JP2006179779A - Double frequency stabilization mode synchronization laser light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize absolute frequencies of all longitudinal modes. <P>SOLUTION: The frequency interval of a longitudinal mode spectrum of laser oscillation by a laser light source 1, and one longitudinal mode oscillation frequency of the longitudinal mode spectrum are independently fed back to the laser light source 1. A signal for adjusting repetition frequencies from a clock extraction circuit 5 and a PLL circuit 6, and a signal for adjusting absolute frequencies from a light filter 2, a frequency reference cell 3, and a phase sensitive detector circuit (optical phase modulator 41, oscillator 42, photodetector 43, lock-in amplifier 44) are independently fed back to the laser light source 1. Thus, stabilization is achieved, and the absolute frequencies of all longitudinal modes are simultaneously stabilized. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a dual-frequency stabilized mode-locked laser light source, and more particularly to a dual-frequency stabilized mode-locked laser light source in which an oscillation frequency and a pulse repetition frequency are accurately fixed in an optical communication light source.

The frequency spectrum of the mode-locked pulse laser is composed of a large number of discrete spectra (longitudinal modes) arranged at regular intervals at the repetition frequency of the output light pulse and having the phases between the modes aligned. Since the “optical ruler” in which the frequency interval is accurately fixed can be used for frequency measurement and time standard, there has recently been a great interest in frequency stabilization technology for mode-locked lasers (Non-patent Document 1, 2).
In general, a mode-locked fiber laser uses a fiber as a resonator, so the resonator length is several meters to several hundred meters longer than other lasers. Therefore, the frequency changes repeatedly due to slight temperature changes and mechanical vibrations. There was a problem that it was easy. Therefore, in order to keep the repetition frequency stable over a long period of time, for example, in Patent Document 1, a clock signal is extracted from a harmonic reproduction mode-locked fiber laser, and a phase comparison is made between the clock frequency and the frequency of an external signal generator (synthesizer). A method is disclosed in which the repetition frequency of the laser is synchronized with an external reference synthesizer by detecting the error by the detector and feeding back the error signal to the cavity length of the laser. By this method, the mode interval can be made constant with high accuracy.
On the other hand, in order to apply a mode-locked laser as an “optical ruler”, it is necessary to accurately stabilize not only the repetition frequency but also the absolute value of the oscillation frequency of each longitudinal mode. In other words, even if the interval between the “light rulers” is constant, the accuracy (absolute accuracy) cannot be obtained if they move left and right. Therefore, for example, in Patent Document 2, one longitudinal mode of a wavelength tunable mode-locked fiber laser is extracted, and the oscillation wavelength is controlled so that the frequency of this monochromatic light matches the resonance frequency of the molecule, thereby stabilizing the absolute frequency. A method for realizing a laser is disclosed.

Th. Udem, J. Reichert, R. Holzwarth, and TW Hansch, “Absolute optical frequency measurement of the cesium D-1 line with amode-locked laser,” Phys. Rev. Lett., Vol. 82, pp. 3568- 3571, 1999. A. Onae, T. Ikegami, K. Sugiyama, F. Hong, K. Minoshima, H. Matsumoto, K. Nakagawa, M. Yoshida, and S. Harada, “Optical frequency link between an acetylene stabilized laser at 1524 nm and an Rbstabilized laser at 778 nm using a two-color mode-locked fiber laser, ”Opt. Commun., Vol. 183, pp. 181-187, Sept. 2000. DJ Jones, SA Diddams, JK Ranka, A. Stent, RS Windeler, JL Hall, and ST Cundiff, “Carrier-envelopephase control of femtosecond mode-locked laser and direct optical frequency synthesis,” Science, vol. 288, pp. 635- 639, 2000. Patent 3350874 "Laser Pulse Oscillator" JP 2001-168438 A “Absolute Wavelength Stabilized Mode-Locked Fiber Laser Light Source”

