JP2000089264A - Reference generating device for wideband light frequency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate the reference of light frequencies over a frequency band exceeding the gain band of a light amplifier. SOLUTION: This device is provided with a master laser 1 generating continuous lights becoming the reference of light frequencies, an optical branching means 2 branching the continuous lights from the laser 1, a light frequency sweeping device 3 outputting a light pulse train having frequencies which are time sequentially different based on continuous lights from one output port of the optical branching means 2, an optical multiplexing means 5 multiplexing the light pulse train and continuous lights from other output port of the means 2, a nonlinear optical medium 6 generating lights having comb-shaped frequency spectra having intervals equivalent to differences among light frequencies of the continuous frequencies from the laser 1 and the light frequency of the light pulses in respective light pulses among the light pulse train by inputting output lights from the means 5 and a light output means outputting lights to be generated in the medium 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a broadband optical frequency reference generator. More specifically, the present invention relates to a technique for generating a high-precision frequency reference for light waves.

[0002]

2. Description of the Related Art A conventional optical frequency sweep apparatus is shown in FIG. 12 (Japanese Patent Application No. 4-36181). As shown in the drawing, continuous light output from the master laser 1 whose absolute frequency is stabilized is pulsed by a pulse cutting optical switch 21 and the light pulse is internally provided in an optical delay line 23 and an optical amplifier 2.
4. By circulating in an optical ring circuit including an optical frequency shifter 26 and an optical switch 27 and having an optical input port 22 and an output port 28, the optical frequency shifter is discretely shifted from the output port 28 in steps of the frequency shift width Δf of the optical frequency shifter. An optical pulse train whose frequency has been swept is obtained.

[0003] This optical pulse train is expected to be applied as an optical frequency reference in an absolute frequency measurement of a light wave or a wavelength division multiplexing (WDM) communication network. The frequency sweep width of the optical frequency sweep device is proportional to the number of times the optical pulse circulates in the optical ring circuit. The number of rounds is limited by spontaneous emission (ASE) noise emitted from the optical amplifier 24 in the optical ring circuit every round.

In order to eliminate the spontaneous emission optical noise and increase the number of rounds, a configuration has been proposed in which an optical bandpass filter (BPF) 25 is inserted into an optical ring circuit (Japanese Patent Application No. 4-262154). According to this configuration, the frequency sweep width increases, but is limited by the transmission bandwidth of the bandpass filter 25, and when the AOS of -120 MHz and the bandpass filter of 5 nm are used, the limit is about 100 GHz (reference). References: K. Shimizu etal., IEEE J.
Quantum Electron., Vol. 31, No. 6, pp. 1038-1046, 19
95).

Further, in order to suppress the instability of the number of rounds caused by the fluctuation of the polarization state of the optical pulse circulating in the optical ring circuit, a depolarizer is inserted in the optical ring circuit in the configuration of FIG. Japanese Patent Application No. 8-305366
No.), and as shown in FIG.
Fabry-Perot containing tattor (FR27)
An Etalon configuration (Japanese Patent Application No. 9-340338) has also been proposed.

In order to further increase the frequency sweep width, a dynamically controllable wavelength tunable BPF is used as a bandpass filter in an optical ring circuit, and is synchronized with a frequency change of an optical pulse orbiting its transmission center frequency. A variable-bandwidth optical frequency sweeping device has been proposed (Japanese Patent Application No. 4-2621).
No. 55). It has been reported that this method dramatically expands the frequency sweep width exceeding 1 THz (Takei et al., To be announced at the 1998 IEICE Society Conference).

[0007]

However, even in the above-mentioned broadband optical frequency sweep device, the frequency sweep width is limited by the gain band of the optical amplifier in the optical ring circuit. The present invention has been made in view of the above problems, and has as its object to generate an optical frequency reference over a frequency band exceeding a gain band of an optical amplifier.

[0008]

According to a first aspect of the present invention, there is provided a broadband optical frequency reference generator which emits a continuous light serving as an optical frequency reference, and a continuous light from the master laser. An optical frequency sweeping device that outputs an optical pulse train having different optical frequencies in a time series based on continuous light from one output port of the optical branching device, and the optical pulse train and the optical pulse train. Optical multiplexing means for multiplexing continuous light from another output port of the optical branching means, and by inputting output light from the optical multiplexing means, for each optical pulse in the optical pulse train, A nonlinear optical medium that generates light having a comb-like frequency spectrum with an interval equal to the difference between the optical frequency of the continuous light and the optical frequency of the optical pulse; Characterized in that it comprises an optical output means for outputting the light.

