WO2007119449A1

WO2007119449A1 - Optical frequency comb generator

Info

Publication number: WO2007119449A1
Application number: PCT/JP2007/055669
Authority: WO
Inventors: Masaaki Hirano; Masashi Onishi; Toshiaki Okuno; Hajime Inaba; Yuta Daimon; Feng-Lei Hong; Kaoru Minoshima; Atsushi Onae; Hirokazu Matsumoto
Original assignee: Sumitomo Electric Industries, Ltd.; National Institute Of Advanced Industrial Science And Technology
Priority date: 2006-03-20
Filing date: 2007-03-20
Publication date: 2007-10-25
Also published as: JP2007256365A

Abstract

Provided is an optical frequency comb generator capable of obtaining an optical comb having a preferable S/N. A pulse laser beam outputted when a laser light source (10) performs mode-synchronized laser oscillation is optically amplified by an optical fiber amplifier (30A) of the inverse-direction excitation method so as to have a high power and is inputted into a high non-linear optical fiber (40A). The pulse laser beam inputted into the high non-linear optical fiber (40A) is broad-banded by a non-linear optical phenomenon in the high non-linear optical fiber (40A) and it is possible to obtain a broad-band optical comb having a preferable S/N.

Description

Specification

Optical frequency comb generator

Technical field

The present invention relates to an optical frequency comb generator that generates light of a plurality of optical frequencies arranged at equal intervals in a certain optical frequency range.

Background art

An optical frequency comb generator generates light of a plurality of optical frequencies (called “optical comb”) arranged at equal intervals in a certain optical frequency range. For example, Patent Document 1 ~ 3. The optical frequency comb generator is used to measure the optical frequency (several hundreds THz) of laser light, and also measures the optical frequency difference (up to several tens of THz) between laser beams having different optical frequencies. Also used when.

[0003] Many documents including Patent Documents 1 to 3 propose optical frequency comb generators having various configurations. Each of the optical frequency comb generators described in Non-Patent Documents 1 to 7 includes a mode-locked laser light source, an optical amplifier, and a highly nonlinear optical fiber, and uses the pulsed laser light output from the mode-locked laser light source. Optical amplification is performed by an optical amplifier, the amplified laser light is guided by a highly nonlinear optical fiber, and the pulsed laser light is output in a broad band by a nonlinear optical phenomenon that appears at the time of the waveguide.

Patent Document 1: Japanese Patent Laid-Open No. 7-58386

Patent Document 2: Japanese Patent Laid-Open No. 11-4037

Patent Document 3: JP 2000-89264 A

Non-Patent Document 1: D. J. Jones, et al., Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis, "Science vol. 288, 635 (20 00).

Patent Document 2: F. Tauser, et al., "Amplified femtosecond pulses from an Er: fiber syst em: Nonlinear pulse shortening and self-referencing detection of the carrier-envelop e phase evolution," Optics Express, vol 11, 594 ( 2003).

Non-Patent Document 3: TR Schibli, K. Minoshima, F.-L. Hong, H. Inaba, A. Onae, H. Mats umoto, I. Hartl, ME Fermann, "Frequency metrology with a turnkey all-fiber syst em," Opt. Lett. 29, 2467-2469 (2004).

Non-Patent Document 4: F. Adler, et al., "Phase-locked two-branch erbium-doped fiber laser system for long-term precision measurements of optical frequencies," Opt. Express 12, 5872-5880 (2004).

Non-Patent Document 5: H. Hundertmark, et. Al., "Phase-locked carrier-envelope offset frequency at 1560 nm," Opt. Express 12, 770-775 (2004).

Non-Patent Document 6: K. Tamura, et al., "Unidirectional ring resonator for selfstarting passi vely mode-locked lasers," Opt. Lett., 18, 220-222 (1993).

Patent Document 7: N. Nakazawa, et al, 'Continuum suppressed, uniformly repetitive 13b fs pulse generation from an erbium-doped fiber laser with nonlinear polarisation rota tion, "Electron. Lett. 29, 1327—1329 (1993).

Disclosure of the invention

Problems to be solved by the invention

[0004] The signal-to-noise ratio (SZN ratio) of the optical comb obtained by a conventional optical frequency comb generator was about 25 dB to 40 dB (resolution band 100 kHz). If this SZN ratio is about 25 dB, it is possible to perform phase synchronization and frequency measurement. However, the SZN ratio may decrease due to environmental fluctuations such as temperature fluctuations, and a poor SZN ratio may affect frequency stability.