However, in the method of Patent Document 2, although the frequency of one specific longitudinal mode extracted can be stabilized, the frequency of other longitudinal modes fluctuates due to the fluctuation of the carrier-envelope offset (CEO) frequency. Therefore, there is a problem that the frequency cannot be stabilized for all longitudinal modes when viewed in the whole spectrum.
The CEO frequency f _CEO is the offset frequency of the entire longitudinal mode derived from the difference between the phase velocity and group velocity of the optical pulse in the laser resonator (FIG. 1). Each longitudinal mode of the mode-locked laser frequency _{f N} of the (N-order mode) is a repetition frequency _{f rep} is also dependent on the CEO frequency _{f CEO} not only, _f N ₌ Nf _{rep + f} CEO using two parameters (1)
It can be expressed as. Here, N is an integer and represents the order of the mode.
As shown in FIG. 2, when the phase of the carrier at the peak of the optical pulse and shifts only .DELTA..PHI _CEO each orbiting the _{resonator, f CEO} is _{f CEO = (ΔΦ CEO / 2π} ) f rep (2)
It is expressed. Here .DELTA..PHI _CEO is the carrier frequency omega _c, cavity length _{l c,} the group velocity _v .DELTA..PHI with _g and the phase velocity _{v p CEO = ω C l C} (1 / v g -1 / v p in the resonator (3)
Given in.
In the fiber laser resonator to cause a difference in group velocity v _g and the phase velocity v _p in the resonator by the group velocity dispersion is present, the difference to vary randomly with temperature and pressure, the CEO frequency Fluctuation occurs. Therefore, in the method described in Patent Document 2, even if the frequency of a certain longitudinal mode can be stabilized, the frequencies of the other longitudinal modes fluctuate due to fluctuations in the CEO frequency, so that the frequency of the entire spectrum is stabilized. It turns out that it is impossible.

Further, when the mode hop f _hop the longitudinal mode frequency resulting in a sudden change to another frequency in order to utilize the harmonic mode-locking occurs, it will also include f _hop in f _CEO. This f _hop also varies with temperature.
In a fundamental wave mode-locked laser having a repetition frequency of about 100 MHz such as a titanium sapphire laser, f _CEO is stabilized by a method called a self-reference method (Non-patent Document 3). In this method, f _CEO is detected by taking a beat with the second harmonic of the Nth order mode and the second Nth mode obtained by extending the spectrum to one octave or more by the nonlinear optical effect. The frequency of the second N-th mode is f _2N = 2Nf _rep + f _CEO (4) from equation (1).
Given in. While the second frequency harmonics of the N-th mode _{f N} is _{2f N = 2Nf rep + 2f CEO} (5)
It is expressed. Therefore, f _CEO can be detected from the beat frequency (f _CEO = 2f _N −f _2N ) in equations (4) and (5). The absolute frequency can be stabilized by converting this change into a voltage signal and feeding it back to the rotation angle of the reflecting mirror in the laser resonator. However, this method has a problem that the configuration is complicated, for example, the second harmonic generation by the nonlinear optical crystal is required. Furthermore, since the output of this laser has a narrow frequency interval of 100 MHz, there is a problem that one longitudinal mode cannot be extracted and used as a highly stable light reference.

In view of the above points, the present invention is particularly capable of double frequency stabilization in which one absolute frequency f and one repetition frequency f _rep are independently stabilized in order to stabilize the CEO frequency f _CEO. It is an object of the present invention to provide a mode-locked laser light source that stabilizes the absolute frequency of all longitudinal modes by providing an integrated feedback mechanism and operating them simultaneously.
That is, for example, in Formula (1), by stabilizing the absolute frequency f and the repetition frequency f _rep of one predetermined longitudinal mode at the same time, it is possible to suppress the fluctuation of the CEO frequency as a result. It becomes possible and the frequency of all longitudinal modes is stabilized. In contrast, since in the conventional patent document 2 or the like technique not stabilize even _{f rep} stabilize the _{f, f rep} is changed with the change of _{f CEO} or mode hopping _{f hop,} frequencies other than f Then, there was a problem that the absolute frequency would change.

