main laser.

[0002]

[0003] Due to factors such as temperature changes, mechanical vibrations, and electrical noise, the output linewidth of a laser expands, and the oscillating center frequency slowly drifts. Laser frequency stabilization refers to the process of eliminating or compensating for these factors to prevent the oscillation frequency from drifting and consequently reduce the oscillation linewidth.

[0004] The most widespread method for stabilizing two laser frequencies while maintaining a constant frequency difference is optical phase locking.

[0005] This optical phase locking method is the most precise because it stabilizes not only the frequency but also the phase of the two lasers; however, it has the disadvantage that it requires a wide servo bandwidth due to the high sensitivity to laser phase changes, and maintaining the locked state is difficult due to disturbances.

[0006] In atomic interferometers, laser frequency stability affects the accuracy of measurements, so the optical phase locking method is the most optimal approach; however, if the phase-locked state cannot be maintained for an extended period, it makes the practical application of atomic interferometers difficult.

[0007] If optical phase stabilization of the two lasers is not required and only frequency stabilization is needed, the offset frequency stabilization method can be used.

[0008] In the case of laser offset frequency stabilization methods known to date, there is a problem in that frequency stability is poor because they generate a frequency-differentiated signal by passing it through a simple filter from the beating signal of two lasers.

[0009]

[0010] The present invention has been devised to solve the above-mentioned problems, and its purpose is to provide a laser offset frequency stabilization method and apparatus that generates a frequency discrimination signal, which is a reference signal for laser frequency offset stabilization, so that the longitudinal laser is stabilized with a constant frequency difference from the main laser.

[0011]

[0012] According to the present invention, a laser offset frequency stabilization device is provided, comprising: a photodetector that receives lasers from a main laser and a secondary laser to generate a beat signal; a power splitter that receives the beat signal from the photodetector and divides it into two; two mixers that each receive the two beat signals divided from the power splitter and mix them with RF signals generated by their respective signal generators to generate a difference frequency signal that is down-converted by each; two low-pass filters that each receive the difference frequency signal from the two mixers and perform low-pass filtering by each; two signal magnitude detectors that each receive the low-pass filter signal from the two low-pass filters and obtain the magnitude of the signal by each; and a differential signal detector that receives the signal magnitude from the two signal magnitude detectors, generates a difference signal, and outputs a frequency discrimination signal.

[0013] Preferably, the frequency discrimination signal of the differential signal detector is applied to the frequency conversion input of the longitudinal laser using a loop filter.

[0014] Preferably, the beat signal generated by the photodetector is converted using a conversion means before being input to the power splitter.

[0015] Preferably, the conversion means is a signal amplifier, and the signal amplifier amplifies the beat signal and inputs it to a power splitter.

[0016] Preferably, the conversion means is a signal generator, and the signal generator is characterized by up-converting or down-converting the frequency of the beat signal and inputting it to a power divider.

[0017] Preferably, the conversion means is an N-frequency multiplier or an N-frequency divider, and is characterized in that the N-frequency multiplier multiplies the frequency of the beat signal by N or the N-frequency divider divides the frequency of the beat signal by N and inputs it to a power divider.

[0018] Meanwhile, according to another aspect of the present invention, a laser offset frequency stabilization method is provided, characterized by comprising: (a) a step of generating a beat signal from a main laser and a secondary laser; (b) a step of dividing the beat signal into two; (c) a step of mixing each of the two divided beat signals with an RF signal to generate two difference frequency signals that are each down-converted; (d) a step of low-pass filtering each of the two difference frequency signals to generate two low-pass filter signals; (e) a step of obtaining each signal magnitude from the two low-pass filter signals to generate two signal magnitudes; and (f) a step of generating a difference signal of the two signal magnitudes to output a frequency discrimination signal.

[0019] Preferably, after step (f), the frequency discrimination signal is applied to the frequency conversion input of the laser.

[0020] Preferably, between steps (a) and (b), the beat signal is transformed through a transformation process.

[0021] Preferably, the above conversion process is characterized by amplifying the beat signal.

[0022] Preferably, the conversion process is characterized by up-converting or down-converting the frequency of the beat signal.

[0023] Preferably, the conversion process is characterized by multiplying the frequency of the beat signal by N or dividing the frequency of the beat signal by N.

[0024]

[0025] According to the present invention, precise frequency stabilization is possible and the offset frequency can be precisely changed by generating a frequency discrimination signal, which is a reference signal for laser frequency offset stabilization, so that the longitudinal laser is stabilized with a constant frequency difference from the main laser.

[0026] In addition, it has the advantage of providing frequency discrimination signals across a wide frequency band while maintaining the accuracy of the atomic interferometer, thereby stably maintaining a frequency stabilization state over a wide frequency range.