[0005] The present invention has been made to solve the above problems, and an object of the present invention is to provide an optical frequency comb generator capable of obtaining an optical comb having a good SZN ratio.

Means for solving the problem

[0006] An optical frequency comb generator according to the present invention includes: (1) a laser light source that oscillates in a mode-locked laser and outputs pulsed laser light; and (2) an amplification optical fiber in which a rare earth element is added to an optical waveguide region And a pumping light source that outputs pumping light having a wavelength capable of pumping the rare earth element added to the amplification optical fiber, and the pulsed laser light output from the laser light source is input to the first end of the amplification optical fiber. The excitation light source power is input to the second end of the amplification optical fiber, and the pulsed laser light input to the first end of the amplification optical fiber is input to the amplification optical fiber. (3) An optical fiber amplifier that amplifies the light at the Aiba and outputs it from the second end of the amplification optical fiber; (3) A pulse laser beam that is amplified and output by the optical fiber amplifier is input to the first end, and nonlinear optics And a first non-linear optical medium (for example, a highly non-linear optical fiber) that outputs broadband light according to a phenomenon.

[0007] In this optical frequency comb generator, the pulsed laser light output by the mode-locked laser oscillation of the laser light source is optically amplified by a reverse pumping type optical fiber amplifier to be high power, One nonlinear optical medium is input. Due to the nonlinear optical phenomenon that occurs in the first nonlinear optical medium, the pulse laser beam input to the first nonlinear optical medium is broadened and output as a broadband pulsed light, and a broadband optical comb with a good SZN ratio is obtained. Be

[0008] An optical frequency comb generator according to the present invention provides: (4) a broadband light output from a first nonlinear optical medium is input, a second harmonic is generated and output with respect to a fundamental wave of the broadband light; (2) Based on the beat frequency of the second harmonic of the broadband light output from the second nonlinear optical medium and the fundamental wave, the carrier envelope 'offset frequency (hereinafter referred to as the “CEO frequency”) CEO frequency detector, and (6) Stabilize and control the wavelength of the pulse laser light output from the laser light source so that the CEO frequency detected by the CEO frequency detector is a predetermined value. It is preferable to further include a CEO frequency stabilization unit. In this case, the broadband light output from the first nonlinear optical medium is input to the second nonlinear optical medium, and the second harmonic is generated with respect to the fundamental wave of the broadband light in the second nonlinear optical medium. . The CEO frequency is detected by the CEO frequency detector based on the beat frequency of the second harmonic and the fundamental wave of the broadband light output from the second nonlinear optical medium. The wavelength of the laser light emitted from the laser light source is stabilized and controlled by the CEO frequency stabilization unit so that the CEO frequency detected by the CEO frequency detection unit becomes a predetermined value.

[0009] An optical frequency comb generator according to the present invention includes: (7) a repetition frequency detection unit that detects a repetition frequency of pulse laser light; and (8) a repetition frequency detected by the repetition frequency detection unit is a predetermined value. In order to achieve the above, it is preferable to further include a repetition frequency stabilization unit that stably controls the repetition frequency of the pulsed laser light output from the laser light source. This place In this case, the repetition frequency detection unit detects the repetition frequency of the pulse laser beam, and the repetition frequency stabilization unit detects the pulse output from the laser light source so that the detected repetition frequency becomes a predetermined value. The repetition frequency of the laser beam is controlled stably.

[0010] An optical frequency comb generator according to the present invention includes (9) combining broadband light output from a first nonlinear optical medium and laser light output from another laser light source, and combining the combined light. It is preferable to further include an optical multiplexing unit that outputs the laser beam thus obtained, and (10) a heterodyne detection unit that heterodyne-detects the laser beam combined and output by the optical multiplexing unit. In this case, the broadband light output from the first nonlinear optical medium and the laser light output from the other laser light source are combined by the optical combining unit and heterodyne detected by the heterodyne detection unit. Thus, the wavelength of the laser beam output from the other laser light source is detected.

[0011] The optical frequency comb generator according to the present invention is provided between the laser light source and the fiber amplifier, and adjusts the polarization of the pulsed laser light output from the laser light source and adjusts the polarization. It is preferable to further include a polarization adjustment unit that outputs the pulse laser beam to the fiber amplifier. In this case, the polarization adjustment unit provided between the laser light source and the fiber amplifier adjusts the polarization of the pulsed laser light output from the laser light source force, and the polarization-adjusted Norlas laser light is transmitted to the fiber. Input to the amplifier. As a result, the CEO frequency f CEO can be detected with high accuracy.