One of the main features of the present invention is that the frequency interval of the longitudinal mode spectrum of the laser oscillation and the oscillation frequency of one longitudinal mode of the longitudinal mode spectrum are stabilized independently in the laser that generates the optical pulse. By doing so, it is a dual frequency stabilized mode-locked pulse laser light source in which all oscillation longitudinal modes are simultaneously stabilized in absolute frequency.
In addition, the present invention provides a method for stabilizing the frequency interval of the longitudinal mode spectrum in a dual-frequency stabilized mode-locked laser light source, using the difference between the repetition frequency of the output light pulse and the frequency from the external reference synthesizer as an error signal. At the same time as feeding back to the laser resonator, a single longitudinal mode frequency is extracted from the optical pulse by a narrow-band optical filter, and an error signal generated when the frequency deviates from the resonance frequency of the molecule is fed back to the laser resonator. be able to.
Further, according to the present invention, in a dual frequency stabilization mode-locked laser light source, an error signal from a molecular resonance line is fed back to a temperature change of the resonator, and the temperature can be controlled with high accuracy.
According to the first solution of the present invention,
A laser that generates optical pulses;
A circuit for stabilizing the oscillation of the laser,
The circuit stabilizes the frequency interval of the longitudinal mode spectrum of the laser oscillation and the oscillation frequency of the longitudinal mode spectrum of the longitudinal mode spectrum independently from each other by the circuit. There is provided a dual frequency stabilized mode-locked pulsed laser source characterized by simultaneously stabilizing the longitudinal mode in absolute frequency.
According to the second solution of the present invention,
A laser light source capable of tuning a repetition frequency and an oscillation frequency by an element that varies the resonator length and an excitation light source whose output is variable;
A clock extraction circuit that outputs a clock signal equal to a repetition frequency of an output light pulse from the laser light source;
A first circuit that repeatedly outputs a difference between a clock signal from the clock extraction circuit and a frequency from a reference synthesizer as a frequency adjustment signal;
An optical filter that extracts one longitudinal mode frequency from an output light pulse from the laser light source;
An oscillator that oscillates a reference signal;
An optical phase modulator that is driven by a reference signal from the oscillator and modulates the phase of the optical signal extracted by the optical filter;
A frequency reference cell that encapsulates an atom or molecule having a stable resonance absorption line and absorbs the optical signal when the center frequency of the optical signal from the optical phase modulator matches the frequency of the resonance line;
A photodetector for converting an optical signal output from the frequency reference cell into an electrical signal;
A second circuit that outputs, as an absolute frequency adjustment signal, an error signal that is obtained by phase-sensitive detection of an electrical signal from the photodetector and is proportional to the amount of deviation of the optical frequency from the resonance line;
With
The frequency interval of the longitudinal mode spectrum of the laser oscillation by the laser light source and the oscillation frequency of one longitudinal mode of the longitudinal mode spectrum are determined from the repetition frequency adjustment signal from the first circuit and the second circuit. Provided is a dual-frequency stabilized mode-locked pulsed laser light source characterized in that all oscillation longitudinal modes are stabilized in absolute frequency by independently feeding back and stabilizing the absolute frequency adjustment signal to the laser light source. The

  According to the present invention, a highly stable longitudinal mode frequency spectrum (optical comb) whose absolute frequency is stabilized can be easily obtained with a single light source. The optical comb whose frequency is accurately controlled as described above can be used not only for frequency standards but also for high-purity microwave generation and high-accuracy phase control of optical frequencies.

1. First Embodiment FIG. 3 is a block diagram showing a schematic configuration of a dual frequency stabilization mode-locked pulsed laser light source of the first embodiment. In the figure, a solid line indicates an optical signal, and a dotted line indicates an electrical signal (the same applies hereinafter).
In the figure, the dual frequency stabilization mode-locked pulse laser light source includes a mode-locked pulse laser light source 1, an optical filter 2, a frequency reference cell 3, an optical phase modulator 41, an oscillator 42, a photodetector 43, and a lock-in amplifier 44. , A clock extraction circuit 5, a PLL (Phase Locked Loop) circuit 6, a piezoelectric element (PZT) 7 for changing the resonator length of the laser light source 1, and an excitation light source 8 for the laser light source 1.
For the mode-locked pulse laser light source 1, it is effective to use a fiber laser using a rare earth-doped fiber such as erbium as a gain medium. The laser light source 1 has a PZT 7 whose resonator length is variable and an excitation light source 8 whose output is variable. By these, the laser light source 1 can tune the repetition frequency and the oscillation frequency (details will be described later).
The optical filter 2 is used to extract one longitudinal mode determined by transmission characteristics from the output light of the mode-locked pulse laser light source 1. For the optical filter 2, it is particularly effective to use an optical filter having a narrow band transmission characteristic such as a fiber Bragg grating.
The laser light source 1 is capable of high repetition operation in the GHz band, for example, and has a wide frequency interval of, for example, 10 to 40 GHz. Therefore, one longitudinal mode can be extracted by the narrow band optical filter 2 and highly stable frequency standard light can be obtained. It has a feature that it can be easily obtained.
Light phase modulator 41 is driven by an oscillator 42, modulates the phase of the optical signal modulation width [Delta] [omega, at the modulation frequency omega _m. The frequency reference cell 3 is a container in which atoms / molecules having a stable resonance absorption line that does not change with time in optical frequency are sealed in a container at an appropriate pressure. For example, stable absorption in a wavelength band of 1.5 μm used for optical communication. Acetylene (C ₂ H ₂ ) or hydrogen cyanide (HCN) molecules with lines can be used. The photodetector 43 converts the optical signal output from the frequency reference cell 3 into an electrical signal. Further, the lock-in amplifier 44 outputs a first derivative signal of the absorption spectrum of the frequency reference cell 3 from the electrical signal after light detection and the reference signal from the oscillator 42, and generates a feedback signal (details will be described later).

