Laser frequency measurement method and device using optical frequency comb

Abstract

To measure the frequency of a laser, the frequency of a beat signal that is generated by the interference between an optical frequency comb, used as the reference of measurement, and the laser to be measured is measured. In such a laser frequency measurement using the optical frequency comb, at least one of a repetition frequency and a CEO frequency of the optical frequency comb is changed so that the frequency of the beat signal becomes a predetermined value, and the frequency of the beat signal is measured, so that the frequency of the laser is measured. This allows measurement of the frequency of laser having large frequency variation and low stability.

Claims

1. A laser frequency measurement device using an optical frequency comb, for measuring a frequency of a laser by measuring a frequency of a beat signal generated by interference between the optical frequency comb, used as a reference of measurement, and the laser to be measured, the laser frequency measurement device comprising: means for generating a frequency change command when the frequency of the beat signal exceeds a predetermined range; means for changing a CEO frequency f.sub.CEO of the optical frequency comb in response to the frequency change command, such that the frequency of the beat signal becomes a value in the predetermined range; means for detecting the beat signal in the predetermined range generated by interference between the laser to be measured and the optical frequency comb including the CEO frequency f.sub.CEO, the CEO frequency f.sub.CEO being changed by the means for changing in response to the frequency change command; and means for measuring the frequency of the beat signal in the predetermined range.

2. The laser frequency measurement device using an optical frequency comb according to claim 1, further comprising: means for measuring the CEO frequency f.sub.CEO.

3. The laser frequency measurement device using an optical frequency comb according to claim 2, further comprising means for measuring absolute frequencies v.sub.laser of the laser from measurement values of the CEO frequency f.sub.CEO, and the frequency of the beat signal.

4. The laser frequency measurement device using an optical frequency comb according to claim 2, wherein the means for measuring the CEO frequency f.sub.CEO measures a change of the CEO frequency f.sub.CEO changed in response to the frequency change command, and the laser frequency measurement device further comprises means for obtaining an absolute frequency v.sub.laser of the optical frequency comb based on the change of the CEO frequency f.sub.CEO.

5. The laser frequency measurement device using an optical frequency comb according to claim 4, wherein the means for determining the oscillation frequency of the laser determines absolute frequencies v.sub.laser of the laser from measurement values of the CEO frequency f.sub.CEO, the frequency of the beat signal in the predetermined range or measured value of the beat signal, and the change of the CEO frequency f.sub.CEO.

6. The laser frequency measurement device using an optical frequency comb according to claim 1, comprising: means for stabilizing the CEO frequency f.sub.CEO by phase synchronization with an f.sub.CEO reference frequency, which is generated for stabilization of the CEO frequency f.sub.CEO; and means for changing the f.sub.CEO reference frequency such that the frequency of the beat signal becomes the value in the predetermined range.

7. The laser frequency measurement device using an optical frequency comb according to claim 6, comprising: means for generating the reference frequencies by using a frequency synthesizer.

8. The laser frequency measurement device using an optical frequency comb according to claim 1, comprising: means for measuring the CEO frequency f.sub.CEO; and means for changing the CEO frequency f.sub.CEO such that the frequency of the beat signal becomes the value in the predetermined range.

9. The laser frequency measurement device using an optical frequency comb according to claim 1, wherein the predetermined range is a frequency range in which the frequency of the beat signal is able to be measured.

10. The laser frequency measurement device using an optical frequency comb according to claim 1, wherein the predetermined range is a band of a band pass filter.

11. The laser frequency measurement device using an optical frequency comb according to claim 10, wherein the frequency of the beat signal is changed to a center of the band of the band pass filter by changing the CEO frequency f.sub.CEO of the optical frequency comb in response to the frequency change command.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein:

(2) FIG. 1 is a graph showing examples of spectra of an optical frequency comb and a laser;

(3) FIG. 2 is a graph showing an example of a spectrum of a beat frequency signal;

(4) FIG. 3 is a block diagram showing the configuration of a first embodiment of the present invention;

(5) FIG. 4 is a drawing of an example of the optical frequency comb used in the first embodiment;

(6) FIGS. 5A to 5C are graphs showing a spectrum of the beat frequency signal after passing through a band pass filter in the first embodiment;

(7) FIG. 6 is a block diagram showing the configuration of a second embodiment of the present invention;

(8) FIG. 7 is a block diagram showing the configuration of a third embodiment of the present invention; and

(9) FIG. 8 is a block diagram showing the configuration of a fourth embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

(10) Embodiments of the present invention will be described below in detail with reference to the drawings. Note that, the present invention is not limited to descriptions of the below embodiments and practical examples. Components of the embodiments and the practical examples described below contain what is easily assumed by those skilled in the art, what is substantially the same, and what is in a so-called equivalent scope. Moreover, the components disclosed in the embodiments and the practical examples described below may be appropriately combined with each other or appropriately selectively used.

