JP2012132711A - Interpulse phase shift measurement device, offset frequency controller, interpulse phase shift measurement method, and offset frequency control method - Google Patents

Abstract

An object of the present invention is to measure a phase shift between pulses of an optical frequency comb light source with high efficiency.
Optical path setting means (12, 13, 14) for branching an optical pulse signal (S) oscillated by an optical frequency comb light source (11) into a pair of optical pulse signals (Sa, Sb) and then reintegrating them. And the oscillation period of the optical frequency comb light source and the pair of light so that the pulse shift number N between the pair of optical pulse signals included in the combined optical pulse signal (Sc, Se, Sf) is an integer of 1 or more. The pulse shift means (14) for setting the relationship with the optical path length difference of the pulse signal and the phase shift amount between the pair of optical pulse signals are set to M phase shift amounts (M: an integer of 3 or more). Phase shift means (17, 15) and detection means (16-0, 16-) for detecting M interference intensities indicated by a pair of optical pulse signals under M phase shift amounts based on the combined optical pulse signal 1, 16-2), the relationship between the M interference strengths and the M Rino and a calculation means for calculating (18) a pulse-to-pulse phase shift of the phase shift relationship with the based optical pulse signal (S).
[Selection] Figure 1

Description

  The present invention relates to an inter-pulse phase shift measuring apparatus for measuring a phase shift between pulses of a carrier envelope (hereinafter simply referred to as “inter-pulse phase shift”) generated in a pulse train generated by an optical frequency comb light source, and an offset frequency. The present invention relates to a control device, a pulse phase shift measuring method, and an offset frequency control method.

  The optical frequency comb is effective as a standard for optical frequency measurement, high-precision spectroscopy, precision length measurement, and the like. Since the absolute frequency of each comb (each mode) of the optical frequency comb needs to be known when using the optical frequency comb, the phase between pulses generated in the pulse train (optical pulse signal) oscillated by the optical frequency comb light source It is necessary to measure the deviation and to make the carrier envelope offset frequency (hereinafter simply referred to as “offset frequency”) of the optical frequency comb known (see Patent Document 1, etc.).

  In the measurement method of Patent Document 1, the optical pulse signal oscillated by the optical frequency comb light source is branched into a pair of optical pulse signals and then reintegrated, and the optical path length difference between the pair of optical pulse signals is changed. The phase shift between pulses is clarified by monitoring the interference intensity and analyzing the change over time.

JP 2004-138524 A

  However, this method requires a complicated operation such as Fourier transform to analyze the time change. Also, it is necessary to increase the number of interference intensity samplings as much as possible.

  Accordingly, the present invention provides an inter-pulse phase shift measuring device, an offset frequency control device, an inter-pulse phase shift measuring method, and an offset frequency control method capable of measuring an inter-pulse phase shift of an optical frequency comb light source with high efficiency. With the goal.

  One aspect of an inter-pulse phase shift measuring apparatus exemplifying the present invention generates a combined optical pulse signal by splitting an optical pulse signal oscillated by an optical frequency comb light source into a pair of optical pulse signals and then reintegrating them. The oscillation frequency of the optical frequency comb light source and the pair of optical pulses so that a pulse shift number N between the optical path setting means and the pair of optical pulse signals included in the combined optical pulse signal is an integer of 1 or more. Pulse shift means for setting the relationship with the optical path length difference of signals, and phase shift means for setting the phase shift amount between the pair of optical pulse signals to M phase shift amounts (M: an integer of 3 or more). Detecting means for detecting M interference intensities indicated by the pair of optical pulse signals under the M phase shift amounts based on the combined optical pulse signal, and the relationship between the M interference intensities and M phase shifts Based on the amount of relationship, and a computing means for the optical frequency comb source is calculated inter-pulse phase shift of the optical pulse signal oscillated.

  An aspect of the offset frequency control apparatus illustrating the present invention is an aspect of an inter-pulse phase deviation measuring apparatus that measures an inter-pulse phase deviation of an optical pulse signal oscillated by an optical frequency comb light source, and the inter-pulse phase deviation measuring apparatus. And a control device that controls the offset frequency of the optical frequency comb light source in accordance with the phase shift between the pulses measured by.

  According to one aspect of the method for measuring the phase shift between pulses, the optical pulse signal generated by the optical frequency comb light source is branched into a pair of optical pulse signals and then reintegrated to generate a combined optical pulse signal. The oscillation period of the optical frequency comb light source and the pair of optical pulses so that a pulse shift number N between the optical path setting procedure and the pair of optical pulse signals included in the combined optical pulse signal is an integer of 1 or more. A pulse shift procedure for setting the relationship with the optical path length difference of the signal, and a phase shift procedure for setting the phase shift amount between the pair of optical pulse signals to M phase shift amounts (M: an integer of 3 or more). A detection procedure for detecting M interference intensities indicated by the pair of optical pulse signals based on the combined optical pulse signal under the M phase shift amounts, a relationship between the M interference intensities, and the M phase shifts Based on the amount of relationship, the optical frequency comb source and a calculation procedure for calculating a pulse-to-pulse phase shift of the optical pulse signal oscillated.

