KR20020053984A - Laser cavity length stabilizing apparatus of using Brillouin signals - Google Patents

Abstract

PURPOSE: A mode locking optical fiber laser oscillating apparatus is provided to stabilize a laser oscillator on a real time basis by stabilizing a length of an optical fiber laser oscillator by feeding a frequency change as an error signal back to the laser oscillator by using the fact that a frequency transition value of a Brillouin signal is sensitive to a temperature. CONSTITUTION: A Brillouin pump(101) generates a Brillouin pump signal to generate a Brillouin signal. A single mode optical fiber(102) generates a Brilliouin stimulated scattering. A directional first optical fiber coupler(103) inserts the Brillouin pump signal into an oscillator. An optical circulator(104) outputs a returning Brillouin signal. A semiconductor laser(112) serves as an excitation pump optical source for an optical pump to an erbium addition optical fiber(111) for a mode locking optical fiber laser oscillation. The second optical fiber coupler(113) couples optical signals. A function generator(114) generates a signal. An optical modulator(115) makes an active mode locking through a signal inputted from the external function generator(114). The third optical fiber coupler(116) monitors a mode locked laser output. An optical isolator(117) proceeds an optical signal in one direction. An optical fiber PZT(121) is electrically and variably shrunken and expanded to stabilize a length of the oscillator. An optical receiver(122) receives the Brillouin signal. A frequency comparator(123) compares the Brillouin signal detected from the optical receiver(122) and the frequency of the signal generated from the function generator(114). A frequency-voltage converter(124) converts the signal output from the frequency comparator(123) into an electric signal. An electric amplifier(125) amplifies the electric signal converted by the frequency-voltage converter(124).

Description

Mode-locking fiber laser resonator {Laser cavity length stabilizing apparatus of using Brillouin signals}

The present invention relates to a mode locking fiber laser resonator device, and more particularly, stabilizing a fiber laser by using a frequency change with a temperature of a Brillouin signal, which is a nonlinear effect of an optical fiber, as an error signal. The present invention relates to a mode locking fiber laser resonator device.

In general, fiber lasers are exposed to interference to the external environment as the fiber length becomes longer, thereby degrading the quality of the pulse. Therefore, the stability problem must be fundamentally solved for the practical use of the fiber laser. In particular, for stable operation of an active mode locking fiber laser in which mode locking is forcibly induced by a phase or amplitude modulator inside the resonator, it is very important to always maintain an accurate fiber length that matches the frequency value of the oscillating laser.

Fiber laser stabilization means that the physical quantity of the optical fiber is detected by detecting changes in the physical quantity that changes according to the external environment such as vibration or temperature, and compensating the physical quantity of the optical fiber in the direction of minimizing these changes. . In the above-described physical quantities, changes in the length of the optical fiber, changes in the polarization state, and the like exist.

In particular, the instability of the laser output due to the change in the optical fiber length occurs mainly in active lasers using harmonic mode locking.

In general, an active fiber laser is composed of an optical modulator, an optical amplifier, and an optical directional coupler that produces a laser output. At this time, the physical distance of each component The refractive index of each component The optical distance of the fiber laser is It is prescribed.

The ring resonator includes the resonator frequency of the laser. There are several species modes depending on the interval of. Modulator frequency in active mode locking lasers with modulators according to external modulation Is present, and the active mode locking condition according to the resonator frequency and the modulator frequency is formed in all the longitudinal modes. Becomes Where N is any positive integer.

The above equation means that the modulator frequency must be maintained to have a constant value in accordance with the resonator frequency in order to induce active mode locking. This mode locking is called harmonic mode locking. If the length of the resonator changes with the change of external conditions, the resonator frequency And the active mode locking conditions change. Therefore, in order to satisfy the mode locking condition, the value of the modulator frequency must be changed according to the change of the resonator frequency.

However, in the case of an optical communication system, the modulation frequency must be kept at a constant value, in which case the value of the resonator frequency must be kept at a constant value, and thus the length of the fiber laser must be kept constant. In other words, it can be concluded that stable laser output can be obtained only when the optical fiber length change according to the external environment is suppressed as much as possible.

