JPH09258286A - Optical frequency comb generation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wide sideband wave generation range more suitable for practical use than conventional one while keeping a frequency interval of a practically sufficiently narrow sideband wave by providing a harmonics generation means and a drive signal generation means obtaining a drive signal making a required frequency a basic frequency from the output signal of the harmonics generation means in a driving device. SOLUTION: The driving device 2 driving an optical frequency comb generator 1 is constituted of the harmonics generation means 3 generating frequency components of e.g. 4GHz and 12GHz and the drive signal generation means 4 superimposing the frequency component of 12GHz on the frequency component of 4GHz and generating the drive signal. An oscillator 21 outputs a signal of 4GHz, and a phase clock oscillator 22 is provided with three times as frequency as the oscillator 21, and outputs the signal whose phase is synchronized with it. A mixer 24 mixes the signals based on the outputs of the oscillator 21 and the phase clock oscillator 22, and an amplifier 25 amplifies the output from the mixer 24 to output the drive signal of the optical frequency comb generator 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for generating an optical frequency reference in a wide band.

[0002]

2. Description of the Related Art FIG. 16 shows the configuration of a conventional optical frequency comb generator. The conventional optical frequency comb generator comprises the optical frequency comb generator 1 and the oscillator 21. An optical frequency comb generator 1 that can be driven by a sinusoidal microwave having a predetermined frequency includes at least one mirror 12a, 12 on each side of an optical phase modulator 11 that utilizes the electro-optic effect.
The optical resonator 12 is configured by arranging b. In such an optical frequency comb generator 1, the microwave synchronized with the time when the input light reciprocates in the optical resonator 12 is used as a drive input from the oscillator 21 to the optical phase modulator 11, so that the optical phase Compared to the case of passing through the modulator 11 only once,
It is possible to apply deep phase modulation of several ten times or more. As a result, higher sidebands can be strongly generated, and the frequency intervals fm of adjacent sidebands are all equal to the frequency fm of the input microwave. Therefore, it is possible to use the spectrums arranged in a comb-like shape on the frequency axis at equal intervals as a relative reference of the optical frequency. If the frequency ν of the input light is known, the frequency of each sideband is the sum of the frequency of the input microwave and the sideband order, added to the frequency of the input light. Optical frequency reference.

By the way, when the optical phase modulator contained in the optical frequency comb generator is capable of maintaining a high phase modulation index over a wide band, the frequency of the input microwave is changed by the incident light within the optical resonator. An optical frequency comb signal can be generated by multiplying the reciprocal of one round trip time (a free spectrum region) by a natural number. An ordinary optical frequency comb generator has a free spectrum range of about 1 to 3 GHz, and a broadband phase modulator has an operating frequency range of DC to 20 GHz.
It is a degree. Therefore, when the free spectrum range is 2 GHz, the driving frequency is 2, 4, 6, ..., 20
It is possible to select GHz.

Conventionally, a specific one is selected from a plurality of such frequencies and the drive signal is a microwave of a single frequency. If the drive frequency is increased with the power of the input light and microwaves kept constant, the sideband frequency interval of the optical frequency comb signal increases, so the frequency at which sidebands with a certain intensity or higher are generated in proportion to the frequency. The range expands. The generated sideband is combined with light from another light source (hereinafter referred to as a local light source) and is received. From the frequency of the received light signal (heterodyne signal), the sideband and light from another light source are received. It is used for the purpose of detecting the frequency difference of. The signal corresponding to the detected frequency difference is used for the optical frequency offset lock control, which will be described later, and is the main use of the optical frequency comb generator.

[0005]

The basic characteristic desired for the optical frequency comb generator is that the power of the input light and the driving microwave is constant (the power of the microwave that can be input to the optical phase modulator). There is a limit.) That is, the frequency range in which sidebands above a certain amount of light are generated is wide.
From this point of view, it is desirable that the optical frequency comb generator including the optical phase modulator that can be driven in a wide band be driven at a higher frequency.