The clock extraction circuit 5 outputs a clock signal equal to the repetition frequency from the output pulse train of the light source 1. Here, as an example, the clock signal indicates a sine wave electric signal instead of a temporal on / off waveform. That is, the clock extraction circuit 5 can obtain a microwave signal having a frequency of 10 GHz from a repeated optical pulse train of 10 GHz, for example. In the laser light source 1, as shown in FIG. 4, the optical modulator 11 is directly driven by the obtained clock signal. The PLL circuit 6 includes a synthesizer 61, a phase comparator 62, and a feedback circuit 63. The synthesizer 61 can be further stabilized by locking to an atomic clock such as cesium or rubidium.
FIG. 4 is a diagram illustrating an example of the configuration of a mode-locked pulse laser light source.
This laser light source 1 uses a fiber laser, and includes PZT 7, excitation light source 8, fiber 9 with peltier (temperature control circuit), erbium fiber 10, optical modulator 11, clock extraction circuit 12, amplifier 13, and WDM 14 to 16. The laser light source 1 generates an optical pulse by reproducing mode synchronization. For feedback control, the PZT unit 7, the excitation light source unit 8, and the optical fiber unit 9 with Peltier are used.

Next, the operation of the laser light source 1 will be described.
When the excitation light generated by the excitation light source 8 is incident on the erbium fiber 10 via the WPM coupler 14, erbium ions are excited and a continuous laser beam oscillates. This laser light is output by the WDM couplers 15 and 16. The clock extraction circuit 12 outputs a clock signal from the output light. The sine wave clock signal is amplified by the amplifier 13 and then supplied to the optical modulator 11. The optical modulator 11 is driven by the clock signal extracted by the clock extraction circuit 12 and plays a role of generating a pulse having the repetition frequency. Therefore, in the mode-locked pulse laser light source 1, the laser beam is intensity-modulated at a frequency synchronized with the sine wave clock signal.

Next, the principle of frequency stabilization according to this embodiment will be described.
First, one longitudinal mode determined by the transmission characteristic is extracted by the optical filter 2 and input to the frequency reference cell 3 via the optical phase modulator 41. Next, an error signal between the longitudinal mode frequency and the resonance frequency of the molecule is detected by the photodetector 43 and the lock-in amplifier 44 and fed back to the excitation current of the excitation light source 8 so that the oscillation frequency matches the resonance frequency of the molecule. The oscillation frequency of the light source 1 is changed by negative feedback in the direction in which the light is emitted. That is, the absolute frequency f of one longitudinal mode is stabilized. Here, the principle that the refractive index of the medium changes and the frequency changes due to the Kramers-Kronig relationship in the erbium fiber 10 that is the amplification medium of the fiber laser due to the power change of the pumping light source is used. Thus, if an appropriate longitudinal mode is extracted by the optical filter 2, the frequency can be stabilized to the frequency of the absorption line. Since the optical filter 2 has a certain degree of wavelength variability, a desired mode can be extracted.
FIG. 7 is a diagram for explaining a method of detecting a deviation amount of the laser oscillation frequency from the absorption line of the frequency reference cell 3 as an error signal by phase sensitive detection.
Hereinafter, the phase sensitive detection is mainly performed by the optical phase modulator 41, the oscillator 42, the photodetector 43, and the lock-in amplifier 44.
As shown in FIG. 7 (a), the optical phase modulator 41, the frequency omega _c of the optical signal modulation width [Delta] [omega, phase modulation at the modulation frequency omega _m. The phase-modulated optical signal is incident on the frequency reference cell 3, and the electrical signal y _sig (t) obtained by optically detecting the optical signal from the frequency reference cell 3 by the photodetector 43 and the frequency ω _m from the oscillator 42 The reference signal y _ref (t) = cos (ω _m t) is input to the lock-in amplifier 44. If the absorption spectrum of the frequency reference cell 3 is f (ω), the electric signal y _sig (t) after light detection is