The invention's effect

According to the present invention, it is possible to provide an optical frequency comb generator capable of obtaining an optical comb having a good SZN ratio.

Brief Description of Drawings

FIG. 1 is a configuration diagram of an optical frequency comb generator 1 according to the present embodiment.

FIG. 2 is a diagram showing an arrangement of optical frequencies of broadband light output from the highly nonlinear optical fibers 4 OA and 40B of the optical frequency comb generator 1 according to the present embodiment.

[FIG. 3] FIG. 3 is a diagram showing the chromatic dispersion characteristics of a highly nonlinear optical fiber of an optical frequency comb generator.

[Figure 4] Figure 4 shows the output from the highly nonlinear optical fiber of the optical frequency comb generator of the example. It is a figure which shows the spectrum of an optical comb.

FIG. 5 is a diagram showing a spectrum of an optical comb output from a highly nonlinear optical fiber of the optical frequency comb generator of Comparative Example 1.

FIG. 6 is a diagram showing a spectrum of an optical comb output from a highly nonlinear optical fiber of the optical frequency comb generator of Comparative Example 2.

FIG. 7 is a diagram showing CEO frequency f signals detected by the optical frequency comb generators of the example and comparative example 1.

CEO

Explanation of symbols

1 Optical frequency comb generator

2 Laser light source

10 Laser light source

11 Amplifying optical fiber

12 Excitation light source

13 drums

14 λ Ζ4 plates

15 λ Ζ2 plates

16 Bolizer

17, 18 Optical isolator

21A, 21B X 4U

22Α, 22Β / 2

30Α, 30Β optical fiber amplifier

31A, 31B amplification optical fiber

32Α, 32Β excitation light source

0Α, 40Β First nonlinear optical medium (high nonlinear optical fiber)

1A, 41B single mode optical fiber

51A, 51B, 51C lenses

52Α Second nonlinear optical medium

53Β Mirror 54A, 54B Half mirror

55A bandpass filter

56A CEO frequency detector

56B heterodyne detector

56C repetitive frequency detector

60 CEO Frequency Stabilization Department

70 Repetitive frequency stabilization section

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

FIG. 1 is a configuration diagram of an optical frequency comb generator 1 according to the present embodiment. The optical frequency comb generator 1 shown in this figure includes a laser light source 10, optical fiber amplifiers 30A and 30B, first nonlinear optical media (high nonlinear optical fibers) 40A and 40B, a second nonlinear optical medium 52A, and a photodiode 56A. , 56B, 56C, CEO frequency stabilization unit 60, repetitive frequency stabilization unit 70, etc.

The laser light source 10 oscillates in a mode-locked laser and outputs pulsed laser light. The amplification optical fiber 11, the excitation light source 12, the drum 13, the λ 4 plate 14, the λ 2 plate 15, and the polarizer 16 A polarization-dependent optical isolator 17 and a polarization-independent optical isolator 18. Among these, the amplification optical fibers 11, λ Ζ4 plate 14, λ Ζ2 plate 15, polarizer 16 and optical isolator 17 are provided on the resonance optical path of the ring type optical resonator.

The amplification optical fiber 11 is an optical fiber in which an Er element is added to an optical waveguide region, and is wound around a drum 13 provided with a piezoelectric element. The pumping light source 12 outputs pumping light having a wavelength of 1.48 m that can pump Er ³⁺ added to the amplification optical fiber 11, and supplies the pumping light to the amplification optical fiber 11.

[0019] The λΖ4 plate 14 and the λΖ2 plate 15 control the polarization of the laser light oscillated by the ring-type optical resonator. The λλ4 plate 14, λΖ2 plate 15, the polarizer 16 and the polarization-dependent optical isolator 17 selectively oscillate pulsed laser light with a ring type optical resonator. On drum 13 The provided piezo element can adjust the length of the amplifying optical fiber 11 wound on the drum 13, thereby adjusting the resonator length of the ring type optical resonator.

In the laser light source 10 configured as described above, the pumping light output from the pumping light source 12 is supplied to the amplification optical fiber 11 and added to the amplification optical fiber 11 so that Er ³⁺ is pumped. Then, light having a wavelength of 1.55 m is emitted from the amplification optical fiber 11. The light emitted from the amplification optical fiber 11 is resonated by the ring optical resonator, and the laser light source 10 oscillates in a laser.