However, since the optical frequency comb signal is used for the purpose of utilizing the heterodyne signal of the sideband contained therein and the light from the local light source, it is practically necessary to increase the driving frequency. This often becomes an obstacle when arbitrarily setting the optical frequency of the local light source. This is mainly due to the limitation of the response band of the light receiver. Usually, a light receiver that responds up to about 2 GHz is inexpensive, and an electric circuit required for processing such as amplification and frequency division of a received light signal is easy to handle and inexpensive. For this reason, it is desired to drive the optical frequency comb generator at a frequency of about 4 GHz or less, which is twice the light receiving band that can be easily realized. In this case, since many sidebands are generated at equal intervals with a frequency interval of about 4 GHz or less, if light can be received in a band of about ± 2 GHz from each sideband, the light of the local light source is generated over the entire sideband generation range. The frequency can be set arbitrarily.

Further, when controlling the oscillation frequency of the local oscillation light source on the basis of the heterodyne signal, it is difficult to directly handle signals up to GHz order, so the signal divided by the prescaler is phase-compared with the signal giving the reference frequency. Often to obtain a control signal. Such a control method is
This is called optical frequency offset lock control because the frequency difference between the two lights that generate the heterodyne signal is fixed to the frequency (offset frequency) that is the reference frequency given for phase comparison multiplied by the frequency division ratio. , Is the main use of optical frequency comb generator. At this time, the signal speed after frequency division is limited by the operation speed of the logic IC that constitutes the phase comparator (usually several tens of MHz is the upper limit), and it is necessary to increase the frequency division ratio as the frequency of the heterodyne signal is higher. . Increasing the frequency division ratio reduces the frequency resolution at the time of phase comparison with respect to the frequency difference between the two lights that generate the heterodyne signal. Therefore, as a result, the stability of the frequency difference between the two lights subjected to the optical frequency offset lock control deteriorates. Therefore, also from this viewpoint, it is desired that the frequency interval of the sideband waves included in the optical frequency comb signal is narrow.

Here, the sideband generation range will be described with reference to typical numerical values. Considering that the oscillating bandwidth of the semiconductor laser in the wavelength band of 1.55 μm is about 100 nm, and the gain band of the erbium-doped fiber amplifier is close to 40 nm, it is desired that the sideband generation range is 10 nm or more. . Typical numbers are 12
When it is driven at about GHz, even if the sideband generation range of 24 nm (3 THz) of -70 dBm or more at an optical input of 10 mW with a wavelength of 1.55 μm and a microwave input of about 2 W is set to about 4 GHz, the sideband The generation range is about 1/3 of about 8 nm (1 THz).

As described above, in the conventional optical frequency comb generator, there is a limit to the power of the microwave that can be input to the optical phase modulator. Therefore, unless the frequency interval of the sideband is widened, the sideband generation range is set. Cannot be widened, and the frequency interval of the sideband cannot be widened due to the relationship with the light receiving band,
A practical sideband generation range was not obtained. An object of the present invention is to realize an optical frequency comb generator capable of obtaining a wider sideband generation range that is more practical than the conventional one while maintaining a narrow practical sideband frequency interval.

[0010]

Therefore, in the present invention, as a microwave drive signal input to an optical frequency comb generator having an optical phase modulator capable of being driven in a wide band, a frequency component equal to the frequency interval of sidebands ( A fundamental wave or its harmonics, or a microwave in which the harmonics are positively superimposed, is used so that a component having a fundamental frequency) is obtained.

That is, an optical frequency comb generator according to the present invention has an optical frequency comb generator and a drive device for driving the optical frequency comb generator, and the drive device has a natural number multiple of a desired frequency. A harmonic wave generating means for generating two or more frequency components and a drive signal generating means for obtaining a drive signal having the desired frequency as a fundamental frequency from an output signal of the harmonic wave generating means are provided.

[0012]

