KR101674795B1 - System and method of measuring a free spectral range of a fabry-perot etalon - Google Patents

Abstract

A system and method for precisely measuring the free motion region of a Februrit etalon is disclosed. The free-running area measuring system includes a PM modulating unit PM-modulating a low-frequency scanning CW laser to fabricate a Fabry-Perot Etalon (FPE), and a PM modulator for transmitting the ferri- A photodetector for detecting a transmitted signal or a reflected signal reflected by the Febraryot etalon, and a detector for detecting the free motion of the Febrary etalon using the detection result and the modulated signal used for the PM modulation Range, FSR).

Description

FIELD AND METHOD OF MEASURING A FREE SPECTRAL RANGE OF A FABRY-PEROT ETALON BACKGROUND OF THE INVENTION [0001]

FIELD OF THE INVENTION The present invention relates to a system and method for measuring the free motion area of a Febraryot etalon.

Ferrierot etalon is widely used in various fields such as various filter implementations, optical frequency stabilization, etc. due to the periodic frequency passing characteristics of free motion region intervals.

In recent years, there have been found cases of application to oscillation mode and frequency stabilization of mode locked lasers by using the periodic frequency pass filter characteristics of the Febriferot etalon. At this time, the mode locking frequency of the laser should coincide with the free motion region of the Febrary ferrite etalon, and therefore accurate measurement of the free motion region is required.

There are various methods of measuring the free motion region, but the measurement accuracy remains at about 0.01%. Therefore, there is a need for a technique that significantly improves the measurement accuracy of the free motion area.

Korean Patent Publication No. 2014-0108112 (Publication Date: September 5, 2014)

SUMMARY OF THE INVENTION The present invention provides a system and method that can precisely measure the free motion region of a Febraryot etalon.

According to an embodiment of the present invention, a free motion area measuring system of a ferriplot etalon is configured to phase-modulate (PM) a low-frequency scanning CW laser to produce a Fabry- -Perot Etalon, FPE); A photodetector for detecting a transmission signal transmitted through the ferri-ferrite etalon or a reflection signal reflected by the ferri-ferrite etalon according to the application; And an FSR measuring unit for measuring a Free Spectral Range (FSR) of the ferroelectric etalon using the detection result and the modulation signal used for the PM modulation.

According to an embodiment of the present invention, there is provided a method of measuring a free motion area of a ferroelectric etalon, comprising: scanning a light frequency at a low frequency; PM-modulating the output laser at a frequency near the FSR region; Detecting a transmission signal transmitted through the ferri-ferrite etalon or a reflection signal reflected by the ferri-ferrite etalon according to the application; And measuring the free motion area (FSR) of the Febrilefot etalon using the detection result and the modulation signal used for the PM modulation.

In the conventional method, scanning light (CW light) is applied to the ferri-ferrite etalon (FPE) at low frequency and the periodic light transmission characteristics of the FPE are observed to observe the free-moving region. However, The free motion region measuring system and method of the Briperot etalon can precisely measure the free motion region by PM modulating an optical output scanned at a low frequency and applying it to a ferriplot etalon.

In particular, the free-running area measurement system and method of the present invention measures the free-motion region using both the transmitted signal passed through the ferri-wheel etalon and the reflected signal reflected from the ferri-wheel etalon, The measurement accuracy can be further improved.

FIG. 1 is a diagram illustrating a free motion area measurement system of a ferriplot etalon according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating a concept of a free motion region measurement principle of a ferri-ferrite etalon according to an embodiment of the present invention.
FIG. 3 is a graph showing the results of the modulation of the PM modulated signal spectrum by spectral amplitudes of the phase modulated signal according to one embodiment of the present invention. And the PM sideband signal is overlapped on the transmission characteristic of one FPE. The orange solid line represents the error characteristic curve amplitude-modulated by the Febriferot etalon inferred from the above.
FIG. 4 is a diagram showing an error signal experimentally obtained in the vicinity of the free operation region. FIG.
5 is a graph showing a change in the peak value of the error signal of FIG. 4 while varying the PM modulation frequency.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention relates to a system and a method for precisely measuring a free spectral range (FSR) of a Fabry-Perot Etalon (FPE). For example, the FSR system has a simple structure It is possible to precisely measure the free running range (FSR) within an error range of 1 ppm (10 ^-6 ) or less. Here, the FSR denotes a periodic frequency pass period of the FPE.