According to a second aspect of the present invention, there is provided a broadband optical frequency reference generator according to the first aspect of the present invention.
An optical amplifier that amplifies the light intensity of the continuous light from the master laser and an optical amplifier that amplifies the light intensity of the optical pulse train in order to remarkably generate a comb-like optical frequency spectrum in the nonlinear optical medium. It is characterized by having.

According to a third aspect of the present invention, there is provided a broadband optical frequency reference generator according to the present invention, wherein each pulse has only a single frequency from an optical pulse train having a different comb-shaped spectrum obtained in the first aspect. An optical band for selecting and outputting only a specific frequency band component among the frequency components of the output light by inputting the output light from the optical output means for the purpose of generating an optical pulse train having It is characterized by having a pass filter.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] FIG. 1 shows a first embodiment which is a basic configuration of the present invention. As shown in FIG. 1, continuous light output from a master laser 1 is split into two by an optical splitter 2, one of which is input to an optical frequency sweeping device 3 and outputs a frequency-swept optical pulse train 4.

The optical pulse train 4 is used to distinguish it from an optical pulse train having a comb-like frequency spectrum that is different in time series, which will be described later, and because it is original frequency information generated by the optical frequency reference according to the present method. , And a monochromatic light frequency sweep light pulse train 4 in the first band. The optical pulse train 4 is multiplexed by the optical multiplexing element 5 with the other of the continuous light from the master laser 1 that has been branched by the optical branching element 2, and is then input to a nonlinear optical medium 6 having a tertiary nonlinearity. You.

Light having a comb-like spectrum that varies in time series due to nonlinear interaction between the output light of the master laser 1 via the nonlinear optical medium 6 and the monochromatic light frequency sweeping light pulse train 4 in the first band. A pulse train, that is, a time-division multiplex comb frequency light pulse train 7 is output. Hereinafter, the nonlinear interaction will be briefly described. Generally 3
The next four-wave mixing in the nonlinear optical medium 6 occurs between light waves satisfying the following phase frequency condition.

[0014] _{k 3 + k 4 = k 1} + k 2 f 3 + f 4 = f 1 + f 2 , _{where, k i, f i (i} = 1~4) represent the phase constant and the frequency of each light wave. FIG. 2 shows the frequency relationship of these four light waves. _One photon of each of the frequencies f ₁ and f ₂ disappears, and one photon having a frequency of f ₃ = 2f ₁ −f ₂ and f ₄ = 2f ₂ −f ₁ is generated one by one. When the intensity of the generated photon is sufficiently strong, the f ₁ and f _3, the same four-wave mixing occurs even between more such f ₂ and f ₄ higher-order sidebands.

By such a chained four-wave mixing, if the dispersion of the nonlinear optical medium is small and the phase condition is satisfied, a comb-like spectrum having a frequency interval of (f ₂ −f ₁ ) is generated. I can do it. Assuming that the frequency of the master laser 1 is f ₀ , the frequency shift by the pulse cutting optical switch 17 is f ₀ , and the frequency shift by the optical frequency shifter 22 is −Δf, after passing through the optical frequency shifter n times, the optical frequency sweeping device The optical pulse frequency f _n ⁽⁻¹⁾ output from the third ^node is expressed as follows. Here, the superscript (-1) represents a primary frequency downshift light pulse train.

F _n ⁽⁻¹⁾ = f _M + f ₀ −nΔf (1) Between the master laser and the output optical pulse train of the optical frequency sweeping device in which the n-th pulse has the frequency of the formula (1). As a result, the n-th light pulse has a comb-like frequency spectrum as shown in FIG. Here, the following equation is obtained. f _n ^(k) = f _M −k · f ₀ + n (k · Δf) The feature is that the spectrum of the k-th order is frequency-shifted in steps of (k · Δf).

As a result, as shown in FIG. 4, optical pulse trains having different comb-like spectra in time series are generated.
As shown in the figure, when the spectrum up to the second order is generated, each optical pulse has four frequency components of f _n ⁽⁻²⁾ , f _n ⁽⁻¹⁾ , f _M , and f _n ⁽⁺¹⁾ . Since the frequency accuracy of each spectrum component is determined by the accuracy of the master laser frequency and the frequency accuracy of the optical frequency shifter, it can be used as a highly accurate optical frequency reference.

[Embodiment 2] (Spontaneous emission light removal) In the basic configuration shown in the first embodiment, a large amount of spontaneous emission noise is contained in the monochromatic sweeping light pulse train 4 output from the optical frequency sweeping device 3. Component, the four-wave mixing between the main light pulse train and the branched light from the master laser 1
A situation arises in which the newly generated sideband component is buried in the spontaneous emission noise and becomes unobservable.