The optical frequency comb generator can be regarded as an optical resonator in which the resonator length expands and contracts slightly in synchronization with the round trip of light. Therefore, the light that has entered the optical resonator with a time difference that is an integral multiple of the time required for the light to make one round trip has the same behavior as a normal optical resonator. In other words, as is well known, when a constant amount of monochromatic light is constantly incident, if the phase transition when the light incident at one time makes one round trip is an integral multiple of 2π, resonance occurs, Ideally, the same amount of transmitted light as the amount of incident light can be obtained. Further, if the phase shift when the light incident at another time makes one round trip is away from an integral multiple of 2π by π / (finesse) or more, the amount of transmitted light is 1/2 or less of that at resonance. Here, finesse is the average number of round trips of light that reciprocates in the resonator at the time of resonance, and the higher the finesse, the steeper the change in the amount of transmitted light with respect to the round trip phase transition of light. In this way, the operation of the optical frequency comb generator can be viewed as a shutter that produces a short light pulse from a constant amount of monochromatic light when viewed in the time domain. When a single frequency microwave is input to the optical frequency comb generator that satisfies the resonance condition without microwave input, the shutter open / close frequency is twice the drive frequency and the shutter open / close time is finesse. And is inversely proportional to the product of the phase modulation index and the driving frequency. Finesse is an optically determined quantity and is considered to be constant. The phase modulation index is proportional to the power of 1/2. Further, although it varies depending on the driving frequency, it may be considered to be almost constant up to about 15 GHz in an ordinary optical phase modulator that can be driven in a wide band. Therefore, when the drive power is constant, the opening / closing time of the shutter is considered to be inversely proportional to the drive frequency. Generally, a pulse that is short in time has a wide spectrum in the frequency domain, and a periodic pulse train has a spectrum only in an integral multiple of the reciprocal of the pulse interval in the frequency domain (expanded to Fourier series). Are known. In other words, the shorter the opening / closing time of the shutter, the wider the spectrum is, and the sidebands are arranged at the same intervals as the driving frequency.

By the way, when the reciprocal phase of light deviates from the resonance condition of the optical resonator by 2π / (finesse) or more, the amount of transmitted light becomes small, and until the reciprocal phase deviates from the resonance condition by π.
The amount of transmitted light hardly decreases. Therefore, when the optical resonator is used as a shutter, in order to shorten the opening / closing time of the shutter, the reciprocal phase, that is, the microwave electric field may be suddenly changed in the vicinity of the resonance condition ± 2π / (finesse). At any point away from the conditions, the transmitted light is hardly affected by any change. From this, it is understood that the microwave drive signal input to the optical frequency comb generator does not have to be a sine wave, and may have a shape close to a rectangular wave or a pulse shape. Rather, considering a sine wave and a waveform in which the amplitude of the sine wave is limited by a limiter,
It can be seen that the sideband generation range is the same and the drive power is less limited when the amplitude is limited.

More specifically, a quantitative description will be made on the case where a third harmonic is superposed on a fundamental wave having a frequency equal to a desired sideband interval. The applied microwave electric field is E (t), the amplitude of the fundamental wave is E1, the angular frequency is ω,
Let the amplitude of the third harmonic be E3, and at time t = 0, the initial phases of the fundamental wave and the third harmonic are both 0 (in sin).
And In addition, the total power is 1. E (t) = E1sinωt + E3sin3ωt (1) (E1 squared) + (E3 squared) = 1 (2) In the above description, the time change of the microwave electric field near the resonance condition. It was shown that the sideband generation range can be expanded by increasing the rate. Therefore, assuming that the resonance condition is satisfied at t = 0, and considering the time change rate of E (t) at this time, E ′ (0) = ω · (E1 + 3 · E3) (′ is the time derivative ) ... (3) Eq is deleted from the equations (2) and (3), and q =
If E ′ (0) / ω is set, q = √ (1− (E3 squared)) + 3 · E3 ... (4) is obtained. Paying attention to 0 ≦ E3 ≦ 1, it can be seen that 1 ≦ q ≦ 3. This indicates that when driven by only the third harmonic, a sideband generation range three times as large as when driven by only the fundamental wave is obtained. Further, it is suggested that when the fundamental wave and the third harmonic are superposed, an intermediate sideband generation range is obtained between the case of only the fundamental wave and the case of only the third harmonic. Rewriting the right side of the equation (4) as electric powers P1 and P3 of the fundamental wave and the third harmonic, q = √ (1-P3) + √ (3 · P3) (5) When P3 is set to 0.1 and 0.3, q is estimated to be 1.9 and 2.5, respectively. This means that the sideband generation range is approximately doubled by only superimposing the third harmonic of power that is about 1/10 of the fundamental wave, and a great effect is obtained.

In the time domain, the fact that the fundamental frequency is constant and the pulse width is narrow means that the light quantity of the optical frequency comb signal is reduced, but the optical input is in the order of mW.
In a normal usage method utilizing sidebands up to about 70 dBm, the effect of broadening the spectrum is dominant, and the effect of light amount reduction is small.