In addition, in this laser light source 10, mode-locked laser oscillation is performed by the action of λ Ζ4 plate 14, λ Ζ2 plate 15, polarizer 16 and polarization-dependent optical isolator 17, and the pulse laser beam obtained by this oscillation is obtained. Is output via the polarization-independent optical isolator 18. Note that the wavelength of the pulsed laser light output from the laser light source 10 depends on the power of the excitation light supplied from the excitation light source 12 to the amplification optical fiber 11. The repetition frequency of the pulsed laser light output from the laser light source 10 depends on the resonator length of the ring optical resonator adjusted by the action of the piezoelectric element provided on the drum 13.

[0022] The pulse laser beam output from the laser light source 10 is branched into two, and after being branched into two, the pulse laser beam is input to the optical fiber amplifier 30 through the four plate 21A and the λ two plate 22 through The other pulse laser beam after being branched is input to the optical fiber amplifier 30 through the λ Ζ4 plate 21B and λ / 2 plate 22Β.

[0023] The λΖ4 plate 21A and the λΖ2 plate 22Α act as a polarization adjusting unit provided between the laser light source 10 and the optical fiber amplifier 30Α, and the pulsed laser light output from the laser light source 10 The polarization laser light is adjusted and the pulsed laser light whose polarization is adjusted is output to the fiber amplifier 30 mm. Similarly, the λΖ4 plate 21B and the λΖ2 plate 22Β act as a polarization adjusting unit provided between the laser light source 10 and the optical fiber amplifier 30Β, and the pulse laser beam output from the laser light source 10 The polarization laser light is adjusted and the pulse laser light whose polarization is adjusted is output to the fiber amplifier 30 mm.

[0024] The optical fiber amplifier 30Α includes an amplification optical fiber 31Α and a pumping light source 32Α. The optical fiber for amplification 31A is an optical fiber in which an Er element is added to the optical waveguide region. The excitation light source 32A has a wavelength of 0.98 m, which can excite Er ³⁺ added to the amplification optical fiber 31A. The pumping light of the band is output, and the pumping light is supplied to the amplification optical fiber 31A. This optical fiber amplifier 30A is a reverse-pumping type in which light to be amplified (pulse laser light) and pumping light are guided in opposite directions in the amplification optical fiber 31A.

That is, the pulse laser beam output from the laser light source 10 and passed through the λλ4 plate 21A and the λΖ2 plate 22A is input to the first end of the amplification optical fiber 31A. The pump light output from the pump light source 32 is input to the second end of the amplification optical fiber 31A. Then, the pulse laser beam input to the first end of the amplification optical fiber 31A is optically amplified in the amplification optical fiber 31A, and the second end force of the amplification optical fiber 31Α also goes to the highly nonlinear optical fiber 40Α. Is output.

[0026] Similarly, the optical fiber amplifier 30B includes an amplification optical fiber 31B and a pumping light source 32B. The amplification optical fiber 31B is an optical fiber in which an Er element is added to the optical waveguide region. The pumping light source 32B outputs pumping light having a wavelength of 0.98 m that can pump Er ³⁺ added to the amplification optical fiber 31B, and supplies the pumping light to the amplification optical fiber 31B. This optical fiber amplifier 30B is of the reverse excitation type in which the light to be amplified (pulse laser light) and the pumping light are guided in opposite directions in the amplification optical fiber 31B.

Each of the highly nonlinear optical fiber 40A and the highly nonlinear optical fiber 40B is an optical fiber that has high nonlinearity and easily exhibits a nonlinear optical phenomenon. A highly nonlinear optical fiber is an optical fiber that has a nonlinear coefficient three times greater than that of a standard single mode fiber used for normal transmission. The nonlinear coefficient (measured by the XPM method) of the highly nonlinear optical fiber is preferably more than lOZwZkm, more preferably more than 20ZwZkm, more preferably more than 5ZwZkm. Further, the zero dispersion wavelength of the highly nonlinear optical fino OA, 40B is preferably 1450 nm or less. The highly nonlinear optical fiber 40A inputs the pulse laser beam amplified and output by the optical fiber amplifier 30A to the first end, and broadens the spectrum of the input pulse laser beam by nonlinear optical phenomenon. Then, the pulsed laser beam (broadband light) that has been broadened is output to the single mode optical fiber 41 A. Similarly, in the highly nonlinear optical fiber 40B, the pulse laser beam output after being amplified by the optical fiber amplifier 30B is input to the first end, and the spectrum of the input pulse laser beam is converted into a broadband signal by a nonlinear optical phenomenon. The wideband pulse array The light (broadband light) is output to the single-mode optical fiber 41B. Here, it is preferable that the broadband light has a band of 1 octave or more.