[0027] In conventional methods, the frequency offset changes according to the magnitude of the beat signal between the two lasers, that is, the change in laser power. However, the apparatus and method according to the present invention have the advantage of having very little change in frequency due to changes in laser power, which not only reduces sensitivity but also provides frequency stability that is more than 10 times better.

[0028] It also has the advantage of being able to change the stabilization frequency range depending on the appropriate selection of the low-pass filter.

[0029]

[0030] FIG. 1 is a drawing for explaining a laser offset frequency stabilization device according to an embodiment of the present invention.

[0031] FIG. 2 is a graph showing signals generated from two signal magnitude detectors as the beat frequency changes in a laser offset frequency stabilization device according to an embodiment of the present invention.

[0032] FIG. 3 is a graph showing the frequency stability measurement results of a frequency offset stabilized laser in a laser offset frequency stabilization device according to an embodiment of the present invention.

[0033]

[0034] Hereinafter, embodiments disclosed in this specification will be described in detail with reference to the attached drawings. Identical or similar components regardless of drawing symbols are assigned the same reference number, and redundant descriptions thereof will be omitted. The suffixes "module" and "part" used for components in the following description are assigned or used interchangeably solely for the ease of drafting the specification and do not inherently possess distinct meanings or roles. Furthermore, in describing the embodiments disclosed in this specification, if it is determined that a detailed description of related prior art could obscure the essence of the embodiments disclosed in this specification, such detailed description will be omitted. Additionally, the attached drawings are intended only to facilitate understanding of the embodiments disclosed in this specification; the technical concept disclosed in this specification is not limited by the attached drawings, and it should be understood that they include all modifications, equivalents, and substitutions that fall within the spirit and technical scope of the invention.

[0035] Terms including ordinal numbers, such as first, second, etc., may be used to describe various components, but said components are not limited by said terms. These terms are used solely for the purpose of distinguishing one component from another.

[0036] When it is stated that one component is "connected" or "connected" to another component, it should be understood that while it may be directly connected or connected to that other component, there may also be other components in between. On the other hand, when it is stated that one component is "directly connected" or "directly connected" to another component, it should be understood that there are no other components in between.

[0037] A singular expression includes a plural expression unless the context clearly indicates otherwise.

[0038] In this application, terms such as “comprising” or “having” are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0039]

[0040] FIG. 1 is a drawing for explaining a laser offset frequency stabilization device according to an embodiment of the present invention.

[0041] A laser offset frequency stabilization device according to an embodiment of the present invention comprises a main laser (11) that generates a main laser, a longitudinal laser (12) that generates a longitudinal laser, and a beat signal (f) which is the difference between the two lasers, which receives the main laser (11) and the longitudinal laser (12). beat A high-speed photodetector (20) that generates ), a power splitter (30) that receives a beat signal from the photodetector (20) and splits it into two, and a first RF signal (f) generated by a first signal generator (51) that receives the split beat signal from the power splitter (30). L =f beat The first frequency signal (f) down-converted by mixing with -α)beat ±f L A first mixer (41) that generates ), a second RF signal (f) generated by a second signal generator (52) that receives a beat signal divided from the power splitter (30), and a beat signal divided from the power splitter (30). H =f beat The second frequency signal (f) down-converted by mixing with +α) beat ±f H It may comprise a second mixer (42) that generates a signal, a first low-pass filter (61) that receives a first frequency signal from the first mixer (41) and performs low-pass filtering, a second low-pass filter (62) that receives a second frequency signal from the second mixer (42) and performs low-pass filtering, a first signal magnitude detector (71) that receives a low-pass filter signal of the first frequency signal from the first low-pass filter (61) and obtains the magnitude of the signal, a second signal magnitude detector (72) that receives a low-pass filter signal of the second frequency signal from the second low-pass filter (62) and obtains the magnitude of the signal, and a differential signal detector (80) that receives the signal magnitude from the first signal magnitude detector (71) and the second signal magnitude detector (72), generates a difference signal, and outputs a frequency discrimination signal.

[0042] The laser offset frequency stabilization method proposed in this invention is a technique in which a slave laser is precisely stabilized with respect to a master laser with a constant frequency difference.

[0043] When the main laser and the secondary laser are combined and incident on a high-speed photodetector (20), a beating signal corresponding to the frequency difference of the main laser is observed. Usually, the frequency of the secondary laser drifts from the frequency of the main laser, so the beating frequency also fluctuates accordingly. When this beating signal is combined with a precise RF signal in a frequency mixer (41, 42), a difference frequency signal corresponding to the difference between the beating frequency and the RF frequency can be obtained.