  One aspect of the offset frequency control method exemplifying the present invention includes a measurement procedure for measuring an inter-pulse phase shift of an optical pulse signal oscillated by an optical frequency comb light source by one aspect of the inter-pulse phase shift measurement method, and the measurement procedure. And a control procedure for controlling the offset frequency of the optical frequency comb light source in accordance with the phase shift between pulses measured in (1).

  According to the present invention, there is provided an inter-pulse phase shift measuring device, an offset frequency control device, an inter-pulse phase measuring method, and an offset frequency control method capable of measuring an inter-pulse phase shift of an optical frequency comb light source with high efficiency. With the goal.

It is a whole block diagram of the offset frequency control apparatus 100 of 1st Embodiment. It is a schematic diagram of an optical pulse signal. It is a schematic diagram of an optical frequency comb. It is a schematic diagram of the optical pulse signals Sa and Sb when the pulse shift number N is zero. It is a schematic diagram of the optical pulse signals Sa and Sb when the pulse shift number N is 1. It is a whole block diagram of the offset frequency control apparatus 200 of 2nd Embodiment. It is a figure explaining operation | movement of 2nd Embodiment. It is a whole block diagram of the offset frequency control apparatus 300 of 3rd Embodiment.

[First Embodiment]
Hereinafter, an offset frequency control apparatus will be described as a first embodiment of the present invention.

  FIG. 1 is an overall configuration diagram of an offset frequency control apparatus 100 according to the present embodiment. As shown in FIG. 1, the offset frequency control device 100 includes a beam splitter 12, fixed reflectors 13 and 14, a polarizing beam splitter 15, photodetectors 16-0, 16-1, and 16-2, and 1/8. A wave plate 17, a phase calculation unit 18, and an offset control unit 19 are provided. The offset frequency control device 100 has a function of controlling the offset frequency of the optical frequency comb light source 11.

  The optical frequency comb light source 11 is an optical frequency comb light source such as a femtosecond optical frequency comb light source or an attosecond optical frequency comb light source, and oscillates an optical pulse whose pulse width is narrowed to femtoseconds or attoseconds. Oscillates repeatedly at period T. Note that FIG. 2 is a schematic diagram of an optical pulse train (optical pulse signal) oscillated from the optical frequency comb light source 11.

The optical frequency comb, which is the Fourier transform of this optical pulse signal, has a constant mode interval (repetition frequency f _rep ) as shown in FIG. The repetition frequency f _rep is the reciprocal (1 / T) of the pulse oscillation period T, for example 50 MHz.

In FIG. 2, reference symbol Δψ _CEO indicates an inter-pulse phase shift of the carrier envelope generated in the optical pulse signal. This inter-pulse phase shift Δψ _CEO determines the offset frequency f _CEO of the optical frequency comb. The relationship between the offset phase shift Δψ _CEO of the optical pulse signal and the offset frequency f _CEO of the optical frequency comb is expressed by a conversion equation (2) described later.

  As shown in FIG. 1, the optical pulse signal S oscillated from the optical frequency comb light source 11 branches into an optical pulse signal Sa that reflects the beam splitter 12 and an optical pulse signal Sb that transmits the beam splitter 12.

  When one optical pulse signal Sa passes through the 1/8 wavelength plate 17 and then goes to the fixed reflecting mirror 13, it is reflected by the fixed reflecting mirror 13 and then passes again through the 1/8 wavelength plate 17 to the beam splitter 12. Return.

  When the other optical pulse signal Sb goes to the fixed reflecting mirror 14, it is reflected by the fixed reflecting mirror 14 and then returns to the beam splitter 12.

  The pair of optical pulse signals Sa and Sb that have returned to the beam splitter 12 are integrated on the beam splitter 12. Hereinafter, the integrated pair of optical pulse signals Sa and Sb are collectively referred to as a combined optical signal. The combined optical signal is branched into a combined optical signal Sc that passes through the beam splitter 12 and a combined optical signal Sd that reflects off the beam splitter 12.

  One synthesized optical signal Sc is incident on the photodetector 16-2. The other combined optical signal Sd enters the polarizing beam splitter 15 and branches into a combined optical signal Se that reflects from the polarizing beam splitter 15 and a combined optical signal Sf that transmits through the polarizing beam splitter 15. One of the combined light signals Se enters the photodetector 16-0, and the other combined light signal Sf enters the photodetector 16-1. Therefore, in the present embodiment, the combined optical signal composed of the pair of optical pulse signals Sa and Sb is branched into three systems of combined optical signals Sc, Se, and Sf.

  Here, consider a case where the fixed reflecting mirror 14 is disposed at a position indicated by a dotted line in FIG. This position is a position at which the optical path length difference between the pair of optical pulse signals Sa and Sb becomes zero.

  In this case, in each of the combined optical signals Sc, Se, and Sf, as shown in FIG. 4, the i-th pulse of one optical pulse signal Sa and the i-th pulse of the other optical pulse signal Sb overlap each other. That is, the pulse shift number N between the pair of optical pulse signals Sa and Sb is zero. In this case, the pair of optical pulse signals Sa and Sb interfere with each other in each of the combined optical signals Sc, Se, and Sf.

  However, in the present embodiment, as indicated by a solid line in FIG. 1, the arrangement destination of the fixed reflecting mirror 14 is set to a position where the optical path length difference between the pair of optical pulse signals Sa and Sb is a value other than zero. ing.