[0028] The single mode optical fiber 41A is connected to the second end (outgoing end) of the highly nonlinear optical fiber 40A. Similarly, the single mode optical fiber 41B is connected to the second end (outgoing end) of the highly nonlinear optical fiber 4OB. The single-mode optical fiber 41A compensates the chromatic dispersion generated in the highly nonlinear optical fiber 40A and the second nonlinear optical medium 52A, and has a fiber length adjusted for the chromatic dispersion compensation. Similarly, the cinder mode optical fiber 41B compensates the chromatic dispersion generated in the highly nonlinear optical fiber 40B, and has a fiber length adjusted for the chromatic dispersion compensation.

[0029] The lens 51A for inputting the broadband light emitted from the emitting end of the single mode optical fiber 41A collects the inputted broadband light and makes it incident on the second nonlinear optical medium 52A. The second nonlinear optical medium 52A is, for example, PPLN (periodically poled lithium niobate crystal), and generates and outputs a second harmonic with respect to a fundamental wave including at least one comb of incident broadband light. The second harmonic band overlaps with the fundamental band. The half mirror 54A inputs the fundamental wave and the second harmonic wave of the broadband light output from the second nonlinear optical medium 52A, transmits a part thereof, reflects the remaining part, and splits into two.

[0030] The bandpass filter 55A that inputs the broadband light transmitted through the half mirror 54A transmits the input broadband light having a specific transmission band. Here, the transmission band is a band having a high optical frequency in the broadband light band output from the highly nonlinear optical fiber 40A. The CEO frequency detector 56A detects the CEO frequency based on the beat frequency of the second harmonic and the fundamental wave of the broadband light that has passed through the bandpass filter 55A. Then, the CEO frequency stabilizing unit 60 adjusts the power of the pumping light supplied from the pumping light source 12 to the amplification optical fiber 11 so that the CEO frequency detected by the CEO frequency detecting unit 56A becomes a predetermined value. By doing so, the wavelength of the pulsed laser beam output from the laser light source 10 is stabilized and controlled.

[0031] The repetition frequency detector 56C that inputs the broadband light reflected by the half mirror 54A detects the repetition frequency of the input broadband light. The repetition frequency stabilization unit 70 then adjusts the repetition frequency detected by the repetition frequency detection unit 56C to a predetermined value. In addition, by adjusting the resonator length of the ring-type optical resonator of the laser light source 10 via the piezoelectric element of the drum 13, the repetition frequency of the pulsed laser light output from the laser light source 10 is controlled to be stabilized.

[0032] The lens 51B for inputting broadband light emitted from the emitting end of the single mode optical fiber 41B collimates the inputted broadband light. In addition, the lens 51C collimates the laser beam output from the laser light source 2 as well. The half mirror 54B combines the broadband light that has reached the lens 51B through the mirror 53B and the laser light that has also reached the lens 51C, and outputs the combined laser light. Then, the heterodyne detection unit 56B detects the wavelength of the laser beam output from the laser light source 2 by heterodyne detection of the laser beam combined and output by the half mirror 54B.

Next, the operation of the optical frequency comb generator 1 according to this embodiment will be described. The pulsed laser light output from the laser light source 10 after mode-locked laser oscillation is a certain repetition frequency f on the time axis.

It is a rep ultra-short pulse optical train (delta function train on the time axis), and when viewed on the optical frequency axis, it is in a certain optical frequency range according to the longitudinal mode of the optical resonator! , Fixed interval f

It is an optical frequency sequence of rep (a delta function sequence on the optical frequency axis, that is, an optical comb) (see Non-Patent Document 1). At regular intervals f when viewed on the optical frequency axis

The rep optical frequency sequence is a measure of optical frequency.

Although the pulse laser beam output from the laser light source 10 is an optical comb, it is limited to a narrow band including a wavelength of 1,55 m. Therefore, the broadband is performed as follows. The pulse laser beam output from the laser light source 10 is branched into two, and one of the branched pulse laser beams is input to the optical fiber amplifier 30 through the λ 4 plate 21A and λ 2 plate 22 and branched. The other pulse laser beam is input to the optical fiber amplifier 30 through the λ 4 plate 21B and the λ 2 plate 22.

[0035] The pulse laser beam input to the optical fiber amplifier 30Α is optically amplified by the reverse-pumped optical fiber amplifier 30Α to high power, and then input to the highly nonlinear optical fiber 40 40. . Due to the nonlinear optical phenomenon that occurs in the highly nonlinear optical fiber 40A, the pulsed laser light input to the highly nonlinear optical fiber 40A becomes broadband light. Similarly, the pulse laser beam input to the optical fiber amplifier 30B After being optically amplified by the optical fiber amplifier 30B to be high power, it is input to the highly nonlinear optical fiber 40B. Due to the nonlinear optical phenomenon that occurs in the highly nonlinear optical fiber 40B, the pulsed laser light input to the highly nonlinear optical fiber 40B becomes broadband light.