と表される。これを周波数ω_ｃのまわりでテイラー展開すると、

となる。ロックインアンプ４４においてｙ_sig（ｔ）とｙ_ref（ｔ）を乗算すると、

となるので、直流成分を抽出すると一次微分信号（ｄｆ／ｄω）_ωｃを検出することができる。ｆ（ω）は分子吸収線の共鳴周波数ω_０においてピークを有するので、一次微分信号はω_ｃがω_０に一致するときちょうどゼロとなり、ω_０の近傍では吸収線からのレーザ発振周波数のずれ量に比例する。したがってこの一次微分信号を誤差信号として用いてレーザの発振周波数をフィードバック制御することにより、レーザの発振周波数を分子吸収線の中心周波数に安定化することができる。周波数ω_cをω_０のまわりで掃引したときに得られる一次微分信号を図７（ｂ）に示す。
また、該縦モード一本の絶対周波数ｆの安定化と同時に、クロック抽出回路５によって共振器内の光パルスからクロック信号を抽出し、ＰＬＬ回路６に入力する。次に、ＰＬＬ回路６において該クロック信号の周波数とシンセサイザ６１の発振周波数との誤差信号を検出し、ＰＺＴ７にフィードバックし、クロック周波数が該シンセサイザの周波数に一致するようにＰＺＴ７の出力をチューニングすることにより、光源１の繰り返し周波数ｆ_ｒｅｐを変化させる。
フィードバックに関しては、例えば、ＰＺＴによる共振器長変化あるいは励起光源８への励起電流変化によって、ｆ_ｒｅｐおよびｆがどのくらい変化するかその割合により最適なパラメータにフィードバックすればよい。一般には、一例として、励起電流を変化させるとｆ_ｒｅｐよりｆ_ＣＥＯ及びｆが大きく変化するので、ＰＺＴ７によりｆ_ｒｅｐを安定化し、励起電流によりｆ_ＣＥＯ及びｆを安定化すればよい。
以上の結果、レーザ光源１の一本の縦モードの絶対周波数ｆが周波数基準セル３内の分子の共鳴周波数に一致し、かつ光パルスの繰り返し周波数ｆ_ｒｅｐがＰＬＬ回路６内の外部シンセサイザ６１の発振周波数に一致した高安定な縦モードスペクトル群を得ることができる。すなわちレーザ光源１のスペクトルを構成する全ての縦モードの絶対周波数が同時に安定化される。
なお、本技術は、ファイバレーザの他にも、半導体を用いたモード同期パルスレーザに、共振器長と励起パワーあるいは温度を制御することにより同様に適用できる（以下の実施の形態においても同様）。