According to one embodiment, the FSR measuring system of the present invention can use a method of measuring the FSR by injecting laser (for example, CW light) modulated by a PM modulator into the FPE. That is, the FSR measurement system can measure the FSR using the PM modulation frequency.

According to another embodiment, the FSR measurement system can measure the FSR using both the transmission wave (transmitted signal) and the reflected wave (reflected signal) of the FPE. It is possible to improve the measurement accuracy by measuring the FSR by using the transmitted wave and the reflected wave simultaneously, rather than using the transmitted wave or the reflected wave only to measure the FSR. In this case, a complicated signal accumulating device may not be necessary. As a result, it was confirmed that FSR measurement using both transmission wave and reflected wave is more accurate than FSR measurement using only transmission wave or reflection wave.

Various embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a view showing a free motion region measuring system of a ferriplot etalon according to an embodiment of the present invention, and FIG. 2 is a diagram illustrating a free motion region measuring method of a ferriplot etalon according to an embodiment of the present invention. Fig. FIG. 3 is a graph showing the results of the modulation of the PM modulated signal spectrum by spectral amplitudes of the phase modulated signal according to one embodiment of the present invention. 4 is a graph showing an error signal experimentally obtained in the vicinity of a free operating region. FIG. 5 is a graph showing the error signal obtained in the vicinity of the free operating region, And the change of the peak value of the error signal.

1, the FPR FSR measuring system of the present embodiment may include a laser output unit 100, a PM modulating unit 102, an FPE 104, an FPE filter 106, and an FSR measuring unit 110 have.

The laser output unit 100 can output a CW laser. According to an embodiment, the laser output unit 100 may scan and apply CW light (continuous wave laser) at a low frequency (for example, 20).

The PM modulating unit 102 can PM-modulate the scanned and outputted laser (CW light) to a high frequency (for example, 10) near the FSR and apply it to the FPE 104. In this case, a transmission signal transmitted through the FPE 104 and a reflection signal reflected from the FPE 104 may occur.

Sideband signals generated by the PM modulation of 10 intervals, for example, the modulation frequency of the PM modulation unit 102 as the transmission signal, are filtered by the periodic frequency pass characteristics of the FPE filter 106, The filtered transmission signal can be detected by a high-speed photo detector (PD, 108). The reflected signal can also be detected by the photodetector 108. However, the transmitted and reflected signals travel through different channels and can be detected by the same photodetector 108 or other photodetectors.

The FSR measuring unit 110 can measure the FSR using at least one of the transmission signal and the reflection signal detected by the photodetector 108.

According to one embodiment, the FSR measuring unit 110 detects an error signal including information on a frequency difference between the PM modulation frequency and the FSR in the FPE transmission signal, and detects a PM modulation Frequency can be determined as the free-running region (FSR).

Hereinafter, a process of measuring the FSR by the FSR measuring unit 110 will be described taking a transmission signal as an example.

Referring again to FIG. 1, the laser output unit 100 may scan and output CW light, for example, at a low frequency of 20, for example.

Then, the PM modulator 102 can PM-modulate the output CW light with, for example, 10 high-frequency waves and apply it to the FPE 104. [ Here, it is assumed that the FPE 104 has a FSR of about 10 GHz as a result of directly examining the periodic transmission characteristics of the scanned CW light by an existing method. Hereinafter, the PM modulation frequency will be referred to as Fpm.

In this case, the transmission signal passed through the FPE 104 is filtered by the FPE filter 106 and then detected by the high-speed photodetector (PD) 108, and the reflected signal reflected by the FPE 104 is also detected by the photodetector (Not shown). Here, the FPE filter 106 can periodically filter (change the component size) each of the PM sideband frequency components as shown in FIG. 2 at FSR intervals.