Therefore, in the second embodiment shown in FIG. 5, the monochromatic sweeping light pulse train 4 output from the optical frequency sweeping device 3 is passed through the optical band-pass filter 9 for removing the spontaneous emission light, so that unnecessary natural light is removed. The emitted light noise component can be removed, and the newly generated sideband component can be observed remarkably. Furthermore, by amplifying the monochromatic light sweeping light pulse train 4 and the branch light from the master laser 1 by the optical amplifier 8 respectively, more efficient four-wave mixing can be performed. In this embodiment, a nonlinear optical fiber 10 is used as an example of the nonlinear optical medium 6.

[Embodiment 3] FIG. 6 shows a third embodiment of the present invention.
As shown in FIG. 5, the time-division multiplexed comb frequency optical pulse train 7 obtained by the configuration shown in FIG. 1 or FIG. A swept light pulse train 12 can be generated.

In the example shown in FIG. 5, the intensity of the branched light from the master laser 1 and the intensity of the monochromatic light frequency sweeping light pulse train 4 in the first band output from the optical frequency sweeping device 3 are adjusted so as to be higher than ± 2 order. By passing through the master laser 1 and an optical bandpass filter that removes the frequency components of the monochromatic light frequency sweeping light pulse train 4 in the first band in a state where the next spectrum is not generated, the frequency shifts in the positive direction in steps of Δf. An optical pulse train can be obtained. By branching the monochromatic light frequency sweeping light pulse train 4 of the first band in advance by the light splitting element 2, the frequency sweep width can be equivalently doubled.

[Embodiment 4] (Multi-stage connection) As shown in FIG. 7, a fourth embodiment of the present invention is obtained by a method obtained by combining the first (or second) and third embodiments. The frequency-swept light pulse train 12 of the monochromatic frequency in the second band is multiplexed with the output light from the master laser 1 again, and four-wave mixing is performed in the second nonlinear optical medium 6.
That is, four-wave mixing is performed in a two-stage cascade connection. With this configuration, a broadband optical frequency reference can be generated more efficiently. From the optical pulse train obtained in this way,
It is also possible to generate a monochromatic light frequency sweeping light pulse train of a new band again by the method of the second embodiment.

An example thereof will be described below. As shown in FIG. 8A, a monochromatic light frequency sweeping light pulse train in the second band having a + 1st-order spectral component generated by the first-stage four-wave mixing. A +2 order spectral component is generated by four-wave mixing of the output light from the master laser 12 and the output light from the master laser 1.
By disposing the band-selecting optical bandpass filter 11 at a position on the frequency axis shown in FIG. 8B, a monochromatic light frequency sweeping light pulse train in the third region as shown in FIG. 13 can be generated. Although FIG. 7 shows two stages, the present configuration can be connected in more stages.

Fifth Embodiment (Optical Frequency Synthesizer) In a fifth embodiment shown in FIG. 9, a time-division multiplexed comb-like frequency optical pulse train 7 obtained by the method shown in the first to fourth embodiments or the fifth embodiment will be described. Monochromatic light frequency sweep light pulse train 15 in N band
(N is an integer) is used as an optical frequency reference for the controlled light source unit 16. Since the monochromatic light frequency sweeping light pulse train 15 in the N-th band is obtained by time-division multiplexing of single light frequency information, the method of Japanese Patent Application No. 4-155254 is applied as it is to obtain the controlled light source. Frequency control is possible. On the other hand, an example of a method of controlling the frequency of the controlled light source using the time-division multiplexed comb-like frequency sweeping optical pulse train 7 will be described.

FIG. 10 is a block diagram of the controlled light source unit. The locking target frequency of the controlled light source is the n-th pulse frequency f _N ^(k) of the ^k- th spectrum. It is assumed that the frequency of the controlled light source can be roughly adjusted by only the temperature control. If a beat signal with a spectrum other than the k-th order is output at the same time in the output signal from the electric band-pass filter 18 having the bandwidth B, it becomes noise and adversely affects the frequency synchronization operation.

Therefore, by setting the synchronization target frequency so as to satisfy the following equation, the beat signal with the other order spectral component at the same point on the time axis is observed after passing through the electric band-pass filter. However, the above problem can be avoided. | F _N ^(k) −f _M |> k · B / 2 (2) FIG. 11 shows the time change of the frequency spectrum of the time-division multiplex comb-like optical pulse train when k = 2. +3
It is assumed that the following spectral components have been observed, and equation (2) holds.

At this time, after passing through the electric band-pass filter, beat signals are observed at two points shown in FIG. 11 on the time axis. The beat signal between the lowest-order frequency spectrum component and the controlled light source can be recognized as a beat signal that appears on the time axis at the point that is the latest from the light pulse incident time.