  In this case, in each of the combined optical signals Sc, Se, and Sf, each pulse of one optical pulse signal Sa and each pulse of the other optical pulse signal Sb do not necessarily overlap each other. That is, the pulse shift number N between the pair of optical pulse signals Sa and Sb is not always zero. In this case, the pair of optical pulse signals Sa and Sb does not always interfere with each other in each of the combined optical signals Sc, Se, and Sf.

However, in the present embodiment, the arrangement destination of the fixed reflecting mirror 14 is set so that the pulse shift number N between the pair of optical pulse signals Sa and Sb becomes an integer of 1 or more. In this case, the optical path length difference between the pair of optical pulse signals Sa and Sb is set to a distance of N pulses (N · C / f _rep ). C is the speed of light.

  In this case, in each of the combined optical signals Sc, Se, and Sf, as shown in FIG. 5, the i-th pulse of one optical pulse signal Sb and the (i + N) -th pulse of the other optical pulse signal Sa overlap each other. (Note that the example shown in FIG. 5 is for N = 1.) In this case, the pair of optical pulse signals Sa and Sb interfere with each other in each of the combined optical signals Sc, Se, and Sf.

  Therefore, in the present embodiment, the interference intensity of the pair of optical pulse signals Sa and Sb is reflected in each of the combined optical signals Sc, Se, and Sf.

In addition, if the number of pulse shifts N is set to an integer equal to or greater than 1 (see FIG. 5), the inter-pulse phase shift Δψ _CEO is proportional to between the interfering pulses of the pair of optical pulse signals Sa and Sb. Since the phase difference (N · Δψ _CEO ) is given, the value of the inter-pulse phase deviation Δψ _CEO is reflected in each of the combined optical signals Sc, Se, and Sf.

Incidentally, if, if a pulse shift number N is 0 (see FIG. 4), a pair of optical pulse signals Sa, during the pulse between interfere with each other among Sb, positions in proportion to the pulse between the phase shift [Delta] [phi] _CEO Since no phase difference (N · Δψ _CEO ) is given, the value of the inter-pulse phase deviation Δψ _CEO is not reflected in each of the combined optical signals Sc, Se, and Sf.

  Furthermore, in the present embodiment, due to the arrangement of the 1/8 wavelength plate 17 and the polarization beam splitter 15, the following difference is made between the combined optical signal Sc, the combined optical signal Se, and the combined optical signal Sf. . That is, the phase shift amount between the pair of optical pulse signals Sa and Sb included in the combined optical signal Se is set to 0, whereas the pair of optical pulse signals Sa and Sb included in the combined optical signal Sf. Is set to π / 2, and the phase shift amount between the pair of optical pulse signals Sa and Sb included in the combined optical signal Sc is set to π.

  Therefore, the photodetector 16-0 can detect the interference intensity of the optical pulse signals Sa and Sb whose phase shift amount is 0 as the combined optical signal Se.

  Further, the photodetector 16-1 can detect the interference intensity of the optical pulse signals Sa and Sb having a phase shift amount of π / 2 as the combined optical signal Sf.

  Further, the photodetector 16-2 can detect the interference intensity of the optical pulse signals Sa and Sb having a phase shift amount of π as the combined optical signal Sc.

  Therefore, the photodetectors 16-0, 16-1, and 16-2 of the present embodiment simultaneously measure the interference intensities of the optical pulse signals Sa and Sb when the phase shift amounts are 0, π / 2, and π, respectively. Can be detected (in parallel).

The photodetector 16-1 is appropriately driven by the phase calculation unit 18 and outputs a signal value _INO . This signal value I _N0 is a time integral value over a predetermined time of the combined optical signal Se. The integration time of the photodetector 16-0 is set to a time sufficiently longer than the pulse oscillation period T = 1 / f _rep .

The photodetector 16-1 is appropriately driven by the phase calculation unit 18 and outputs a signal value I _N1 . This signal value I _N1 is a time integration value over a predetermined time of the combined optical signal Sf. The integration time of the photo detector 16-1 is set equal to that of the photo detector 16-0.

The photodetector 16-2 is appropriately driven by the phase calculation unit 18 and outputs a signal value I _N2 . This signal value I _N2 is a time integration value over a predetermined time of the combined optical signal Sc. The integration time of the photo detector 16-2 is set equal to that of the photo detector 16-0.

In this embodiment, the pair of optical pulse signals Sa is applied so that the signal values I _N0 , I _N1 , and I _N2 output from the photodetectors 16-0, 16-1, and 16-2 can be directly applied to an arithmetic expression described later. , Sb amplitude combinations are assumed to be common among the combined optical signals Se, Sf, Sb. If this is made common, the DC component and the amplitude γ can be made common among the combined optical signals Se, Sf, and Sb.

The phase calculation unit 18 drives the photodetectors 16-0, 16-1, and 16-2, and outputs signal values I _N0 , I _N1 , and I that are output in parallel from the photodetectors 16-0, 16-1, and 16-2. _By taking in _N2 and applying it to the following arithmetic expression (1), the current inter-pulse phase shift Δψ _CEO is calculated and output to the offset control unit 19.