Subsequently, the FSR measuring section 110 mixes the signal detected by the photodetector 108 with the phase modulated signal provided for the PM modulation of the PM modulating section 102 to obtain the magnitude (that is, amplitude) of the modulated frequency component, And phase signals.

Then, the FSR measuring unit 110 scans and applies the frequency signal provided to the laser output unit 100 to the extracted amplitude signal to obtain an error signal as shown in FIG.

The FSR measuring unit 110 can measure the FSR by observing the obtained error signal as a function of the modulation frequency. More specifically, when the CW light is PM-modulated near the FSR of the FPE 104 and applied to the FPE 104, the PM side band frequency components of the transmission signal transmitted through the FPE 104 are And is modulated by the periodic filtering characteristic of the FSR interval of the FPE 104. [ The FSR measuring unit 110 may detect the information corresponding to the difference between the FSR frequency of the FPE 104 and the modulation frequency of the PM modulating unit 102 by analyzing the modulation characteristics, ) Can be measured.

Figure 2 conceptually illustrates the principle of the above measurement method. When the interval (Fpm) between the spectral lines of the modulation frequency of the PM modulation unit 102 is larger than the FSR (hereinafter referred to as "FPE-FSR ") of the FPE 104, ) Are as follows.

One. Fpm  > FPE - FSR If

2 (a) shows a case where Fpm is larger than FPE-FSR.

1) When the carrier frequency of the CW light (carrier) is smaller than the peak value (peak value) of FPE 104 (orange line), the low frequency component is suppressed by the periodic frequency characteristic curve of FPE 104 Becomes smaller) and the high frequency component is emphasized (the amplitude becomes larger). That is, the amplitude of the signal of the uniform amplitude modulated with the pure PM is large in the high frequency range and small in the low frequency range. Here, the amplitude modulation frequency becomes equal to the PM modulation frequency.

2) When the carrier frequency of the CW light coincides with the passing peak value of the FPE 104 (black line), the low frequency and high frequency components are lost due to the periodic frequency characteristic curve of the FPE 104, The frequency component passes through the FPE 104 without loss. Therefore, the transmission signal of the FPE 104 is amplitude-modulated twice as much as Fpm (the Fpm component does not appear due to the symmetrical loss of high frequency and low frequency components).

3) If the carrier frequency of the CW light is larger than the pass peak value of the FPE (blue line), the input modulated signal passes through the FPE 104, then the low frequency component is emphasized and the high frequency component is suppressed. In this way, a signal amplitude-modulated with the same frequency as Fpm is amplitude-modulated similarly to the case of 1), but the phase is 180 degrees different.

Taken together, if the magnitude of the Fpm component of the envelope of the transmission signal of the FPE 104 according to the carrier frequency of the CW light source is expressed, an error signal as shown in FIG. 3 can be predicted.

3, in order to make the amplitude modulation characteristics of the PM side band due to the periodic frequency passing curve (dotted line) of the FPE 104 more visible, it is preferable that the side sideband signals are superimposed on the transmission characteristics of one FPE 104 Respectively. If the carrier frequency is less than the peak value of the FPE passing characteristic, the sideband signals are amplitude modulated by the high-pass filter as indicated by the red arrows on the frequency shift. If they match, they are modulated by the bandpass filter as shown by the black arrow, And is amplitude modulated with a low pass filter as shown. If the magnitude of the amplitude modulation signal is represented by the optical frequency transition, it can be easily seen that the differential signal appears as a first derivative of the FPE transmission characteristic curve like the error signal shown in the experiments of FIGS.

2. Fpm  = FPE - FSR If

2 (b) shows a case where Fpm is equal to FPE-FSR. All PM modulated sideband components have the same loss (gain) due to the periodic passing characteristics of the FPE. That is, the amplitude modulation does not occur even if the frequency of the CW light does not coincide with the pass peak value of the FPE. That is, it appears as if there is no frequency variation in the sampling curve shown in FIG. This means that the PM modulation frequency component does not appear in the envelope detection signal regardless of the carrier frequency variation.

3. Fpm  < FPE - FSR If

FIG. 2 (c) shows a case where Fpm is smaller than FPE-FSR. In this case, a phenomenon opposite to the case of 1 appears.