(11) FIG. 3 shows a first embodiment of a laser frequency measurement device using an optical frequency comb according to the present invention.

(12) An optical frequency comb 100 is stabilized by an f.sub.rep control signal and an f.sub.CEO control signal from a controller 110. A frequency counter 120 measures a stabilized repetition frequency f.sub.rep, and a frequency counter 130 measures a stabilized CEO frequency f.sub.CEO. A personal computer (PC) 140 receives measurement values thereof, and monitors the oscillation frequency of the optical frequency comb 100.

(13) A photodetector 160 detects a beat signal generated by a laser 150 to be measured and the optical frequency comb 100. A frequency counter 170 measures the frequency of the beat signal, and the PC 140 receives a measurement value thereof.

(14) Then, when the beat frequency exceeds a predetermined range, a frequency change command is sent to the controller 110, so that the controller 110 sends the f.sub.rep control signal and/or the f.sub.CEO control signal to the optical frequency comb 100.

(15) In the drawing, a reference numeral 152 refers to a mirror, and a reference numeral 154 refers to a half mirror.

(16) An example of the optical frequency comb 100 will be described with reference to FIG. 4, which cites FIG. 1 of Patent Literature 1. In an optical frequency comb oscillator 10, first, light excited by an LD (laser diode) 12 generates in a ring resonator a laser that supports a plurality of longitudinal modes. Then, by adjusting a plane of polarization of the laser orbiting in the ring resonator with the use of polarizing elements (14, 15, and 16), such as a wave plate and a polarizing plate disposed in the ring resonator, phase synchronization among the plurality of longitudinal modes occurs, and a pulsed laser is generated. The frequency spectrum of the pulsed laser at this time is in the shape of a comb having a repetition frequency of f.sub.rep. The repetition frequency f.sub.rep can vary by varying the length of the resonator. Thus, in a technique of FIG. 4, the repetition frequency f.sub.rep is changed by adjustment of an extension amount of an optical fiber 11 by a PZT 13. On the other hand, the CEO frequency f.sub.CEO can vary by varying the excitation power, and hence is controlled by changing an injected current from a driver to the LD 12.

(17) In the drawing, a reference numeral 2 refers to a laser light source. Reference numerals 17 and 18 each refer to an optical isolator. Reference numerals 21A and 218 each refer to a /4 plate. Reference numerals 22A and 22B each refer to a /2 plate. Reference numerals 30A and 30B each refer to an optical fiber amplifier. Reference numerals 31A and 31B each refer to an optical fiber for amplification. Reference numerals 32A and 32B each refer to an excitation light source. Reference numerals 40A and 40B each refer to a highly-non-linear optical fiber. Reference numerals 41A and 41B each refer to a single mode optical fiber. Reference numerals 51A, 51B and 51C each refer to a lens. A reference numeral 52A refers to a non-linear optical medium. A reference numeral 53B refers to a mirror. Reference numerals 54A and 54B each refer to a half mirror. A reference numeral 55A refers to a band pass filter. A reference numeral 56A refers to a CEO frequency detector. A reference numeral 56B refers to a heterodyne detector. A reference numeral 56C refers to a repetition frequency detector. A reference numeral 60 refers to a CEO frequency stabilizer. A reference numeral 70 refers to a repetition frequency stabilizer.

(18) In an optical frequency comb disclosed in Patent Literature 1, the CEO frequency stabilizer 60 and the repetition frequency stabilizer 70 are used for stabilizing the frequencies. However in this embodiment, the CEO frequency stabilizer 60 and the repetition frequency stabilizer 70 are used in an opposite manner to change the frequencies.

(19) In this embodiment, if the oscillation frequency of the laser 150 varies and the beat frequency exceeds, or is likely to exceed, the measurable frequency range (in a case where a state of FIG. 5A is changed to a state of FIG. 5B, which schematically show spectra of the beat frequency signal), the PC 140 sends the oscillation frequency change command. In response to the command, the controller 110 sends the f.sub.rep and/or f.sub.CEO control signals, to perform frequency control so as to compensate a variation of the beat frequency f.sub.B.