なお、この演算式（１）は、位相シフト量の組み合わせが（０、π／２、π）の組み合わせであった場合に適用される演算式である。演算式（１）の説明は、後述する。

This arithmetic expression (1) is an arithmetic expression applied when the combination of phase shift amounts is a combination of (0, π / 2, π). The description of the arithmetic expression (1) will be described later.

Further, the offset control unit 19 converts the inter-pulse phase shift Δψ _CEO to the offset frequency f _CEO by applying the inter-pulse phase shift Δψ _CEO sent from the phase calculation unit 18 to the following conversion equation (2). As a result, the current offset frequency f _CEO becomes known.

そして、オフセット制御部１９は、現時点のオフセット周波数ｆ_ＣＥＯを予め決められた目標値と比較し、現時点のオフセット周波数ｆ_ＣＥＯと目標値との差分を算出すると、その差分が小さくなる方向に、光周波数コム光源１１に含まれる共振器の光路長差を、調整する。なお、共振器（レーザ媒質を含む）の光路差を微調整するには、共振器におけるウェッジプレート（ガラス板であって、プリズム形状）を抜き差しすればよい。但し、その調整は、ＦＳＲ（自由スペクトル領域）の範囲内で行われるものとする。これによって、光周波数コム光源１１のオフセット周波数ｆ_ＣＥＯは、目標値に近づけられる。

Then, the offset control unit 19 compares the current offset frequency f _CEO with a predetermined target value, and calculates the difference between the current offset frequency f _CEO and the target value. The optical path length difference of the resonator included in the frequency comb light source 11 is adjusted. In order to finely adjust the optical path difference of the resonator (including the laser medium), a wedge plate (a glass plate and a prism shape) in the resonator may be inserted and removed. However, the adjustment is performed within the range of FSR (free spectrum region). As a result, the offset frequency f _CEO of the optical frequency comb light source 11 is brought close to the target value.

Furthermore, in the present embodiment, a series of processes (loops) including the driving of the photodetectors 16-0, 16-1, and 16-2, the calculation by the phase calculation unit 18, and the adjustment by the offset control unit 19 described above. ) Is repeated at an appropriate frequency. As a result, the offset frequency f _CEO of the optical frequency comb light source 11 is feedback-controlled and stabilized near the target value.

  As described above, in the offset frequency control apparatus 100 of this embodiment, the pulse oscillation of the optical frequency comb light source 11 is performed so that the pulse shift number N between the pair of optical pulse signals Sa and Sb becomes an integer of 1 or more. The relationship between the period T and the optical path length of the pair of optical pulse signals Sa and Sb is set in advance.

The offset frequency control apparatus 100 of the present embodiment sets the phase shift amount between the pair of optical pulse signals Sa and Sb to three phase shift amounts 0, π / 2, and π, The relationship between the three signal values I _N0 , I _N1 , and I _N2 acquired under the phase shift amounts 0, π / 2, and π, and the relationship between these three phase shift amounts 0, π / 2, and π ( Here, based on the calculation formula (1)), the current inter-pulse phase deviation Δψ _CEO is calculated.

Therefore, the offset frequency control apparatus 100 of the present embodiment simply applies the three signal values I _N0 , I _N1 , and I _N2 to the predetermined arithmetic expression (1) in order to measure the inter-pulse phase shift Δψ _CEO. Only computation is required. Therefore, the offset frequency control apparatus 100 of this embodiment can measure the inter-pulse phase shift Δψ _CEO with high efficiency, and can perform feedback control of the offset frequency f _CEO at a high response speed.

In addition, the offset frequency control apparatus 100 according to the present embodiment sets the pair of optical pulse signals Sa and Sb when setting the phase shift amount between the pair of optical pulse signals Sa and Sb to a plurality of kinds of phase shift amounts. By adding a difference in the polarization state between the two, the combined optical signal is branched into a plurality of systems of combined optical signals, and the phase difference between the pair of optical pulse signals Sa and Sb is shifted between the plurality of systems of combined optical signals. . Therefore, the offset frequency control apparatus 100 of the present embodiment can simultaneously acquire (in parallel) a plurality of signal values necessary for calculation. Therefore, the offset frequency control device 100 of the present embodiment, the measurement time of the pulse between the phase shift [Delta] [phi] _CEO, the processing time of each loop of the thus feedback control can be very short.

  In this embodiment, the pulse shift number N between the pair of optical pulse signals Sa and Sb is set to an integer of 1 or more. However, if the pair of optical pulse signals Sa and Sb can interfere with each other, The pulse shift number N may be slightly deviated from the integer (that is, the pulse shift number N may be a value represented by (integer + τ) with respect to a minute value τ). That is, in this embodiment, there is an advantage that the accuracy required for the alignment of the fixed reflecting mirror 14 is low.

  Further, in the present embodiment, the offset frequency control device 100 can be made compact by reducing the distance from the beam splitter 12 to the fixed reflecting mirror 14 as much as possible. Therefore, when downsizing of the offset frequency control apparatus 100 is required, it is desirable to set the pulse shift number N between the pair of optical pulse signals Sa and Sb to 1.

  In the present embodiment, the combination of phase shift amounts is a combination of (0, π / 2, π), but other combinations may be used. For example, a combination of (π, 3π / 2, 2π), a combination of (0, π / 4, π / 2, 3π / 4), or a combination of (0, π / 2, π, 3π / 2) may be used. . The number M of phase shift steps may be three or more. However, it is assumed that the above-described arithmetic expression (1) is appropriately changed according to the employed combination.