1) When the carrier frequency of the CW light is smaller than the transmission peak value of the FPE 104 (orange line), the high frequency component is suppressed (the amplitude becomes small) and the low frequency component is emphasized )do. That is, the amplitude of the pure-PM-modulated signal of uniform amplitude becomes low in the low frequency range and low in the high frequency range. Of course, this amplitude modulation frequency becomes equal to the PM modulation frequency.

2) When the carrier frequency of the CW light coincides with the transmission peak value of the FPE 104 (black line), the low frequency and high frequency components are lost due to the periodic passing characteristic curve of the FPE, Without passing through the FPE 104. As a result, the transmission signal of the FPE 104 is amplitude-modulated to twice the PM modulation frequency (there is no PM modulation frequency component due to the symmetrical loss of high frequency and low frequency components).

3) When the carrier frequency of the CW light is larger than the peak value of the FPE 104 (blue line), the input PM modulated signal passes through the FPE 104, and then the high frequency component is emphasized as in the case of 1) The ingredients are inhibited.

Taken together, it can be seen that the Fpm component of the envelope modulated signal has a negative value (ie, a phase difference of 1 and 180 degrees) of the first derivative of the FPE pass characteristics shown in FIG.

In sum, the error signal of FIG. 3 according to the modulation frequency increases as the distance from the FPE-FSR increases (larger or smaller becomes phase), and becomes smaller as the modulation frequency approaches. That is, the modulation frequency at which the error signal of FIG. 3 disappears is the FSR of the FPE 104.

On the other hand, not only the transmission signal of the FPE 104 but also the reflection signal have similar modulation characteristics. However, since the reflection signal becomes small when the transmission signal becomes large, the error signal of the reflection signal can have a phase difference of 180 degrees from the error signal of the transmission signal.

The FSR measurement system of the present invention can improve the accuracy of the FSR measurement by measuring not only the transmission signal but also the reflection signal.

Hereinafter, an experiment will be performed in which a phase (frequency) modulation is performed at 10 GHz using a FPR 104 having a FSR of about 10 GHz and a finesse of about 270, and a carrier is scanned at a low frequency (20 Hz).

Fig. 4 shows the error signal described in Fig. 3 obtained by applying a phase-modulated CW optical signal to the FPE 104 at a frequency in the vicinity of the FSR of the FPE 104 (when the FSR is slightly larger than that of the FSR). As described above, the first-order differential form of the transmission characteristic curve of the FPE 104 can be seen, and it can be confirmed that the error signal obtained from the transmission signal and the reflection signal has a phase difference of 180 degrees.

It can be seen that a phase difference of 180 degrees occurs when the PM modulation frequency is larger than that of FPE-FSR or when the phase modulation is smaller than FPE-FSR. As shown in FIGS. 3 and 4, when the PM modulation frequency is equal to the FPE-FSR, no error signal is generated.

Therefore, the FSR measurement system of the present invention measures the frequency of the point where the error signal of FIG. 4 disappears (that is, passes through 0) by observing the change of the error signal while varying the PM modulation frequency, The frequency of the point is the frequency of the FSR.

In the above, FSR was measured using only the transmission signal, but FSR can be measured using both the transmission signal and the reflection signal to improve the accuracy. 5 shows the peak value of the error signal of the transmission signal and the error signal of the reflection signal as a function of the PM modulation frequency.

In the case of using only the transmission signal or the reflection signal, the zero-pass frequency is 9.995765 GHz and 9.99579 GHz, respectively. However, when both the transmission signal and the reflection signal are used, 9.995771 GHz appears when the error signal crosses, . In order to measure the accuracy between the two values, reproducibility of the two methods by repeated measurement was observed. In the case of using only the transmission signal, a reproducibility error of about 10 KHz was shown. However, when both the transmission signal and the reflection signal were used, the measurement reproducibility error (0.6 ppm) of about 6 KHz was recorded. That is, the method of specifying the FSR using both the error signal of the transmission signal and the error signal of the reflection signal is more accurate than the method of measuring the FSR using only one.

It can be seen that the method of measuring the FSR using both the transmission signal and the reflection signal improves the accuracy of the reproducibility by about 30%.