Further, the drive signal waveform may change due to the phase difference between the fundamental wave and the higher harmonic wave, and the spectrum of the optical frequency comb signal may become asymmetric. The most prominent example is the case where the second harmonic of the power of 1/4 of the fundamental wave is superimposed at the initial phase 0 or π (in sin display) (Figs. 5 and 6). In this case, the optical frequency comb signal is used. Almost no lower or upper sidebands are generated. However, normally, both sidebands of the optical frequency comb signal are not used at the same time, and even when the even harmonics are present or the initial phase of the fundamental wave and the harmonics is zero.
Even if it is not, it is practical. Furthermore, since the optical frequency comb generator is non-linear with respect to the drive signal, it is necessary to consider the influence of the direct current (bias) electric field (FIG. 7). However, applying a bias electric field is equivalent to changing the round-trip phase of the optical resonator. Therefore, even if the temperature of the optical frequency comb generator is changed and the resonator length is changed, the same effect can be obtained. To be If the frequency of the incident light is variable, it may be changed.

Further, when only the second harmonic and the third harmonic are superposed without including the fundamental wave, the fundamental wave component which is the difference frequency appears in the output due to the (secondary) nonlinearity ( 10 (a), (b)). As described above, there is a large degree of freedom in the amplitude and the initial phase for mixing harmonics, but in principle it is necessary that the sidebands corresponding to the fundamental frequency be generated with sufficient intensity. Therefore, for example, it is not desirable to mix the fundamental wave and the third harmonic wave at a ratio of 1: 9.

Simple examples of drive signals are shown in FIGS. FIG. 8 is an example in which the second harmonic is superimposed on the fundamental wave, FIG. 9 is an example in which the third harmonic is superimposed on the fundamental wave, and FIG. 10 is an example in which the third harmonic is superimposed on the second harmonic. Reference numeral 11 is an example in which the second harmonic and the third harmonic are superimposed on the fundamental wave. In these figures, the solid line shows the signal waveform and the broken line shows the first derivative of the signal waveform. The chain line shows the bias, and an optical pulse is generated near the intersection with the solid line. Therefore, the value of the first derivative at this point indicates the spread of the output spectrum. The positive and negative of the first derivative correspond to the upper and lower sidebands, respectively. Further, in FIG. 12, the total power is 2, and the fundamental wave and 1
The maximum derivative is shown when two harmonics are superposed. Naturally, the higher the higher harmonic, the larger the differential coefficient, that is, the spread of the spectrum.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION An optical frequency comb generator according to the present invention comprises an optical frequency comb generator 1 and its driving device (driving signal source) 2 as shown in FIGS.
The optical frequency comb generator 1 is an optical resonator having a built-in optical phase modulator 11 and modulates incident light with a microwave drive signal as an input to generate sidebands up to high order. Moreover, the optical frequency comb generator 1 can be driven by a microwave having a specific frequency, and at least 1
It can be driven by an integer multiple of two or more. For example, it can be driven at 4 GHz or 12 GHz. This condition is satisfied in the optical frequency comb generator with the built-in optical phase modulator that can be driven in a wide band. In addition to the above, an electrode of an optical phase modulator that can be driven in a wide band, which is processed so as to generate microwave resonance, or one which uses a microwave waveguide, can be used as long as it satisfies this condition.

The drive signal source 2 is for generating a periodic microwave drive signal for driving the optical frequency comb generator 1, and has a frequency component capable of driving the optical frequency comb generator 1. It includes. For example, 4,
For an optical frequency comb generator that can be driven at 8, 12, or 16 GHz, when the desired sideband spacing is 4 GHz, normally the 4 GHz component is the fundamental wave and the 12 GHz frequency component (third harmonic), etc. Use what includes. in this case,
Even if the drive signal has a fundamental wave of 2 GHz component, 4,
It suffices to include a frequency component of 8 GHz or the like, but it is disadvantageous under a constant drive power condition because it includes an unnecessary frequency component of 2, 6 GHz or the like that does not contribute to the driving of the optical frequency comb generator. Not practical. Also, 8 GHz
The signal obtained by superimposing 12 GHz and 12 GHz can be used because the fundamental frequency is 4 GHz, which is the greatest common divisor of the signals. Thus, the drive signal does not necessarily include the fundamental wave component. Then, a plurality of harmonics may be superposed, and three or more frequency components may be superposed.