[Explanation of Equation (1)]
Hereinafter, the arithmetic expression (1) will be described.

The signal values I _N0 , I _N1 , and I _N2 included in the arithmetic expression (1) are the combined optical signal Se (the combined optical signal with the phase shift amount 0) and the combined optical signal Sf (the combined light with the phase shift amount π / 2). Signal) and the combined optical signal Sc (the combined optical signal with the phase shift amount π), and are expressed by the following equation (3).

但し、ＤＣは、合成光信号Ｓｅ、Ｓｆ、ＳｃのＤＣ成分であり、γ（τ）は、合成光信号Ｓｅ、Ｓｆ、Ｓｃの振幅である（なお、τは、１対の光パルス信号Ｓａ、Ｓｂのうち互いに干渉するパルス同士のズレである。）。

However, DC is the DC component of the combined optical signal Se, Sf, Sc, and γ (τ) is the amplitude of the combined optical signal Se, Sf, Sc (where τ is a pair of optical pulse signals Sa. , Sb is a deviation between pulses that interfere with each other.

  Therefore, the following formula (4) is established.

したがって、以下の式（５）が成り立つ。

Therefore, the following formula (5) is established.

この式（５）から、上述した演算式（１）が導出されるのは明らかである。

It is clear that the above-described arithmetic expression (1) is derived from this expression (5).

  As a result of the above, it is clear that the arithmetic expression (1) can be used when the combination of phase shift amounts is a combination of (0, π / 2, π).

[Second Embodiment]
Hereinafter, an offset frequency control apparatus will be described as a second embodiment of the present invention. Since this embodiment is a modification of the first embodiment, only differences from the first embodiment will be described.

  FIG. 6 is an overall configuration diagram of the offset frequency control apparatus 200 of the present embodiment. As shown in FIG. 6, the offset frequency control device 200 of the present embodiment is the same as the offset frequency control device 100 of the first embodiment, except that the ８ wavelength plate 17 and the polarization beam splitter 15 are omitted, and three photodetectors 16-0. , 16-1 and 16-0 are provided with a single photodetector 16, and a fixed reflecting mirror 14 is provided with a moving reflecting mirror 14a. It is assumed that the position calculation function of the movable reflecting mirror 14a is also added to the phase calculation unit 18 of the present embodiment.

  The optical pulse signal S emitted from the optical frequency comb light source 11 branches into an optical pulse signal Sa that reflects the beam splitter 12 and an optical pulse signal Sb that transmits the beam splitter 12.

  When one optical pulse signal Sa goes to the fixed reflecting mirror 13, it is reflected by the fixed reflecting mirror 13 and then returns to the beam splitter 12. When the other optical pulse signal Sb goes to the moving reflecting mirror 14a, it is reflected by the moving reflecting mirror 14a and then returns to the beam splitter 12.

  The pair of optical pulse signals Sa and Sb that have returned to the beam splitter 12 are integrated on the beam splitter 12. Hereinafter, the integrated pair of optical pulse signals Sa and Sb are collectively referred to as a combined optical signal Sg. The combined optical signal Sg enters the photodetector 16.

The photodetector 16 is appropriately driven by the phase calculator 18 and outputs a signal value I. This signal value I is a time integration value over a predetermined time of the combined optical signal Sg. The integration time of the photodetector 16 is set to a time sufficiently longer than the pulse oscillation period T = 1 / f _rep .

  And the phase calculating part 18 of this embodiment switches the pulse shift number N between 1 pair of optical pulse signals Sa and Sb, in order to perform the phase shift of 1 pair of optical pulse signals Sa and Sb. For this purpose, the phase calculation unit 18 of this embodiment switches the optical path length difference between the pair of optical pulse signals Sa and Sb while fixing the pulse oscillation period T of the optical frequency comb light source 11.

Here, the pulse shift number N is assumed to be switched between 1, 2, 3, and 4. In that case, the phase calculation unit 18 of the present embodiment changes the optical path length difference between the pair of optical pulse signals Sa and Sb by switching the placement destination of the movable reflecting mirror 14a between the four positions. Minute distance (C / f _rep ), distance for two pulses (2 · C / f _rep ), distance for three pulses (3 · C / f _rep ), distance for four pulses (4 · C / F _rep ).

Further, the phase calculation unit 18 of the present embodiment drives the photodetector 16 when the pulse shift number N is 1 captures the signal value I ₁ with (see FIG. 7 (a).), Pulse shift number N is 2 It drives the photodetector 16 captures the signal value I ₂ in (see FIG. 7 (b).) when it is, to drive the photodetector 16 when the pulse shift number N is 3 captures the signal value I ₃ to (not When the pulse shift number N is 4, the photodetector 16 is driven to take in the signal value I ₄ (not shown).

Further, the phase calculation unit 18 of the present embodiment calculates the current inter-pulse phase shift Δψ _CEO by applying the signal values I ₁ , I ₂ , I ₃ , and I ₄ to the following calculation formula (6). And output to the offset control unit 19. The operation of the offset control unit 19 is the same as that of the first embodiment.

