JP5668265B2 - Optical frequency comb generating apparatus and optical frequency comb generating method - Google Patents

Description

  The present invention relates to an optical frequency comb generating apparatus and an optical frequency comb generating method capable of generating a broadband optical frequency comb that can change a comb interval (for example, GHz order) and a spectrum shape.

  As an optical frequency comb generator, conventionally, an optical pulse OP from a femtosecond laser 81 is propagated to an optical fiber (for example, a photonic crystal fiber) 83 via an optical amplifier 82 as shown in FIG. A technique for generating a broadband optical frequency comb (super continuum light) SC is known (see Patent Document 1, Patent Document 2, and Non-Patent Document 1).

  As shown in FIG. 7B, this technique can generate an optical frequency comb SC having a very wide band (for example, 10 to 50 THz), but the peak intensity of the optical pulse OP determines the bandwidth. Therefore, the repetition frequency is kept low (for example, 100 MHz or less), and a pulse with a high peak power is incident on the optical fiber 83.

  Since the comb interval of the optical frequency comb SC is determined by the above repetition frequency, it is usually about 100 MHz or less. Also, since the center frequency of the optical frequency comb SC is determined by the laser specifications, it is difficult to change. Furthermore, the optical spectrum shape is determined by the shape of the optical pulse OP and the characteristics of the optical fiber.

  Therefore, since it is difficult to adjust the comb interval and the center frequency of the optical frequency comb SC, the spectrum shape is determined by the laser and the fiber used, and there is no mechanism for controlling the absolute frequency of each comb. The accuracy is low.

In addition, as an optical frequency comb generator, as shown in FIG. 8A, a resonance type phase modulation having a structure in which a phase modulator is installed in an optical resonator (schematically shown by mirrors M1 and M2). A technique using the device 92 is known.
In this method, an optical frequency comb having a band of, for example, about 3 THz is generated from laser light from the semiconductor laser 91 by repeatedly generating sidebands with a resonator (Patent Document 3, Patent Document 4, Patent Document 5, Patent). Reference 6 and Non-Patent Document 2).

  In this technique, the characteristics of the optical frequency comb SC (see FIG. 8B) are determined by the modulation frequency of the resonance type phase modulator 92. An optical frequency comb SC having a wide interval can be generated by using, for example, an RF signal of about 10 GHz generated by the RF signal generator 921 as the modulation signal. However, the comb interval frequency is fixed to the basic resonance frequency determined by the time for which light makes one round (reciprocating) operation of the resonator, or an integral multiple thereof. The optical spectrum shape according to this technique has a shape in which the light intensity decreases exponentially as the distance from the center frequency decreases to the low frequency side and the high frequency side.

PCT / JP2002 / 000579 (Broadband optical spectrum generator and pulsed light generator) JP2007-279704 (optical fiber and broadband light source) JP-A-7-58386 (optical frequency comb generator) JP-A-9-258286 (optical frequency comb generator) JP-A-10-65245 (optical frequency comb generator) JP 2002-341394 (optical frequency comb generator and method for manufacturing the same)

H. Nishioka, W.H. Odajima, K .; Ueda, and H.C. Takama, “Ultraband flat continuum generation in multichannel propagation of terawat Ti: sapphire laser pulses,” Opt. Lett. , Vol. 20, no. 24, pp. 2505 — 2507, 1995. M.M. Kouragi, K .; Nakagawa, and M.K. Otsu, “Wide-span optical frequency generator for accumulator optical frequency difference measurement,” IEEE J. Quantum Electron. , Vol. 29, pp. 2693-2701, Oct. 1993. Y. Tanaka, R.A. Kobe, T .; Shioda, H .; Tsuda and T.A. Kurokawa, “Generation of 100-Gb / s packets having 8-Bit return-to-zero patterns using an optical pulse with a pocket of Elouptable. Technol. Lett. , Vol. 21, no. 1, pp. 39-41, 2009.

  In the technique shown in FIG. 7A, as described above, the short optical pulse OP from the femtosecond laser 81 is propagated to the optical fiber 83, so that supercontinuum light (optical frequency comb SC having a wide interval) is generated. Can be generated in a wide band (bandwidth of 10 to 100 THz or more).