As a method of generating such a drive signal, a method of superimposing a fundamental wave or a harmonic wave (the structure thereof is shown in FIG. 1) and a method of distorting the fundamental wave (the structure thereof is shown in FIG. 2). .) The configuration shown in FIG. 1 is the first embodiment, and includes a harmonic wave generating means 3 and a drive signal generating means 4.
And can be separated. The configuration shown in FIG. 2 is
In the second embodiment, the harmonic generation means 3 and the drive signal generation means 4 are inseparable and integrated.

In any of the methods, a sufficiently significant effect can be seen for driving only the fundamental wave, under the condition that the total power of the driving signal is constant, the distortion rate of the driving signal (harmonic The ratio of total power to total power) is 3% to 100% (1
00% is when superimposing different harmonics on the harmonics)
And when the round-trip phase transition of the incident light is an integral multiple of 2π, the time rate of change of this phase transition is in the range of about 1.5 to 5 times that of driving only the fundamental wave. Is. Under such conditions, the frequency spacing of the sidebands should be 4
Even when the frequency is set to GHz, a sideband generation range of 16 nm or more can be easily realized (conventionally, about 8 nm as described above). Even if the time change rate of the phase transition is increased five times or more, the average output light amount decreases due to the narrow time width of the output optical pulse, and the incident light is dispersed into many sidebands. The influence of the decrease in light intensity of each sideband due to
The range of generation of sidebands having an intensity of -70 dBm or more only slightly increases, and decreases to about 10 times or more.
Further, when the frequency interval of the sidebands is 4 GHz, 20 G is required to realize a time change rate of phase transition of 5 times or more.
It becomes necessary to handle harmonics of Hz or higher, which is not economically effective.

FIG. 13 shows the maximum sideband order (that is, the effective sideband generation range) above the detectable level with respect to the product of the round-trip phase modulation index and finesse, with the normalized detectable level as a parameter. (Because the finesse is considered constant here), the width of the output optical pulse becomes narrower as the round-trip phase modulation index increases, and the spectrum spreads and the average output light quantity also decreases. As a result, when extremely deep modulation is applied, sidebands with a certain intensity or more decrease. In FIG. 14, the product of the round-trip phase modulation index and the finesse is normalized as 100. Even if the product of the round-trip phase modulation index and the finesse can be effectively increased to 5 times or more by superimposing the harmonic, the highest detectable order does not increase at the normalized detectable level −55 dB.

Here, the normalizable detection level is the detection limit light amount of the light receiving system with respect to the maximum transmitted light amount, and is currently about 1 to 5% for an incident light amount of several mW, which is 0.1 mW.
The maximum transmitted light amount is around (-10 dBm), and the detection limit light amount of the light receiving system is about -60 to -65 dBm. Therefore, the typical normalized detectable level is -55 dB (-65 dB).
It is about − (− 10)). Also, the maximum amount of transmitted light is the amount of light obtained by multiplying the incident light amount by the maximum transmittance of the optical frequency comb generator (transmittance at the resonance peak), and the detection limit light amount of the light receiving system has a predetermined SN ratio (usually around 25 dB). Minimum light intensity,
The reciprocal phase modulation index is an effective value corresponding to the slope when passing through the resonance point in the case of non-sinusoidal drive. In the current sine wave drive, the round-trip phase modulation index is π to 2π (rad).
It is a degree. Finesse is about 20-30. Therefore, the product of the round-trip phase modulation index and the finesse is about 60 to 180.

[0025]

[Embodiment] Here, the drivable frequencies of the optical frequency comb generator are 2, 4, 6, 8, 10, 12, 14, and 16 GHz.
Then, a case where the desired sideband spacing is 4 GHz will be described. These drive frequencies are typical values of an optical frequency comb generator having a built-in waveguide type optical phase modulator that can be driven in a wide band. In other cases, the driving signal source can be freely implemented according to the drivable frequency of the optical frequency comb generator.