なお、この演算式（６）は、パルスシフト数Ｎの組み合わせが（１、２、３、４）の組み合わせであった場合に適用される演算式である。演算式（６）の説明は、後述する。

This calculation formula (6) is a calculation formula applied when the combination of the pulse shift numbers N is a combination of (1, 2, 3, 4). The description of the arithmetic expression (6) will be described later.

  Also in the present embodiment, a series of processes (loops) including the switching of the pulse shift number N and the multiple driving of the photodetector 16, the calculation by the phase calculation unit 18, and the adjustment by the offset control unit 19 as described above. ) Is repeated at an appropriate frequency.

  Therefore, the offset frequency control apparatus 200 according to this embodiment switches the phase shift between the pair of optical pulse signals Sa and Sb by switching the pulse shift number N (specifically, 1) instead of branching the combined optical signal. This is different from the offset frequency control apparatus 100 of the first embodiment in that it is performed by switching the optical path length difference between the pair of optical pulse signals Sa and Sb.

  Since switching of the pulse shift number N requires a certain time, each loop of the feedback control of the present embodiment requires more time than that of the first embodiment. However, as long as the switching speed of the pulse shift number N can be increased, an effect similar to that of the first embodiment can be obtained in this embodiment.

  In the present embodiment, the pulse shift number N between the pair of optical pulse signals Sa and Sb is switched between the four pulse shift numbers 1, 2, 3, and 4. However, the four pulse shift numbers If the interval is exactly 1, each pulse shift number may be slightly deviated from the integer (that is, the four pulse shift numbers are (1 + τ) with respect to a minute value τ, (It may be a value represented by (2 + τ), (3 + τ), (4 + τ)).

  In the present embodiment, the combination of the pulse shift numbers N is (1, 2, 3, 4), but other combinations may be used. For example, a combination of (2, 3, 4, 5), a combination of (3, 4, 5, 6), or a combination of (0, 1, 2, 3) may be used. The number M of phase shift steps may be four or more. However, the arithmetic expression (6) is appropriately changed according to the employed combination.

Incidentally, when the combination of the pulse shift numbers N is a combination of (0, 1, 2, 3), that is, instead of the signal value I ₄ when the pulse shift number is 4, the pulse shift number is 0. When acquiring the signal value I ₀ at a certain time, the arithmetic expression (7) may be used instead of the arithmetic expression (6).

なお、この演算式（７）は、パルスシフト数Ｎの組み合わせが（０、１、２、３）の組み合わせであった場合に適用される演算式である。演算式（７）の説明は、後述する。

This arithmetic expression (7) is an arithmetic expression applied when the combination of the pulse shift numbers N is a combination of (0, 1, 2, 3). The description of the arithmetic expression (7) will be described later.

[Explanation of Equation (6)]
Hereinafter, the arithmetic expression (6) will be described.

The signal values I ₁ , I ₂ , I ₃ , and I ₄ included in the arithmetic expression (6) are the combined light signal Sg when the pulse shift number N is 1 and the combined light when the pulse shift number N is 2. The signal Sg, the combined optical signal Sg when the pulse shift number N is 3, and the combined optical signal Sg when the pulse shift number N is 4, are expressed by the following equation (8).

但し、ＤＣは、合成光信号ＳｇのＤＣ成分であり、γ（τ）は、合成光信号Ｓｇの振幅である（なお、τは、１対の光パルス信号Ｓａ、Ｓｂのうち互いに干渉するパルス同士のズレである。）。

However, DC is a DC component of the combined optical signal Sg, and γ (τ) is the amplitude of the combined optical signal Sg (where τ is a pulse that interferes with each other in the pair of optical pulse signals Sa and Sb. Misalignment between each other.)

  Therefore, the following formula (9) is established.

したがって、以下の式（１０）が成り立つ。

Therefore, the following formula (10) is established.

これらの式（９）、（１０）より以下の式（１１）が導出される。

From these equations (9) and (10), the following equation (11) is derived.

この式（１１）を変形すると、演算式（６）が導出されるのは明らかである。

It is clear that the equation (6) is derived by transforming the equation (11).

  As a result of the above, it is clear that the arithmetic expression (6) can be used when the combination of the pulse shift numbers N is a combination of (1, 2, 3, 4).

[Explanation of Equation (7)]
Hereinafter, the arithmetic expression (7) will be described.

The signal values I ₀ , I ₁ , I ₂ , and I ₃ included in the arithmetic expression (7) are the combined light signal Sg when the pulse shift number N is 0, and the combined light when the pulse shift number N is 1. The signal Sg, the combined optical signal Sg when the pulse shift number N is 2, and the combined optical signal Sg when the pulse shift number N is 3, are expressed by the following equation (12).

但し、ＤＣは、合成光信号ＳｇのＤＣ成分であり、γ（τ）は、合成光信号Ｓｇの振幅である（なお、τは、１対の光パルス信号Ｓａ、Ｓｂのうち互いに干渉するパルス同士のズレである。）。

However, DC is a DC component of the combined optical signal Sg, and γ (τ) is the amplitude of the combined optical signal Sg (where τ is a pulse that interferes with each other in the pair of optical pulse signals Sa and Sb. Misalignment between each other.)

  Therefore, the following equation (13) is established.