FIG. 2 is a diagram showing an arrangement of optical frequencies of broadband light output from the highly nonlinear optical fibers 40A and 40B as the nonlinear medium of the optical frequency comb generator 1 according to the present embodiment. . As shown in this figure, the broadband light output from the highly nonlinear optical fibers 40A and 40B has a constant interval f in a certain optical frequency range when viewed on the optical frequency axis.

The optical frequency train of rep is an optical comb.

[0037] The spread of the spectrum of an optical comb, which is a measure of optical frequency, over one octave is extremely important from the viewpoint of application to optical frequency measurement. In other words, is it possible to observe the offset frequency (f) of the optical comb by a technique called f-to-2f interference?

CEO

(See Non-Patent Document 2). Figure 2 shows the modes on the frequency axis of an optical comb that extends over one octave.

[0038] On the optical frequency axis, the modes of combs arranged at regular intervals f are virtually represented by optical frequency rep

When extending to the vicinity of the point where it becomes 0, there is an offset frequency (f). This is the time axis

CEO

This is the difference between the optical carrier frequency of adjacent pulses and the envelope of the pulse! The power of becoming “carrier—envelope-like” is called “carrier—envelope of fset frequency (CEO frequency)” f. CEO frequency f can be observed

CEO CEO

In the optical comb, the CEO frequency f is zero, and the frequency f (

CEO

n) can be described by the following equation (1), and can be expressed by two rep CEO parameters of repetition frequency f and CEO frequency f.

f (n) = n X f + f

rep CEO

[0039] The fact that the optical comb spreads over the optical frequency range 90 of one octave or more means that the nth mode and the 2nth mode exist simultaneously in the optical comb. The optical frequency of the second harmonic of the nth mode is expressed by the following equation (2), and the optical frequency of the 2nth mode is expressed by the following equation (3). The difference between the two is f as shown in the following equation (4),

CEO

It can be measured by measurement. This is a technique called f-to-2 sweating. 2f (n) = 2n X f + 2f--(2)

rep CEO

f (2n) = 2n X f + f--(3)

rep CEO

2f (n) -f (2n)

= (2n X f + 2f)-(2n X f + f)

rep CEO rep CEO

= f… (

CEO

Specifically, the CEO frequency f is detected as follows. That is, highly nonlinear

CEO

The broadband light that has been broadened to 1 octave or more in the optical fiber 40A is input to the second nonlinear optical medium 52A via the single mode optical fiber 41A and the lens 51A. In the second nonlinear optical medium 52A, the second harmonic is generated with respect to the fundamental wave of the incident broadband light. Of the fundamental wave and the second harmonic of the broadband light, the light transmitted through the half mirror 54A and the bandpass filter 55A is received by the CEO frequency detection unit 56A.

The CEO frequency detector 56A detects the CEO frequency f based on the beat frequencies of the second harmonic wave and the fundamental wave of the broadband light. In addition, CEO Frequency Stabilization Department

CEO

60, CEO frequency f detected by CEO frequency detector 56A becomes a predetermined value.

CEO

As described above, the power of the pumping light supplied from the pumping light source 12 to the amplification optical fiber 11 is adjusted, whereby the wavelength of the pulsed laser light output from the laser light source 10 is stably controlled.

[0042] The repetition frequency f appearing in the above equation (1) is the wideband rep reflected by the half mirror 54A.

Measurement can be performed easily by directly receiving the range light with the high-speed repetition frequency detector 56C. Then, the repetition frequency stabilization unit 70 replies through the piezoelectric element of the drum 13 so that the repetition frequency f detected by the repetition frequency detection unit 56C becomes a predetermined value.

Thus, the resonator length of the ring type optical resonator of the laser light source 10 is adjusted, whereby the repetition frequency f of the pulsed laser light output from the laser light source 10 is stably controlled.

rep

[0043] By controlling the repetition frequency f and the CEO frequency f obtained as described above to be equal to the reference frequency generated by the frequency source force synchronized with the international atomic time, which is the frequency rep CEO standard, The ultra-short pulse laser “optical comb” can be used as an “optical frequency measure”. Furthermore, the laser light output from the laser light source 2 is highly nonlinear light. The broadband light output from the fiber 40B and the half mirror 54B are combined, and heterodyne detection is performed by the heterodyne detection unit 56B, so that the wavelength of the laser light output from the laser light source 2 can be detected with high accuracy.