It is expressed. If this is expanded to Taylor around the frequency ω _c ,

It becomes. Multiplying y _sig (t) and y _ref (t) in the lock-in amplifier 44,

Therefore, when the DC component is extracted, the primary differential signal (df / dω) _ωc can be detected. Since f (ω) has a peak at the resonance frequency ω ₀ of the molecular absorption line, the first-order differential signal is exactly zero when ω _c matches ω ₀ , and the laser oscillation frequency shift from the absorption line in the vicinity of ω _0. Proportional to quantity. Therefore, the laser oscillation frequency can be stabilized at the center frequency of the molecular absorption line by feedback control of the laser oscillation frequency using the first derivative signal as an error signal. FIG. 7B shows a primary differential signal obtained when the frequency ω _c is swept around ω ₀ .
At the same time as the stabilization of the absolute frequency f of one longitudinal mode, the clock extraction circuit 5 extracts the clock signal from the optical pulse in the resonator and inputs it to the PLL circuit 6. Next, an error signal between the frequency of the clock signal and the oscillation frequency of the synthesizer 61 is detected in the PLL circuit 6, fed back to the PZT 7, and the output of the PZT 7 is tuned so that the clock frequency matches the frequency of the synthesizer. Thus, the repetition frequency f _rep of the light source 1 is changed.
Regarding feedback, for example, it is only necessary to feed back to an optimum parameter depending on the ratio of how much f _rep and f change due to a change in resonator length due to PZT or a change in excitation current to the excitation light source 8. In general, as an example, since the varying the excitation current _{f rep} than _{f CEO} and f is largely changed, the _{f rep} stabilized by pZT7, may be stabilized _{f CEO} and f by the excitation current.
As a result, the absolute frequency f of one longitudinal mode of the laser light source 1 matches the resonance frequency of the molecule in the frequency reference cell 3, and the repetition frequency f _{rep of the} optical pulse is equal to that of the external synthesizer 61 in the PLL circuit 6. A highly stable longitudinal mode spectrum group matching the oscillation frequency can be obtained. That is, the absolute frequencies of all longitudinal modes constituting the spectrum of the laser light source 1 are stabilized at the same time.
In addition to the fiber laser, the present technology can be similarly applied to a mode-locked pulse laser using a semiconductor by controlling the resonator length and pumping power or temperature (the same applies to the following embodiments). .

2. Second Embodiment FIG. 5 is a block diagram illustrating an example of a schematic configuration of a dual frequency mode-locked laser light source according to a second embodiment.

  As shown in FIG. 5, that is, contrary to the first embodiment, the output signal of the frequency reference cell 3 (the error signal of the longitudinal mode and the absorption line of the numerator) is sent to the PZT 7 and the output signal of the PLL circuit 6 ( The same stabilization can be realized by feeding back the clock and synthesizer error signals) to the excitation light source 8 respectively.

3. Third Embodiment FIG. 6 is a block diagram illustrating an example of a schematic configuration of a dual frequency mode-locked laser light source according to a third embodiment.
As shown in FIG. 6, since most of the frequency change is caused by a minute temperature change, the error signal reflects the temperature change. Therefore, there is a method of feeding back this error signal to the Peltier fiber 9 of the laser resonator. Since f _CEO and f are particularly sensitive to temperature changes, feedback is applied to the Peltier fiber 9 in FIG. 6 as a countermeasure to the temperature changes. This also completely controls the laser temperature and suppresses mode hops and f _CEO and f.
In order to control f _CEO and f, not only the power of the excitation light source but also an element for changing the phase may be inserted into the resonator, and feedback may be provided there.

  The dual frequency stabilization mode-locked fiber laser light source of the present invention is an ultra-high stability optical pulse light source in the optical communication wavelength band, such as ultrafast optical time division multiplex transmission, wavelength division multiplex transmission, coherent transmission utilizing the phase of light, etc. A wide range of applications to optical communication technology can be considered. In addition to these, the present laser light source can be used for optical frequency standards and high-purity microwave generation. In other words, since a spectrum sequence in which the frequency interval of the longitudinal mode is accurately fixed and the oscillation frequency of all longitudinal modes is always constant is output, it can be applied to a high-precision optical frequency standard technology as a measure of light. is there. In addition, since the harmonic reproduction mode-locked pulse fiber laser has a repetition frequency of about several tens of GHz and a frequency in the microwave to THz band, it is possible to generate a highly stable micro wave by generating beats of two different longitudinal modes. Waves can be generated.

It is a figure which shows typically the optical spectrum of a mode synchronous laser light source. It is a figure explaining the carrier envelope offset phase in the output pulse of a mode locked laser. It is a block diagram which shows an example of schematic structure of the dual frequency mode synchronous laser light source of 1st Embodiment. It is a figure which shows an example of a structure of a mode synchronous fiber laser light source. It is a block diagram which shows an example of schematic structure of the dual frequency mode synchronous laser light source of 2nd Embodiment. It is a block diagram which shows an example of schematic structure of the dual frequency mode synchronous laser light source of 3rd Embodiment. It is a figure explaining the method to detect the deviation | shift amount of the laser oscillation frequency from the absorption line of the frequency reference cell 3 as an error signal by phase sensitive detection.