(20) Taking a beat frequency that is generated by the interference between the optical frequency comb and the laser to be measured having the spectra of FIG. 1 as an example, if the beat frequency f.sub.B increases due to increase in the frequency .sub.laser, the repetition frequency f.sub.rep is increased or the CEO frequency f.sub.CEO is increased under control, so that the beat frequency f.sub.B is changed to be in the predetermined range, for example, returned to the center of the BPF. In the spectrum of the beat frequency, the state of FIG. 5B is changed to a state of FIG. 5C.

(21) Measuring the repetition frequency f.sub.rep and/or the CEO frequency f.sub.CEO changed at this time by the frequency counters 120 and 130 facilitates obtainment of the absolute frequencies of the optical frequency comb 100, and therefore it is possible to measure the absolute frequency .sub.laser of the laser 150 to be measured without any problems.

(22) To stabilize the oscillation frequencies of the optical frequency comb, there is a method by which the repetition frequency f.sub.rep and/or the CEO frequency f.sub.CEO are synchronized in phase to f.sub.rep and/or f.sub.CEO reference frequencies, respectively, each of which is generated in accordance with the repetition frequency f.sub.rep and/or the CEO frequency f.sub.CEO. In a method for generating the reference frequencies in this case, a frequency synthesizer is used. An input of a highly accurate standard frequency of 10 MHz makes it possible to generate the reference frequencies with great accuracy. Thus, the optical frequency comb that is stabilized by phase synchronization with the reference frequencies constitutes the laser frequency measurement device with high accuracy. FIG. 6 shows a second embodiment of the present invention for realizing this method.

(23) Upon receiving the measurement value of the beat frequency signal, the PC 140 sends a reference frequency change command to frequency synthesizers 200 and/or 210, which generate the f.sub.rep or f.sub.CEO reference frequencies respectively, so that the beat frequency is controlled to fall within the predetermined frequency range. How to change the repetition frequency f.sub.rep and/or the CEO frequency f.sub.CEO in accordance with an actual change of the beat frequency f.sub.B is the same as that of the first embodiment.

(24) According to the second embodiment, since the accuracy of frequency stabilization by the phase synchronization is much higher than the accuracy of frequency measurement by the frequency counters, it is possible to perform frequency measurement with higher accuracy than the first embodiment.

(25) FIG. 7 shows a system configuration that more easily achieves accuracy sufficient for practical use, according to a third embodiment.

(26) An error remaining in stabilization control of the repetition frequency f.sub.rep is multiplied by the number n of orders thereof. Since n is a value of several millions, extremely high accuracy is required of stabilization of the repetition frequency f.sub.rep. On the contrary, an effect of an error remaining in stabilization control of the CEO frequency f.sub.CEO is much smaller than accuracy required for stabilization of the repetition frequency f.sub.rep. Accordingly, in this embodiment, the repetition frequency f.sub.rep is stabilized by phase synchronization with the f.sub.rep reference frequency having high accuracy, with the use of the frequency synthesizer 200. On the other hand, the CEO frequency f.sub.CEO is controlled by sending a command signal from the PC 140 to an LD controller 300 so that the beat frequency f.sub.B measured by the frequency counter 170 falls within the predetermined range. The frequency counter 130 measures the controlled CEO frequency f.sub.CEO. If the beat frequency f.sub.B varies due to a variation in the oscillation frequency of the laser 150, the CEO frequency f.sub.CEO is changed so as to compensate a variation of the beat frequency f.sub.B.

(27) This holds promise of an effect of extending a measurable range f.sub.B of the beat frequency f.sub.B to a range from f.sub.CEOf.sub.B to +f.sub.CEO+f.sub.B, even with limitations of an f.sub.CEO oscillation frequency range of f.sub.CEO.

(28) Note that, as a fourth embodiment shown in FIG. 8, a method may be combined in which the PC 140 commands the frequency synthesizer 200 to generate the f.sub.rep reference frequency, and to change the f.sub.rep reference frequency, so that the beat frequency f.sub.B varies. The combination allows measurement of a variable laser frequency in a wide range, out of the range from f.sub.CEOf.sub.B to +f.sub.CEO+f.sub.B.

(29) It should be apparent to those skilled in the art that the above-described embodiments are merely illustrative which represent the application of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and the scope of the invention.