By applying this signal to the method of Japanese Patent Application No. 4-155254, the frequency of the controlled light source can be controlled. Further, by dividing the optical pulse train by the optical branching element and distributing it to the plurality of controlled light source units 16, the absolute frequencies of the plurality of controlled light source units 16 can be simultaneously controlled stably, which is effective as a light source in a WDM communication network. .

[0029]

As described above in detail, by using the broadband optical frequency reference generator of the present invention, it is possible to supply a highly accurate optical frequency reference over a wide frequency band in a time division multiplexed form. For example, by using it as an optical frequency reference for a WDM optical communication network, the quality of communication can be further improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment (claim 1) of a broadband optical frequency reference generator of the present invention.

FIG. 2 is a configuration diagram showing a frequency arrangement of four-wave mixing light.

FIG. 3 is a frequency spectrum diagram of an n-th output light pulse.

FIG. 4 is a time-division multiplexed comb frequency pulse train diagram.

FIG. 5 is a block diagram showing a second embodiment (claim 2) of the broadband optical frequency reference generator of the present invention.

FIG. 6 is a block diagram showing a third embodiment (claim 3) of the broadband optical frequency reference generator of the present invention.

FIG. 7 is a configuration diagram showing a fourth embodiment of the broadband optical frequency reference generator of the present invention.

FIG. 8 is a configuration diagram illustrating a frequency arrangement example of an optical bandpass filter for band selection in a fourth embodiment.

FIG. 9 is a block diagram showing a fifth embodiment of the broadband optical frequency reference generator of the present invention.

FIG. 10 is a block diagram of a controlled light source unit.

FIG. 11 is an explanatory diagram showing a time-division multiplex comb frequency pulse train and frequency locking of a controlled light source.

FIG. 12 is an explanatory diagram showing an optical frequency sweeping device (ring type) which is a conventional optical frequency reference generating device.

FIG. 13 is an explanatory diagram showing an optical frequency sweep device (FP resonator type) which is a conventional optical frequency reference generation device.

[Explanation of symbols]

 Reference Signs List 1 master laser 2 optical branching device 3 optical frequency sweeping device 4 monochromatic optical frequency sweeping pulse train 5 optical multiplexing device 6 nonlinear optical medium 7 time-division multiplexing comb-like frequency sweeping optical pulse train 8 optical amplifier 9 optical band-pass filter for removing spontaneous emission light DESCRIPTION OF SYMBOLS 10 Nonlinear optical fiber 11 Optical bandpass filter for band selection 12 Monochromatic light frequency sweeping light pulse train in second band 13 Monochromatic light frequency sweeping light pulse train in third band 14 Broadband optical frequency reference generator of the present invention 15 Nth Monochromatic light frequency sweeping light pulse train in band 16 Controlled light source unit 17 Photodetector 18 Electric bandpass filter (BPF) 19 Feedback circuit 20 Controlled light source 21 Pulse cutout optical switch 22 Input port 23 Optical delay line 24 Optical amplifier 25 Optical bandpass filter (BPF) 26 Optical frequency shifter 27 Optical switch Pitch 28 output ports 29 synchronization control circuit 30 the optical isolator 31 Faraday rotator (FR) 32 polarizer 33 mirror 34 optical amplifying gain medium 35 Faraday rotator mirror (FRM) 36 pumping light laser 37 pumps Hikari Mitsumochi isolator 38 WDM coupler

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Tsuneo Horiguchi 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo F-term in Nippon Telegraph and Telephone Corporation (reference) 2K002 AA02 AB12 BA02 EA10 EA30 HA31

Claims (3)

[Claims] 1. A master laser that emits continuous light serving as an optical frequency reference, an optical splitter that splits continuous light from the master laser, and a continuous light from one output port of the optical splitter. An optical frequency sweeping device that outputs an optical pulse train having different optical frequencies in time series, an optical multiplexing unit that multiplexes the optical pulse train and continuous light from another output port of the optical branching unit, and the optical multiplexing unit. By inputting the output light from the optical pulse train, each optical pulse in the optical pulse train has a comb-like frequency spectrum having an interval equal to the difference between the optical frequency of the continuous light from the master laser and the optical frequency of the optical pulse. A nonlinear optical medium that generates light;
And a light output means for outputting light generated in the nonlinear optical medium. 2. The broadband optical frequency reference generator according to claim 1, wherein the intensity of continuous light from the master laser is reduced in order to remarkably generate a comb-like optical frequency spectrum in the nonlinear optical medium. A broadband optical frequency reference generator, comprising: an optical amplifier for amplifying; and an optical amplifier for amplifying the light intensity of the optical pulse train. 3. The broadband optical frequency reference generator according to claim 1, wherein the output light from the optical output means is input to select only a component of a specific frequency band from the frequency components of the output light. A broadband optical frequency reference generator, comprising: an optical bandpass filter that outputs the output.