[0044] If the beat signal is divided into two and combined into two RF signals with slightly different frequencies in a frequency mixer (41, 42), the frequency of the two output difference frequency signals differs by the frequency of the two RF signals.

[0045] When these difference frequency signals are each passed through a low-pass filter (61, 62), the magnitude of the low-pass filter (61, 62) signal varies according to the beat frequency fluctuation. After each signal magnitude is acquired using a signal magnitude detector (71, 72) and the difference signal is obtained through the differential circuit of a differential signal detector (80), this signal becomes a frequency discriminator that is proportional to the change in beat frequency. Since it is a differential signal of two signals, it is insensitive to various environmental changes such as laser power, so it can be used as a precise reference signal for laser frequency offset stabilization.

[0046] Referring to FIG. 1, a laser offset frequency stabilization method according to an embodiment of the present invention is described as follows.

[0047] First, as a first step, the main laser generated from the main laser (11) and the longitudinal laser generated from the longitudinal laser (12) are incident on the photodetector (20) to produce a beat signal (f) which is the difference between the two lasers. beat Creates ).

[0048] Next, as a second step, the beat signal from the photodetector (20) is input to the power splitter (30) to split it into two, and the two split beat signals are used to generate a first RF signal (f) from the first signal generator (51) and the second signal generator (52). L =f beat -α) and the second RF signal (f H =f beat +α) and the first frequency signal (f) down-converted by mixing in the first mixer (41) and the second mixer (42), respectively. beat ±f L ) and the second frequency signal (f beat ±f H Creates ).

[0049] Here, the first mixer (41) and the second mixer (42) are identical, and the two split beat signals of the photodetector (20) are applied to the RF ports of the first mixer (41) and the second mixer (42), respectively, and the first RF signal and the second RF signal generated by the first signal generator (51) and the second signal generator (52) are input to the LO ports of the first mixer (41) and the second mixer (42), respectively, so that the down-converted first frequency signal and the second frequency signal are created at the IF ports of the first mixer (41) and the second mixer (42), respectively.

[0050] Next, as a third step, the first frequency signal and the second frequency signal generated from the first mixer (41) and the second mixer (42), respectively, are applied to the first low-pass filter (61) and the second low-pass filter (62), respectively, to perform low-pass filtering.

[0051] Next, as a fourth step, the low-pass filter signals of the first and second frequency signals generated from the first low-pass filter (61) and the second low-pass filter (62) are applied to the first signal magnitude detector (71) and the second signal magnitude detector (72), respectively, to obtain the signal magnitudes, and these two signal magnitudes are applied to the differential signal detector (80) to generate a difference signal and output a frequency discrimination signal.

[0052] Stabilization is achieved by feeding the above frequency discrimination signal back into the frequency conversion input of the laser (12) using an appropriate loop filter.

[0053] Here, the beat signal (f) generated in the first step above beat ) can be converted using a conversion means so that a stable difference frequency signal can be obtained in the mixer (41) before being input to the power splitter (30). The conversion means may be placed between the photodetector (20) and the power splitter (30).

[0054] The above conversion means may be a signal amplifier, and the signal amplifier may amplify the beat signal and input it to the power splitter (30).

[0055] In addition, the above conversion means may be a third signal generator, and the third signal generator may up-convert the frequency of the beat signal and input it to the power splitter (30).

[0056] In addition, the above conversion means may be a third signal generator, and the third signal generator may down-convert the frequency of the beat signal and input it to the power splitter (30).

[0057] In addition, the above conversion means may be an N-frequency multiplier, and the N-frequency multiplier may input the frequency of the beat signal to the power divider (30) after multiplying it by N.

[0058] In addition, the above conversion means may be an N-frequency divider, and the N-frequency divider may divide the frequency of the beat signal into N and then input it to the power divider (30).

[0059] Meanwhile, the signal magnitude detector (71, 72) that acquires the magnitude of the signal in the above 4th step detects the magnitude of the low-pass filter signal.

[0060] Such signal magnitude detectors (71, 72) can be configured with any means capable of detecting the magnitude of the vibrating RF, and can be configured with an RMS detector that converts the signal magnitude into RMS (Root-Mean-Square), a power detector that detects the power of the signal, an envelope detector that detects the envelope of the signal, etc.

[0061]

[0062] FIG. 2 is a graph showing signals generated from two signal magnitude detectors as the beat frequency changes in a laser offset frequency stabilization device according to an embodiment of the present invention.

[0063] In the test shown in Fig. 2, an RMS detector was used as a signal magnitude detector (71, 72).

[0064] The upper graph of Fig. 2 is the beat signal (f beat It represents the signals generated by the two RMS detectors as ) changes.