[0044] Compared to a conventional apparatus that employs a bidirectional pumping type optical fiber amplifier that supplies pumping light from both the forward and reverse directions to the amplification optical fiber, the present embodiment described above In such an optical frequency comb generator 1, since the optical fiber amplifiers 30A and 30B are of the reverse excitation type, the SZN ratio of the obtained optical comb is excellent.

[0045] The optical frequency comb generator 1 according to the present embodiment employs a common optical path interferometer when generating the second harmonic in the second nonlinear optical medium 52A. This is because the broadband optical comb generated by the highly nonlinear optical fiber 40A is collected by the lens 51A and is incident on the second nonlinear optical medium 52A all together. Then, the second harmonic and the short wavelength portion of the long wavelength portion of the broadband optical comb generated in the second nonlinear optical medium 52A are directly incident on the CEO frequency detection portion 56A without being separated.

[0046] However, since the highly nonlinear optical fiber 40A and the second nonlinear optical medium 52A have chromatic dispersion, generally the pulse light on each of the long wavelength side and the short wavelength side is shifted in time, so that the CEO frequency detector Incident on 56A. Therefore, in this embodiment, by adjusting the length of the single-mode optical fiber 41A provided at the subsequent stage of the highly nonlinear optical fino OA, the chromatic dispersion for the optical combs in the respective wavelength bands becomes substantially the same. It is doing so. Here, the configuration is simplified, and the CEO frequency detector 56A receives the pulsed light emitted from the second nonlinear optical medium 52A as it is. This makes it easier and more robust to detect the CEO frequency f.

CEO

[0047] Also, in the optical frequency comb generator 1 according to the present embodiment, the λΖ4 plate 21A and the λΖ2 plate 22Α for polarization adjustment are provided in front of the optical fiber amplifier 30A, and the optical fiber A λ Ζ4 plate 21B and λ Β2 plate 22Β for polarization adjustment are provided in front of the amplifier 30Β. By adjusting the azimuth of the optical axis of these wave plates, the power of the nth and 2nth modes used in f-to-2f interference can be increased, and the CEO frequency f

CEO can be detected with high accuracy.

[0048] The present invention is not limited to the above-described embodiment, and various modifications are possible. Les The one light source 10 is an optical fiber laser light source having a ring-type optical resonator structure in the above embodiment, but may be a mode-locked laser light source having another configuration. For example, a semiconductor laser that oscillates in a mode-locked laser It may be a light source or a solid laser light source.

Example

Next, a more specific example of the optical frequency comb generator 1 according to the present embodiment will be described in comparison with a comparative example. The optical frequency comb generator of the embodiment has the configuration shown in FIG. The optical frequency comb generator of Comparative Example 1 is obtained by replacing the optical fiber amplifiers 30A and 30B with a bidirectional pump type in the configuration shown in FIG. The optical frequency comb generator of Comparative Example 2 is obtained by replacing the optical fiber amplifiers 30A and 30B with the forward pumping type in the configuration shown in FIG.

[0050] The optical fiber used as the highly nonlinear optical fino OA, 40B in this example and comparative examples 1 and 2 had the chromatic dispersion characteristics shown in FIG. This optical fiber has a fiber length of 20 cm, a transmission loss of 0.62 dBZkm, a total loss including the connection loss when a single-mode optical fiber is connected to both ends, 0.1 to 2 dB, and a zero dispersion wavelength. a 1447Nm, the wavelength dispersion + 3.91psZnmZkm, a distributed Ro-loop is + 0.032psZnm ² Zkm, the cutoff wavelength of 1560 nm, the effective sectional area is 10.4 m ^2, the nonlinear coefficient γ is 21W ^_1 km ^_1 . Of these parameters, the one that depends on the wavelength is the value at the wavelength of 1550 nm.

FIG. 4 is a diagram illustrating a spectrum of an optical comb output from the highly nonlinear optical fiber of the optical frequency comb generator according to the embodiment. FIG. 5 is a diagram showing the spectrum of the optical comb output from the highly nonlinear optical fiber of the optical frequency comb generator of Comparative Example 1. FIG. 6 is a diagram showing the spectrum of the optical comb output from the highly nonlinear optical fiber of the optical frequency comb generator of Comparative Example 2. In the reverse pumping example, the power of light input to the highly nonlinear optical fiber was 45 mW, and the power of light output from the highly nonlinear optical fiber was 19 mW. In Comparative Example 1 of the bidirectional pumping method, the power of light input to the highly nonlinear optical fiber was 84 mW, and the power of light output from the highly nonlinear optical fiber was 33 mW. In Comparative Example 2 of the forward pumping method, the power of the light input to the highly nonlinear optical fiber is 40 mW, which is output from the highly nonlinear optical fiber. The power of light was 18mW.