  However, in this technique, (a) the comb interval is 100 MHz or less and cannot be made variable. (B) The center frequency and the optical spectrum shape are determined by the specifications of the femtosecond laser and the nonlinear fiber, and are difficult to change. (C) Since there is no mechanism for controlling the absolute frequency of each comb, the absolute frequency accuracy is low, and (d) a femtosecond laser is extremely expensive. For this reason, there are problems that the application range is limited and it is difficult to reduce the cost.

  As described above, the technique of FIG. 8A uses sideband generation by a phase modulator, and only a few sidebands are generated by simple modulation. Therefore, a phase modulator is installed in the resonator. Thus, an optical frequency comb can be generated by the enhancement effect of further generating sidebands from the generated sidebands.

  However, in this technique, (a) it is difficult to make the frequency variable because the comb interval is limited by the resonance frequency of the optical resonator. (B) The comb band is limited by the RF signal power to be applied. There is a problem that the upper limit is about 3 THz. For this reason, there exists a problem that an application range is narrow.

  As shown in FIG. 9, the present invention can generate a wideband (for example, 10 to 100 THz) optical frequency comb having a comb interval frequency on the order of GHz at low cost, and can change the frequency interval and the spectrum shape. An object of the present invention is to provide an optical frequency comb generator and an optical frequency comb generation method capable of increasing the absolute frequency accuracy.

The gist of the present invention is as follows.
(1) a laser;
A seed comb generator that modulates the emitted light of the laser and generates seed comb light;
An optical pulse generator for generating an optical pulse from the seed comb light;
A pulse compression unit that compresses the light pulse emitted from the light pulse generation unit to generate a short light pulse with a shorter pulse width; and
An optical fiber that receives a short light pulse emitted from the pulse compression unit and generates supercontinuum light; and
An optical spectrum analyzer for measuring the spectrum of the supercontinuum light;
An optical frequency comb generator comprising:
An optical frequency comb generator, wherein a measurement result of the optical spectrum analyzer is fed back to the optical pulse generator.
(2)

  The optical frequency comb generator according to (1), wherein the pulse compression unit is an optical fiber for pulse compression.

(3)
The optical frequency comb generator according to (1), wherein the pulse compression unit is an optical phase modulator, and the measurement result is fed back to the optical pulse generation unit and the pulse compression unit.
(4)

  The optical amplifier according to any one of (1) to (3), wherein an optical amplifier is disposed at at least one position after the pulse compression unit, the optical pulse generation unit, and the seed comb generation unit. Frequency comb generator.

(5)
The seed comb generator according to any one of (1) to (4), wherein the seed comb generator is modulated by a sine wave signal having a constant frequency or a variable frequency.

(6)
A seed, and a seed comb generator that modulates light emitted from the laser and generates seed comb light;
An optical pulse generator for generating an optical pulse from the seed comb light;
A pulse compression unit that compresses the light pulse emitted from the light pulse generation unit to generate a short light pulse with a shorter pulse width; and
An optical fiber that receives a short light pulse emitted from the pulse compression unit and generates supercontinuum light; and
A pulse waveform measuring instrument for measuring the short light pulse;
An optical frequency comb generator comprising:
The optical frequency comb generator is characterized in that the measurement result of the pulse waveform measuring device is fed back to the optical pulse generator and the pulse compressor.

(7)
The optical frequency comb generator according to (6), wherein an optical amplifier is disposed in at least one of the subsequent stages of the pulse compressor, the optical pulse generator, and the seed comb generator.

(8)
The seed comb generator according to (6) or (7), wherein the seed comb generator is modulated by a sine wave signal having a constant frequency or a variable frequency.

(9)
The seed comb light is generated by modulating the light emitted from the laser,
Next, an optical pulse is generated from the seed comb light,
Furthermore, the light pulse is compressed to generate a short light pulse with a shorter pulse width,
Thereafter, in the optical frequency comb generating method for generating supercontinuum light by passing the short light pulse through an optical fiber,
An optical frequency comb generator for measuring the spectrum of the supercontinuum light and generating the desired supercontinuum light by feeding back the spectrum measurement result.