A first embodiment of the present invention (corresponding to the first embodiment) will be described with reference to FIG. In this embodiment, the fundamental wave is the 4 GHz component, and the third harmonic is 1
The drive signal in which 2 GHz is superimposed is input to the optical frequency comb generator 1. The optical frequency comb generator 1 receives the drive signal from the drive signal source 2 with respect to the incident light, performs a modulation operation, and generates an optical frequency comb signal including many sideband wave components. The drive signal source 2 includes an oscillator 21, a phase locked oscillator 22, a phase shifter 23, a mixer 24 and an amplifier 2.
Consists of five. The oscillator 21 outputs a 4 GHz signal which is a fundamental wave. The phase-locked oscillator 22 refers to a part of the signal output from the oscillator 21, and outputs a signal having a frequency three times that of the signal and having a phase synchronized with the referred signal. The phase shifter 23 is provided in the middle of the signal path from the oscillator 21 to the mixer 24 in order to set the phase difference between the fundamental wave and the triple wave included in the finally obtained drive signal to a desired value. . Normally, this phase difference may be set to 0 (in sin display) as in Expression (1). The mixer 24 is
The signals based on the outputs of the oscillator 21 and the phase locked oscillator 22 are mixed. The amplifier 25 amplifies the output from the mixer 24 and outputs a drive signal for the optical frequency comb generator 1.

The internal configuration of the drive signal source 2 has a large degree of freedom, and various forms are possible. For example, instead of the phase-locked oscillator 22, a frequency tripler (generally a frequency multiplier) is used to amplify and use the multiplied output. The phase shifter 23 arranged at the point A in the figure can be moved to the points B and C, or can be omitted if not necessary. Regarding the arrangement of the amplifier 25, the fundamental wave and the third harmonic wave may be amplified independently and then mixed. In the arrangement shown in FIG. 3, the amplifier 2
5 is required to be able to amplify at least 4 GHz and 12 GHz, and a wide band amplifier is generally used. However, when amplifying the fundamental wave and the third harmonic wave independently, each amplifier may have a narrow band.

Since the response band of the optical frequency comb generator 1 is set to 16 GHz, an example in which the third harmonic is superposed is shown. However, when the response band of 20 GHz or more is provided, the fifth harmonic (20 GHz). ) Is added,
The sideband generation range can be expanded. Further, although it becomes complicated, it is possible to extend this embodiment and superimpose a plurality of harmonics.

Next, as a second embodiment of the present invention (corresponding to the second embodiment), the portion of the drive signal source 2 is shown in FIG.
It was shown to. The optical frequency comb generator 1 is omitted because it is the same as that of the first embodiment. The drive signal source 2 of this embodiment outputs the drive signal input to the optical frequency comb generator 1 to 4
It is generated through the limiting amplifier 26 with the GHz component as the fundamental wave. The oscillator 21 generates a 4 GHz signal which is a fundamental wave. Limiting amplifier 26
Is to limit the amplitude while amplifying the fundamental wave signal. If necessary, the limiting amplifier 26
Is amplified by the amplifier 25 to generate a drive signal.
A log amplifier is typically used as the limiting amplifier 26, but a normal amplifier may be used by being saturated, or a limiter may be used although it does not have an amplifying function. The drive signal generated in this manner has a waveform close to a trapezoid, and includes only odd harmonics when ideal amplitude limitation is performed. Similarly, instead of the limiting amplifier 26, a frequency multiplier such as a frequency tripler or an electric signal comb generator may be used to directly amplify the output to obtain a drive signal. When the phase difference between the fundamental wave and harmonics becomes a problem when using an element with such non-linearity, an element with frequency dispersion such as a phase shifter is used for the main harmonics. It is also possible to add a transmission line and perform phase compensation.

FIG. 15 shows the result of an experiment using an optical frequency comb generator having substantially the same structure as that of the first embodiment. In the figure, (a) shows only the fundamental wave (3.81 GHz, 200 mW), (b) shows only the third harmonic (25 mW), and (c) shows the fundamental wave (3.81 GHz, 200 mW) with the third harmonic ( 25m
7 shows an output light spectrum (envelope) when driven by a superposed wave in which W) is superposed. In this example, the drive power in (c) is only 1.1 times or more, but the envelope slope is about 1.5 times gentle.

[0031]

The optical frequency comb generator of the present invention positively outputs the fundamental wave or its harmonics or harmonics so that a frequency component equal to the frequency interval of sidebands can be obtained as a microwave drive signal. Since it is decided to use the superposed microwaves, an optical frequency comb generator that can obtain a wider sideband generation range that is more practical than the conventional one while maintaining a narrow practical sideband frequency interval. Could be realized.