[0052] As can be seen by comparing Figs. 4 to 6, in Comparative Example 2 (Fig. 6) of the forward pumping method, the lower limit of the spectrum of light output from the highly nonlinear optical fiber is about 1050 nm. Only spread. In contrast, in the reverse pumping example (Fig. 4), the lower limit of the spectrum of the light output from the highly nonlinear optical fiber extends to about lOOOnm, and the bi-directional pumping method comparison example 1 (Fig. 4). In 5), the lower limit of the spectrum of light output from a highly nonlinear optical fiber has expanded to about 960 nm. In both the examples and comparative examples 1 and 2, the upper limit of the spectrum of light output from the highly nonlinear optical fiber extended to 2000 nm or more. In the reverse pumping example (Fig. 4) and the bi-directional pumping comparison example 1 (Fig. 5), the spectrum of the light output from the highly nonlinear optical fiber is in the band exceeding one octave.

FIG. 7 is a diagram showing CEO frequency f signals detected in the optical frequency comb generators of the example and the comparative example 1. As can be seen from this figure, bidirectional excitation system

CEO

Compared with Comparative Example 1 in the reverse excitation method, the noise floor is low and the SZ is high.

An N ratio signal was obtained. As described above, the optical frequency comb generator 1 according to the present embodiment is such that the optical fiber amplifiers 30A and 30B are of the reverse direction pumping system as compared with the conventional apparatus employing the bidirectional pumping type optical fiber amplifier. Therefore, the SZN ratio of the obtained optical comb is excellent.

Industrial applicability

The present invention can be used for an optical frequency comb generator that generates light of a plurality of optical frequencies arranged at equal intervals in a certain optical frequency range.

Claims

The scope of the claims

[1] a laser light source that emits pulsed laser light by mode-locked laser oscillation;

An optical fiber for amplification in which a rare earth element is added to the optical waveguide region; and an excitation light source that outputs excitation light having a wavelength capable of exciting the rare earth element added to the optical fiber for amplification. The output pulsed laser light is input to the first end of the amplification optical fiber, the excitation light output from the excitation light source is input to the second end of the amplification optical fiber, and the amplification optical fiber An optical fiber amplifier for optically amplifying the pulsed laser light input to the first end in the optical fiber for amplification and outputting from the second end of the optical fiber for amplification;

A first nonlinear optical medium that inputs the pulsed laser light amplified and output by the optical fiber amplifier to the first end and outputs it as broadband light by a nonlinear optical phenomenon; and an optical frequency comb comprising: Generator.

[2] A second nonlinear optical medium that receives the broadband light output from the first nonlinear optical medium, generates a second harmonic with respect to the fundamental wave of the broadband light, and outputs the second harmonic.

CEO detecting carrier 'envelope' offset frequency (hereinafter referred to as “CEO frequency”) based on the beat frequency of the second harmonic and fundamental wave of the broadband light output from the second nonlinear optical medium. A frequency detector;

A CEO frequency stabilizing unit that stably controls the wavelength of the pulsed laser light output from the laser light source so that the CEO frequency detected by the CEO frequency detecting unit becomes a predetermined value;

The optical frequency comb generator according to claim 1, further comprising:

[3] a repetition frequency detector for detecting a repetition frequency of the pulse laser beam;

A repetition frequency stabilization unit that stably controls the repetition frequency of the pulse laser beam output from the laser light source so that the repetition frequency detected by the repetition frequency detection unit has a predetermined value;

The optical frequency comb generator according to claim 1, further comprising:

[4] An optical combining unit that combines the broadband light output from the first nonlinear optical medium and the laser light output from another laser light source, and outputs the combined laser light; A heterodyne detection unit for heterodyne detection of the laser beam combined and output by the optical multiplexing unit;

The optical frequency comb generator according to claim 1, further comprising:

Polarization adjustment that is provided between the laser light source and the fiber amplifier, adjusts the polarization of the pulsed laser light output from the laser light source power, and outputs the polarization-adjusted pulsed laser light to the fiber amplifier. The optical frequency comb generator according to claim 1, further comprising a unit.