(10)
The method for generating an optical frequency comb according to (9), wherein the pulse compression is performed by an optical fiber for pulse compression.

(11)
The optical frequency comb generation method in which the pulse compression is performed by an optical phase modulator, wherein the spectrum measurement result is fed back to generate the supercontinuum light having a desired shape. Optical frequency comb generation method.

(12)
The optical frequency comb generation method according to any one of (9) to (11), wherein the seed comb light is generated by being modulated by a sine wave signal having a constant frequency or a variable frequency.

(13)
The seed comb light is generated by modulating the light emitted from the laser,
Next, an optical pulse is generated from the seed comb light,
Furthermore, the optical pulse is phase-modulated to generate a short optical pulse with a shorter pulse width,
Thereafter, in the optical frequency comb generating method for generating supercontinuum light by passing the short light pulse through an optical fiber,
An optical frequency comb generator that measures the short optical pulse and feeds back a measurement result to generate the supercontinuum light having a desired shape.

(14)
The method of generating an optical frequency comb according to (13), wherein the seed comb light is generated by being modulated by a sine wave signal having a constant frequency or a variable frequency.

[1] The frequency interval of the comb can be made variable in the order of GHz.
[2] An arbitrary center optical frequency can be set, and the absolute frequency accuracy can be increased.
[3] An optical frequency comb having an arbitrary spectral shape in a wide band can be created. For this reason, all very wide applications (high-precision spectroscopy, three-dimensional measurement, optical communication, etc.) are possible.

It is explanatory drawing which shows one Embodiment of the optical frequency comb generator of this invention using the optical phase modulator as a pulse compression part. It is a figure which shows the specific example of the seed comb generation | occurrence | production part used in this invention, (A) is a structural example by one optical modulator, (B) is comprised from two optical modulators connected to the tandem. FIG. It is a figure which shows an example with the optical pulse production | generation part used in this invention. It is a figure which shows typically how an optical spectrum and a time waveform change in each process of FIG. It is a figure which shows embodiment of the optical frequency comb generator of this invention whose pulse compression part is an optical fiber for pulse compression. It is a figure which shows embodiment of the optical frequency comb generator of this invention which measures and feeds back the short optical pulse output from the pulse compression part, without measuring the super continuum light output from an optical fiber. It is explanatory drawing of a prior art, (A) is a figure which shows the example which propagates the optical pulse from femtosecond laser to an optical fiber via an optical amplifier, and produces | generates a broadband optical frequency comb, (B) is emitted light It is a figure which shows the spectrum of (broadband optical frequency comb). It is explanatory drawing of a prior art, (A) and the example which produces | generates an optical frequency comb with a narrow band from the laser beam from a semiconductor laser by repeatedly producing | generating a sideband with a resonator using the resonance type phase modulator 92. The figure shown, (B) is a figure which shows the spectrum of an emitted light (narrow-band optical frequency comb). It is a figure which shows the spectrum of the optical frequency comb of the broadband (for example, 10-100 THz) with the comb space | interval frequency of GHz order achieved by this invention.

An embodiment of the present invention will be described with reference to FIG.
In FIG. 1, an optical frequency comb generator 1A includes a laser 11, a seed comb generator 12, an optical pulse generator 13, a pulse compressor 14, an optical amplifier 15, an optical fiber 16, and an optical spectrum analyzer 17. And.

  As the laser 11, a semiconductor laser, a gas laser, a solid laser, or the like that outputs continuous light can be used. For example, a laser whose oscillation wavelength is locked to an acetylene absorption line used as an absolute frequency standard can be used. The optical frequency comb SC (super continuum light) that is finally generated has a frequency component at a certain frequency interval with reference to the frequency of the light source. Com can be generated. Alternatively, a tunable laser can be used as the laser 11 to set the center wavelength of the finally generated optical frequency comb SC to an arbitrary wavelength.