This means that in the system using the optical frequency comb generator of the present invention, the handling of the heterodyne signal can be facilitated and the system can be realized at low cost. Further, in the optical frequency offset lock control system in which the optical frequency comb generator is mainly used, the stability of the frequency difference between the two lights that generate the heterodyne signal is increased.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a second embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 5 shows a drive signal (microwave) sin ωt + (1 /
2) It is an example when sin2ωt, where (a) is a diagram showing transmittance-reciprocal phase characteristics of the optical resonator, (b) is a diagram showing superimposed waveforms, and (c) is a transmission along the time axis. Diagram showing changes in light intensity, (d) spectrum of output light (envelope)
FIG.

FIG. 6 shows a drive signal (microwave) as sinωt− (1 /
2) It is an example when sin2ωt, where (a) is a diagram showing transmittance-reciprocal phase characteristics of the optical resonator, (b) is a diagram showing superimposed waveforms, and (c) is a transmission along the time axis. Diagram showing changes in light intensity, (d) spectrum of output light (envelope)
FIG.

FIG. 7 shows a drive signal (microwave) sin ωt− (1 /
2) It is an example when cos2ωt, where (a) is a diagram showing the transmittance-reciprocal phase characteristics of the optical resonator, (b) is a diagram showing superimposed waveforms, and (c) is a transmission along the time axis. Diagram showing changes in light intensity, (d) spectrum of output light (envelope)
FIG.

FIG. 8 is an example of a drive signal in which a second harmonic is superimposed on a fundamental wave, (a) shows a waveform (solid line) of y = sinx + sin2x and its first derivative (dotted line), and (b) shows y. = S
It is a figure which shows the waveform (solid line) of inx-cos2x and its 1st-order differentiation (dotted line).

FIG. 9 is an example of a drive signal in which a third harmonic is superimposed on a fundamental wave, (a) shows a waveform of y = sinx + sin3x (solid line) and its first derivative (dotted line), and (b) shows y. = S
It is a figure which shows the waveform (solid line) of inx-cos3x and its 1st-order differentiation (dotted line).

FIG. 10 is an example of a drive signal in which a third harmonic is superimposed on a second harmonic, (a) shows a waveform of y = sin2x + sin3x (solid line) and its first derivative (dotted line), (b) FIG. 4 is a diagram showing a waveform of y = cos2x + cos3x (solid line) and its first derivative (dotted line).

FIG. 11 is an example of a drive signal in which a second harmonic and a third harmonic are superimposed on a fundamental wave, y = cosx + cos2x +
It is a figure which shows the waveform (solid line) of cos3x and its 1st-order differentiation (dotted line).

FIG. 12 is a diagram showing a relationship between normalized harmonic power and maximum derivative when an nth harmonic is superimposed on a fundamental wave.

FIG. 13 is a diagram showing a relationship between (round-trip phase modulation index) × (finesse) with the standardized detectable level as a parameter and the highest detectable level (one side) of the comb signal.

FIG. 14 is a diagram showing the relationship between (round-trip phase modulation index) × (finesse) [relative value] and the highest detectable order (one side) [relative value] of the comb signal with the normalized detectable level as a parameter. .

FIG. 15 is a diagram showing an experimental result of the optical frequency comb generator of the present invention.

FIG. 16 is a diagram showing a configuration of a conventional optical frequency comb generator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical frequency comb generator 2 Driving device (driving signal source) 3 Harmonic generation means 4 Driving signal generation means 11 Optical phase modulator 12 Optical resonator 12a Mirror 12b Mirror 21 Oscillator 22 Phase lock oscillator 23 Phase shifter 24 Mixer 25 Amplifier 26 Limiting amplifier

Claims (1)

[Claims] 1. An optical frequency comb generator for receiving an incident light and generating an optical frequency comb signal, the optical frequency comb generator (1) and a driving device (2) for driving the optical frequency comb generator. And a harmonic generating means (3) for generating two or more frequency components that are natural numbers times the desired frequency, and a basic frequency of the desired frequency from the output signal of the harmonic generating means. An optical frequency comb generator which outputs an optical frequency comb signal at intervals of a fundamental frequency, including a drive signal generating means (4) for obtaining a drive signal having a frequency.