Explanation of symbols

1 mode-locked laser light source 2 optical filter 3 frequency reference cell 41 optical phase modulator 42 oscillator 43 photodetector 44 lock-in amplifier 5 clock extraction circuit 6 PLL circuit 61 synthesizer 62 phase comparator 63 feedback circuit 7 PZT
8 Excitation light source 9 Peltier fiber 10 Erbium fiber

Claims (7)

A laser that generates optical pulses;
A circuit for stabilizing the oscillation of the laser,
The circuit stabilizes the frequency interval of the longitudinal mode spectrum of the laser oscillation and the oscillation frequency of the longitudinal mode spectrum of the longitudinal mode spectrum independently from each other by the circuit. A dual-frequency stabilized mode-locked pulsed laser light source characterized by simultaneously stabilizing the longitudinal mode in absolute frequency. The dual frequency stabilized mode-locked pulsed laser light source according to claim 1,
The circuit is
While using the difference between the repetition frequency of the output light pulse and the frequency from the external reference synthesizer as an error signal and feeding back to the laser resonator of the laser,
By extracting one longitudinal mode frequency from the optical pulse by a narrow band optical filter and feeding back an error signal generated when the frequency deviates from the resonance frequency of the molecule to the laser resonator of the laser,
A dual frequency stabilized mode-locked pulsed laser light source characterized in that all oscillation longitudinal modes are simultaneously stabilized in absolute frequency. The dual frequency stabilized mode-locked pulsed laser light source according to claim 1,
A dual-frequency stabilized mode-locked pulsed laser light source, further comprising a circuit for feeding back an error signal from a molecular resonance line to a temperature change of the resonator and controlling the temperature with high accuracy. A laser light source capable of tuning a repetition frequency and an oscillation frequency by an element that varies the resonator length and an excitation light source whose output is variable;
A clock extraction circuit that outputs a clock signal equal to a repetition frequency of an output light pulse from the laser light source;
A first circuit that repeatedly outputs a difference between a clock signal from the clock extraction circuit and a frequency from a reference synthesizer as a frequency adjustment signal;
An optical filter that extracts one longitudinal mode frequency from an output light pulse from the laser light source;
An oscillator that oscillates a reference signal;
An optical phase modulator that is driven by a reference signal from the oscillator and modulates the phase of the optical signal extracted by the optical filter;
A frequency reference cell that encapsulates an atom or molecule having a stable resonance absorption line and absorbs the optical signal when the center frequency of the optical signal from the optical phase modulator matches the frequency of the resonance line;
A photodetector for converting an optical signal output from the frequency reference cell into an electrical signal;
A second circuit that outputs, as an absolute frequency adjustment signal, an error signal that is obtained by phase-sensitive detection of an electrical signal from the photodetector and is proportional to the amount of deviation of the optical frequency from the resonance line;
With
The frequency interval of the longitudinal mode spectrum of the laser oscillation by the laser light source and the oscillation frequency of one longitudinal mode of the longitudinal mode spectrum are determined from the repetition frequency adjustment signal from the first circuit and the second circuit. A dual frequency stabilized mode-locked pulsed laser light source characterized in that all oscillation longitudinal modes are stabilized in absolute frequency by feeding back and stabilizing the absolute frequency adjustment signal to the laser light source independently. The dual frequency stabilized mode-locked pulsed laser light source according to claim 4,
With the repetition frequency adjustment signal of the first circuit, the repetition rate is stabilized by making the resonator length of the laser light source variable,
A dual frequency stabilized mode-locked pulsed laser light source characterized in that the absolute frequency is stabilized by changing the output of the pumping light source of the laser light source in accordance with the absolute frequency adjustment signal of the second circuit. The dual frequency stabilized mode-locked pulsed laser light source according to claim 4,
With the repetition frequency adjustment signal of the first circuit, the output of the excitation light source of the laser light source is made variable to stabilize the repetition frequency,
A dual frequency stabilized mode-locked pulsed laser light source characterized in that the absolute frequency is stabilized by making the resonator length of the laser light source variable by the absolute frequency adjusting signal of the second circuit. The dual frequency stabilized mode-locked pulsed laser light source according to claim 4,
The laser light source further includes an element for temperature control,
A dual-frequency stabilized mode-locked pulsed laser light source that feeds back an absolute frequency adjustment signal of the second circuit to the element and controls temperature.

Images