  The seed comb generator 12 can modulate the light emitted from the laser 11 and generate the seed comb light PL. As shown in FIG. 2 (A), the seed comb generation unit 12 is supplied from one optical modulator, or from the first modulator 12a and the second modulator 12b connected in tandem as shown in FIG. 2 (B). Can be configured. An RF sine wave signal of, for example, a GHz order (10 GHz in FIGS. 2A and 2B) generated by the RF signal generator 121 is applied to the optical modulator 12.

  The optical pulse generator 13 receives the seed comb light PL and generates an optical pulse OP. An example of the optical pulse generator 13 is shown in FIG. In FIG. 3, the optical pulse generator 13 includes an optical circuit 131, a current controller 132, and an optical circulator 133.

  In FIG. 3, an optical circuit 131 includes an arrayed waveguide grating (AWG) 1311 that demultiplexes light of each frequency component of the seed comb light PL into different channel waveguides C, and an intensity modulator 1312. A phase modulator 1313 is integrated. Regarding this optical circuit 13, there is Non-Patent Document 3 (Non-Patent Document 3).

  The waveform of the optical signal is formed by superimposing various frequency components, and if the respective phases are aligned, it becomes a pulse, and if the phases are different, it does not become a pulse. The optical pulse generator 13 can synthesize an optical pulse OP having an arbitrary shape by individually adjusting the amplitude and phase of each frequency component of the seed comb light PL. FIG. 4 schematically shows how the light spectrum and the time waveform change in each process.

  The pulse compressor 14 widens the spectrum band of the pulsed light emitted from the optical pulse generator 13 and compresses the pulse width to generate the short optical pulse NP. Although the optical phase modulator is used in FIG. 1 for compressing the optical pulse OP, an optical fiber for pulse compression can be used as will be described later.

  When an optical phase modulator is used as the pulse compression unit 14, the spectrum band can be expanded by applying phase modulation to the optical pulse OP, and the width of the optical pulse OP can be compressed. At this time, an RF sine wave signal is applied from the RF signal generator 140 to the optical phase modulator. The frequency of this RF sine wave signal is set to the same frequency as the RF sine wave signal applied to the modulator by the seed comb generator.

  The short optical pulse NP that has been pulse-compressed has a high peak power because its energy is condensed to a short pulse width. As the peak power increases, the optical frequency comb SC (super continuum light) emitted from the optical fiber 16 becomes higher in efficiency and has a wide band as will be described later.

  The optical amplifier 15 amplifies the short optical pulse that has been pulse-compressed, and further increases the peak power.

  The optical fiber 16 propagates the amplified short optical pulse NP and generates an optical frequency comb SC. That is, a broadband optical frequency comb SC is generated by the optical nonlinear effect while the amplified short optical pulse NP enters and propagates in the optical fiber.

  The operation of the optical frequency comb generator 1 and the optical frequency comb generation method of the present invention, that is, control of the spectral band and spectral shape of the optical frequency comb SC will be described.

  In order to obtain an optical frequency comb SC having a desired spectral band and spectral shape, it is necessary to optimally control signals applied to the intensity modulator 1312 and the phase modulator 1323 of the optical pulse generator 13 referred to in FIG. is there.

  Further, when an optical phase modulator is used for the pulse compression unit 14, it is necessary to optimally control the voltage of the RF sine wave signal applied to the optical phase modulator of the pulse compression unit 14. Therefore, in order to perform optimal control, as shown in FIG. 1, the spectrum of the optical frequency comb SC is observed by the optical spectrum analyzer 17, and the difference from the target spectrum waveform is used as an error signal to generate an optical pulse. By feeding back to the unit 13 and the pulse compression unit 14, the SC light having a spectrum close to the target spectrum waveform is automatically generated. As this control algorithm, for example, a genetic algorithm can be used.

  FIG. 5 shows an optical frequency comb generator 1B in which the pulse compression unit 14 is a pulse compression optical fiber (indicated by reference numeral 141). In FIG. 5, an optical amplifier (indicated by reference numeral 151) is also added to the previous stage of the optical fiber 141. In the optical frequency comb generator 1B, when the optical fiber 141 is used as the pulse compression unit 14, the pulsed light emitted from the optical pulse generation unit 13 propagates through the optical fiber 141, and the light of the optical fiber 141 is propagated during this propagation. Non-linear effects (particularly the optical Kerr effect) broaden the pulse spectrum and compress the pulse width.