It will be apparent to those skilled in the art that various modifications, additions and substitutions are possible, without departing from the spirit and scope of the invention as defined by the appended claims. Should be regarded as belonging to the following claims.

100: laser output unit 102: PM modulation unit
104: フ ェ ブ リ ペ ロ ト エ ト ラ ン 106: FPE filter
108: photodetector 110: FSR measuring unit

Claims (14)

A PM modulator for PM-modulating the scanned and outputted CW laser to apply it to a Fabry-Perot Etalon (FPE);
A photodetector for detecting a transmission signal transmitted through the ferri-ferrite etalon or a reflection signal reflected by the ferri-ferrite etalon according to the application; And
And an FSR measuring unit for measuring a Free Spectral Range (FSR) of the Talbot etalon using a result detected by the photodetector and a modulation signal used for the PM modulation Free - motion area measurement system of Februrferot etalon. The system of claim 1, further comprising a laser output unit for scanning the CW laser and applying the laser to the PM modulator. 3. The method of claim 2,
Further comprising an FPE filter periodically filtering the PM sideband frequency components (sideband frequency components of the modulation frequency) of the transmission signal at intervals of FSR. 3. The apparatus of claim 2, wherein the FSR measuring unit detects an error signal including information on a magnitude of a modulation frequency component (amplitude of the modulation signal) in the transmission signal according to a carrier frequency of the CW laser, And determines a modulation frequency at a point where the free running region (FSR) disappears as the free operating region (FSR). 5. The receiver of claim 4, wherein the error signal includes information on a size of the modulated frequency component according to an interval between spectral lines of the modulated frequency and a difference between the free operating regions (FSR) Free motion area measurement system for talons. 6. The method of claim 5, wherein the amplitude of the modulated frequency component in the error signal is different depending on whether the carrier frequency of the CW laser is larger than the frequency of the peak peak of the Febraryet etalon. Free motion area measurement system of etalon. The FSR measuring apparatus of claim 2, wherein the FSR measuring unit detects information corresponding to a difference between a freely operating frequency of the ferroelectric etalon and a modulation frequency of the PM modulating unit, (FSR) of the free ferrite etalon is measured. 3. The apparatus of claim 2, wherein the FSR measuring unit measures the free-running region (FSR) using both the error signal of the transmission signal and the error signal of the reflection signal, wherein the error signal corresponds to a carrier frequency of the CW laser And the magnitude of the modulated frequency component. Performing PM modulation on the scanned and output laser and applying it to a ferri-ferrite etalon;
Detecting a transmission signal transmitted through the ferri-ferrite etalon or a reflection signal reflected by the ferri-ferrite etalon according to the application; And
And measuring the free-running region (FSR) of the ferri-feret etalon using the detected reflected signal and the modulated signal used for the PM modulation. &Lt; RTI ID = 0.0 &gt; Area measurement method. 10. The method of claim 9, wherein measuring the free-
Mixing the detected reflected signal and the modulated signal to obtain an amplitude signal representing a magnitude (amplitude) of a modulated frequency component of the modulated signal;
Obtaining an error signal including information on a magnitude of a modulation frequency component according to a carrier frequency of the laser by scanning and applying a frequency signal used for generating the laser to the amplitude signal; And
And analyzing the obtained error signal to measure the free-running region (FSR). 11. The method of claim 10, wherein a modulation frequency at a point where the error signal disappears is determined as the free operating region (FSR). 11. The method as claimed in claim 10, wherein information corresponding to the difference between the freely operating frequency of the Febraryot etalon and the modulation frequency is detected, and the free operating region (FSR) is measured through the detected information A method of measuring the free motion area of a ferri - ferrite etalon. 10. The method of claim 9,
Periodically filtering each PM sideband frequency component of the transmission signal at an FSR interval; And
And detecting the filtered transmission signal with a photodetector. &Lt; Desc / Clms Page number 19 &gt; 10. The method according to claim 9, wherein the free-running region (FSR) is measured using both the error signal of the transmission signal and the error signal of the reflection signal, Wherein the information of the free motion region of the ferri-etal etalon is stored in the memory.

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