[0065] Also, the graph below in FIG. 2 shows the differential signal of the two RMS detector outputs. A first RF signal (f) generated by a first signal generator (51) and a second signal generator (52) L =f beat -α) and the second RF signal (f H =f beatThe sensitivity of the frequency differential signal can be optimized by appropriately adjusting the frequency difference between +α). This signal becomes a frequency differential signal that generates a change in voltage according to the change in frequency. The zero-crossing frequency of the frequency differential signal, that is, the stabilizing offset frequency, is the first RF signal (f L ) and the second RF signal (f H The center of ) ((f L +f H ) / 2). If this signal is applied to the frequency conversion input of the laser (12) using an appropriate loop filter, stabilization will be achieved.

[0066]

[0067] FIG. 3 is a graph showing the frequency stability measurement results of a frequency offset stabilized laser in a laser offset frequency stabilization device according to an embodiment of the present invention.

[0068] Figure 3 shows the results of measuring the frequency stability of a frequency offset stabilized laser. Short-term stability from an integration time of 1 ms to 10 s was measured using a 53100A phase analyzer, and from 1 s to 10 4 s is the result measured with the frequency counter 53132A.

[0069] When a 1.9 MHz LPF is used as the low-pass filter (61, 62), the stability at an integration time of 1 s is 5x10 -15 We were able to obtain this. This is a result that is 10 times better than the existing method using filters.

[0070]

[0071] Optimal embodiments have been disclosed in the drawings and specification as described above. Specific terms have been used herein, but they are used only for the purpose of describing the invention and are not intended to limit the meaning or the scope of the invention as described in the claims. Therefore, those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the invention should be determined by the technical spirit of the appended claims.

Claims

1. A photodetector that receives incident lasers from a main laser and a secondary laser and generates a beat signal; A power splitter that receives a beat signal from the above photodetector and divides it into two; Two mixers, each receiving two beat signals divided from the power divider and mixing them with RF signals generated from their respective signal generators to generate respective down-converted difference frequency signals; Two low-pass filters that each receive a difference frequency signal from the two mixers above and each perform low-pass filtering; Two signal magnitude detectors, each receiving a low-pass filter signal from the two low-pass filters and acquiring the magnitude of the respective signal; and A laser offset frequency stabilization device characterized by including a differential signal detector that receives signal magnitudes from the two signal magnitude detectors above, generates a difference signal, and outputs a frequency discrimination signal.

2. In Paragraph 1, A laser offset frequency stabilization device characterized by applying the frequency discrimination signal of the above differential signal detector to the frequency conversion input of a longitudinal laser using a loop filter.

3. In Paragraph 1, A laser offset frequency stabilization device characterized in that the beat signal generated by the above photodetector is converted using a conversion means before being input to the power divider.

4. In Paragraph 3, A laser offset frequency stabilization device characterized in that the above-mentioned conversion means is a signal amplifier, and the signal amplifier amplifies the beat signal and inputs it to a power splitter.

5. In Paragraph 3, A laser offset frequency stabilization device characterized in that the above-mentioned conversion means is a signal generator, and the signal generator up-converts or down-converts the frequency of a beat signal and inputs it to a power divider.

6. In Paragraph 3, A laser offset frequency stabilization device characterized in that the above-mentioned conversion means is an N-frequency multiplier or an N-frequency divider, and the N-frequency multiplier multiplies the frequency of a beat signal by N or the N-frequency divider divides the frequency of a beat signal by N and inputs it to a power divider. 7.(a) A step of generating a beat signal from the lasers of the main laser and the secondary laser; (b) a step of splitting the above beat signal into two; (c) a step of generating two difference frequency signals, each down-converted by mixing the RF signal with each of the two split beat signals; (d) a step of generating two low-pass filter signals by low-pass filtering each of the two difference frequency signals; (e) a step of generating two signal magnitudes by obtaining the respective signal magnitudes from two low-pass filter signals; and (f) a step of generating a difference signal of two signal magnitudes and outputting a frequency discrimination signal; a laser offset frequency stabilization method characterized by including 8. In Paragraph 7, After the above (f) step, A laser offset frequency stabilization method characterized by applying the above-mentioned frequency discrimination signal to the frequency conversion input of a longitudinal laser.

9. In Paragraph 7, Between the above steps (a) and (b), A laser offset frequency stabilization method characterized by the above-mentioned beat signal being converted through a conversion process.

10. In Paragraph 9, A laser offset frequency stabilization method characterized by the above conversion process amplifying a beat signal.

11. In Paragraph 9, A laser offset frequency stabilization method characterized by the above conversion process up-converting or down-converting the frequency of a beat signal.

12. In Paragraph 9, A laser offset frequency stabilization method characterized by the above conversion process multiplying the frequency of a beat signal by N or dividing the frequency of a beat signal by N.

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