FIG. 6 shows an optical frequency comb generator 1C that measures the short optical pulse NP output from the pulse compression unit without measuring the output of the optical fiber (optical frequency comb SC). The optical frequency comb generator 1 </ b> C has a configuration in which the optical spectrum analyzer 17 is removed from the optical frequency comb generator 1 </ b> A of FIG. 1 and a pulse waveform measuring device 18 is provided on the emission side of the pulse compressor 14.
The pulse waveform measuring device 18 measures the characteristics of the short optical pulse NP and feeds back the measurement result to the optical pulse generator 13 and the pulse compression unit 14. The time waveform of the short optical pulse NP output from the pulse compressing unit 14 is observed by the pulse waveform measuring device 18, and the difference from the target time waveform of the short optical pulse NP is used as an error signal, and the optical pulse generating unit 13 and By feeding back to the pulse compression unit 14, an optical frequency comb SC having a wide-band spectrum is automatically generated.

1A, 1B, 1C Optical frequency comb generator 11 Laser 12 Type comb generator 12a First modulator 12b Second modulator 13 Optical pulse generator 14 Pulse compression unit 15 Optical amplifier 16 Optical fiber 17 Optical spectrum analyzer 18 Pulse waveform measurement 131 Optical circuit 132 Current control circuit 133 Circulator 1311 Arrayed waveguide grating (AWG)
1312 Intensity modulator 1313 Phase modulator 140 RF signal generator 141 Optical fiber for pulse compression

Claims (10)

Laser,
A seed comb generator that modulates the emitted light of the laser and generates seed comb light;
An optical pulse generator for generating an optical pulse from the seed comb light;
A pulse compression unit that compresses the light pulse emitted from the light pulse generation unit to generate a short light pulse with a shorter pulse width; and
An optical fiber that enters a short light pulse emitted from the pulse compression unit and generates supercontinuum light; and
An optical spectrum analyzer for measuring the spectrum of the supercontinuum light;
An optical frequency comb generator comprising:
An optical frequency comb generator, wherein a measurement result of the optical spectrum analyzer is fed back to the optical pulse generator.   2. The optical frequency comb generator according to claim 1, wherein the pulse compression unit is an optical fiber for pulse compression.   2. The optical frequency comb generator according to claim 1, wherein the pulse compression unit is an optical phase modulator, and the measurement result is fed back to the optical pulse generation unit and the pulse compression unit.   The optical frequency comb according to any one of claims 1 to 3, wherein an optical amplifier is arranged at least at one stage subsequent to the pulse compression unit, the optical pulse generation unit, and the seed comb generation unit. Generator.   5. The optical frequency comb generator according to claim 1, wherein the seed comb generator is modulated by a sine wave signal having a constant frequency or a variable frequency. Generating a seed comb light by modulating the laser light emitted by Taneko arm onset raw part,
Next, an optical pulse is generated from the seed comb light by the optical pulse generator,
Furthermore, the optical pulse is compressed by a pulse compression unit to generate a short optical pulse with a shorter pulse width,
Thereafter, in the optical frequency comb generating method for generating supercontinuum light by passing the short light pulse through an optical fiber,
A method of generating an optical frequency comb, comprising: measuring a spectrum of the supercontinuum light; and feeding back a spectrum measurement result to the optical pulse generator to generate the supercontinuum light having a desired spectrum shape.   7. The method of generating an optical frequency comb according to claim 6, wherein the pulse is compressed by an optical fiber for pulse compression. An optical frequency comb generation method in which the pulse compression is performed by an optical phase modulator,
7. The optical frequency comb generation method according to claim 6, wherein the spectrum measurement result is further fed back to the pulse compression unit to generate the supercontinuum light having a desired spectrum shape.   9. The optical frequency comb generation according to claim 6, wherein optical amplification is performed in at least one of the following stages of the pulse compression unit, the optical pulse generation unit, and the seed comb generation unit. Method.   10. The optical frequency comb generation method according to claim 6, wherein the seed comb light is generated by being modulated by a sine wave signal having a constant frequency or a variable frequency.

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