また、以下の式（１４）も成り立つ。

Moreover, the following formula | equation (14) is also formed.

これらの式（１３）、（１４）より以下の式（１５）が導出される。

From these equations (13) and (14), the following equation (15) is derived.

この式（１５）を変形すると、上述した演算式（７）が導出されるのは明らかである。

It is clear that the equation (7) described above is derived by transforming the equation (15).

As a result of the above, it is clear that the arithmetic expression (7) can be used when the combination of the pulse shift numbers N is a combination of (0, 1, 2, 3).
[Third Embodiment]
Hereinafter, an offset frequency control apparatus will be described as a third embodiment of the present invention. Since this embodiment is a modification of the second embodiment, only differences from the second embodiment will be described.

FIG. 8 is an overall configuration diagram of the offset frequency control device 300 of the present embodiment. As shown in FIG. 8, the offset frequency control device 300 of the present embodiment is the same as the offset frequency control device 200 of the second embodiment, but includes a fixed reflecting mirror 14 instead of the moving reflecting mirror 14a. In addition, it is assumed that a control function of the repetition frequency f _rep of the optical frequency comb light source 11 is added to the phase calculation unit 18 of the present embodiment.

  In the present embodiment, the behavior of the pair of optical pulse signals Sa and Sb is the same as that in the second embodiment. However, in the present embodiment, the arrangement destination of the fixed reflecting mirror 14 is fixed at a position where the optical path length difference between the pair of optical pulse signals Sa and Sb becomes an arbitrary value other than zero.

  The phase calculation unit 18 of the present embodiment also switches the pulse shift number N between the pair of optical pulse signals Sa and Sb in order to perform the phase shift of the pair of optical pulse signals Sa and Sb.

However, in order to switch the pulse shift number N, the phase calculation unit 18 of this embodiment sets the pulse oscillation period T of the optical frequency comb light source 11 while fixing the optical path length difference between the pair of optical pulse signals Sa and Sb. (Ie, the repetition frequency f _rep of the optical frequency comb light source 11 is switched).

Again, the pulse shift number N is assumed to be switched between 1, 2, 3, and 4. In that case, the phase calculation unit 18 of the present embodiment sets the repetition frequency f _rep of the optical frequency comb light source 11 to the first frequency f _rep1 , the second frequency f _rep2 , the third frequency f _rep3 , and the fourth frequency f. Switch between _rep4 .

The first frequency f _rep1 is a frequency at which the pulse shift number N is 1, that is, the i-th pulse of the optical pulse signal Sb and the (i + 1) -th pulse of the optical pulse signal Sa in the combined optical signal Sg. Overlapping frequency.

The second frequency f _rep2 is a frequency at which the pulse shift number N is 2, that is, the i-th pulse of the optical pulse signal Sb and the (i + 2) -th pulse of the optical pulse signal Sa in the combined optical signal Sg. Are frequencies that overlap each other.

The third frequency f _rep3 is a frequency at which the pulse shift number N is 3, that is, the i-th pulse of the optical pulse signal Sb and the (i + 3) -th pulse of the optical pulse signal Sa in the combined optical signal Sg. Are frequencies that overlap each other.

The fourth frequency f _rep4 is a frequency at which the pulse shift number N is 4, that is, the i-th pulse of the optical pulse signal Sb and the (i + 4) -th pulse of the optical pulse signal Sa in the combined optical signal Sg. Are frequencies that overlap each other.

That is, the offset frequency control apparatus 300 of the present embodiment switches the pulse shift number N not the switching of the optical path length difference between the pair of optical pulse signals Sa and Sb, but the repetition frequency f _rep of the optical frequency comb light source 11. It differs from the offset frequency control device 200 of the second embodiment in that it is performed by switching.

Here, the switching of the repetition frequency f _rep of the optical frequency comb light source 11 is easier than the switching of the optical path length difference between the pair of optical pulse signals Sa and Sb. Therefore, in the present embodiment, the measurement time of the inter-pulse phase shift Δψ _CEO (that is, the processing time of each loop of feedback control) can be shortened compared to that of the second embodiment.

  In the present embodiment, the pulse shift number N between the pair of optical pulse signals Sa and Sb is switched between the four pulse shift numbers 1, 2, 3, and 4. However, the four pulse shift numbers If the interval is exactly 1, each pulse shift number may be slightly deviated from the integer (that is, the four pulse shift numbers are (1 + τ) with respect to a minute value τ, (It may be a value represented by (2 + τ), (3 + τ), (4 + τ)).

  In the present embodiment, the combination of the pulse shift numbers N is (1, 2, 3, 4), but other combinations may be used. For example, a combination of (2, 3, 4, 5) or a combination of (3, 4, 5, 6) may be used. The number M of phase shift steps may be four or more. However, the arithmetic expression (6) is appropriately changed according to the employed combination.

[Supplement of each embodiment]
Note that the offset frequency control device according to any of the embodiments can be mounted on various devices that can be applied to the optical frequency comb light source, such as a spectroscopic device or a length measuring device.

  DESCRIPTION OF SYMBOLS 100, 200, 300 ... Offset frequency control apparatus, 11 ... Optical frequency comb light source, 12 ... Beam splitter, 13, 14 ... Fixed reflector, 15 ... Polarizing beam splitter, 16 ... Photo detector, 17 ... 1/8 wavelength plate, 18 ... Phase calculation unit, 19 ... Offset control unit

Claims (11)

An optical path setting means for generating a combined optical pulse signal by branching the optical pulse signal generated by the optical frequency comb light source into a pair of optical pulse signals and then reintegrating the optical pulse signal;
Difference in optical path length between the oscillation period of the optical frequency comb light source and the pair of optical pulse signals so that a pulse shift number N between the pair of optical pulse signals included in the combined optical pulse signal is an integer of 1 or more. A pulse shift means for setting the relationship between
Phase shift means for setting the phase shift amount between the pair of optical pulse signals to M phase shift amounts (M: an integer of 3 or more);
Detecting means for detecting M interference intensities indicated by the pair of optical pulse signals based on the combined optical pulse signal under the M phase shift amounts;
Calculating means for calculating a phase shift between pulses of an optical pulse signal oscillated by the optical frequency comb light source based on the relationship between the M interference intensity and the relationship between the M phase shift amounts. To measure the phase deviation between pulses. In the inter-pulse phase shift measuring device according to claim 1,
The phase shift means includes
The synthesized optical pulse signal is branched into M-system signals, and the M phase shift amounts are simultaneously set by making the phase difference of the pair of optical pulse signals different between the M-system signals. An apparatus for measuring a phase deviation between pulses. In the inter-pulse phase shift measuring device according to claim 2,
The phase shift means includes
By branching the combined optical pulse signal into three signals and setting the phase difference of the three signals to three phase differences of 0, π / 2, and π, three phase shift amounts can be obtained. Set at the same time,
The computing means is
By applying the three interference intensities I _N0 , I _N1 , and I _N2 detected under the three phase differences of 0, π / 2, and π to the following equation (1), the phase shift between pulses is calculated. Calculate Δψ _CEO

An apparatus for measuring a phase deviation between pulses. In the inter-pulse phase shift measuring device according to claim 1,
The phase shift means includes
The M phase shift amount is sequentially set by switching the pulse shift number N between the pair of optical pulse signals between the M pulse shift numbers. apparatus. In the inter-pulse phase shift measuring device according to claim 4,
The phase shift means includes
By switching the pulse shift number N between four pulse shift numbers 1, 2, 3, and 4, the four phase shift amounts are sequentially set,
The computing means is
By applying the four interference intensities I ₁ , I ₂ , I ₃ , and I ₄ detected under the four pulse shift numbers ₁ , ₂ , ₃ , and ₄ to the following equation (6), Calculate the phase shift between pulses Δψ _CEO

An apparatus for measuring a phase deviation between pulses. In the inter-pulse phase shift measuring device according to claim 5,
The phase shift means includes
By setting the pulse shift number N to 0 instead of 4, the four pulse shift numbers 0, 1, 2, 3 are set in sequence,
The computing means is
By applying the four interference intensities I ₀ , I ₁ , I ₂ , and I ₃ detected under the four pulse shift numbers ₀ , ₁ , ₂ , and ₃ to the following equation (7), Calculate the phase shift between pulses Δψ _CEO

An apparatus for measuring a phase deviation between pulses. In the inter-pulse phase shift measuring device according to claim 4,
The phase shift means includes
The inter-pulse phase shift measuring apparatus, wherein the pulse shift number N is switched by switching an optical path length difference between the pair of optical pulse signals while fixing a pulse oscillation period of the optical frequency comb light source. In the inter-pulse phase shift measuring device according to claim 4 or 5,
The phase shift means includes
The pulse shift number N is switched by switching the pulse oscillation period of the optical frequency comb light source while fixing the optical path length difference between the pair of optical pulse signals to a value other than zero. measuring device. The inter-pulse phase shift measuring device according to any one of claims 1 to 8, which measures an inter-pulse phase shift of an optical pulse signal oscillated by an optical frequency comb light source,
An offset frequency control device comprising: a control device that controls an offset frequency of the optical frequency comb light source in accordance with an inter-pulse phase shift measured by the inter-pulse phase shift measurement device. An optical path setting procedure for generating a combined optical pulse signal by branching the optical pulse signal oscillated by the optical frequency comb light source into a pair of optical pulse signals and then reintegrating them;
Difference in optical path length between the oscillation period of the optical frequency comb light source and the pair of optical pulse signals so that a pulse shift number N between the pair of optical pulse signals included in the combined optical pulse signal is an integer of 1 or more. A pulse shift procedure to set the relationship between
A phase shift procedure for setting the phase shift amount between the pair of optical pulse signals to M phase shift amounts (M: an integer of 3 or more);
A detection procedure for detecting M interference intensities indicated by the pair of optical pulse signals under the M phase shift amounts based on the combined optical pulse signal;
And a calculation procedure for calculating an inter-pulse phase shift of an optical pulse signal oscillated by the optical frequency comb light source based on the relationship between the M interference intensity and the relationship between the M phase shift amounts. To measure the phase deviation between pulses. A measurement procedure for measuring an inter-pulse phase shift of an optical pulse signal oscillated by an optical frequency comb light source by the inter-pulse phase shift measuring method according to claim 10,
A control procedure for controlling the offset frequency of the optical frequency comb light source in accordance with the phase shift between pulses measured in the measurement procedure.

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