In order to stabilize the mode-locked fiber laser, it is necessary to find the noise components in the signal according to the change of the optical fiber length by temperature and to compensate in the direction of minimizing these changes. Therefore, the noise component present in the laser output signal should be checked first. very important. Therefore, the laser stabilization method can be classified according to these noise components.

Looking at the prior art of the fiber laser stabilization by the above classification can be summarized into three major.

First, the first proposed method is to match the phase difference between the modulation frequency and the laser output.

This method was first attempted in the prior patent 'Mode-loked fiber ring laser stabilization' proposed by Xuekang Shan (US Pat. No. 5,590,142), which is a noise source for stabilizing the length of a laser resonator. It uses the phase difference between the electrical phase extracted from the signal and the signal applied to the modulator for mode locking.

The above stabilization method focuses on showing the relative phase difference between the optical signal and the modulator according to the small detuning of the optical fiber length.

The second stabilization method is shown in the prior patent 'Mode-lockeder stabilization method and apparatus (US Patent No. 5,646,774) proposed by Hidehiko Takara, and stabilizes the laser resonator using attenuation vibration frequency.

Attenuation oscillation occurs when the mode locking condition is not satisfied when the resonator length changes in the mode locking fiber laser, and the laser output becomes unstable. The frequency component of the attenuation vibrations in the RF spectrum of these signals In addition, its harmonic component ( ) Is detected.

In an ideal quaternary excitation level system, the harmonic vibration frequency is to be. Where r is the proportional constant between the pump rate to the upper level and the threshold pump rates, Is the decay time of the resonator, Is the survival time of the upper level.

If you consider a rare earth-added fiber laser, r = 1 to 10, = 1 ms to 10 ms = 1 ms to 10 ms, so It can be seen that has a value of approximately 10 ~ 500 kHz.

Similar error components include supermodes. The sideband of an optical modulator, which occurs during modulation of an active fiber laser, is given a given modulation frequency. In one species mode coinciding with, energy transfer occurs to several species modes located in both neighbors around the species mode. These coupled slave modes are supermodes, which can have a maximum of N sets and the time interval between neighboring slave mode sets Phosphorus form.

These supermodes each oscillate independently at their own oscillation frequency, and sometimes competitively oscillate by the position of the active element or the spatial and spectral hole burning of the gain medium. At this time, if only one super mode is oscillated, the laser resonator is circulated at N equal time intervals and N pulses are output at one round trip time.

However, when multiple supermodes are excited, N to one pulses are output in various forms per period. Thus, the best form of harmonic mode locking to obtain a stable laser output regardless of the relative amplitude and phase between supermodes is to use two internal resonant modulators.

That is, one modulator produces N = 1, that is, only one pulse per round trip, and another N >> 1 synchronized modulator operates at the mode locking modulation frequency to use to generate the ideal pulse. do. In this case, looking at the spectrum with only one supermode excited, the mode locking frequency, that is, the modulation frequency And their coordination ingredients, Only the back light is present, but if the mode locking conditions are not fully satisfied, several supermodes have a fundamental resonant frequency. It can be seen that on the RF spectrum at intervals of.

Therefore, in unstable harmonic mode locking, the attenuation vibration frequency component mentioned above And their harmonic components, i.e. unnecessary supermode components and their harmonic components, will be created, and the interference between them will produce another very many components. Thus, in this case, the change in the resonant length of the laser not only destabilizes the laser output, but also causes attenuation vibration components and super mode components to occur on the spectrum.

For this reason, it is meant that changing the length of the resonator in the direction of suppressing the above components means moving in the direction that satisfies the mode locking condition as much as possible.

Finally 'R. The dead port of a 2x2 optical modulator, as described in the article published by Kiyan, in stabilisation of actively mode locked Er doped fiber laser by minimising interpulse noise power (electronics letters, Vol. 34 pp 2410-2411). The laser resonator is stabilized by minimizing the light energy passing through the furnace.

The stabilization method utilizes the fact that the two outputs of the dual port optical modulator are complementary to each other. If the fiber laser length and the frequency of the modulator match exactly, the laser output through the dead port of the optical modulator is minimized. Experimentally it can be seen that the change in resonator length is proportional to the light output passing through the dead port of the modulator. Therefore, it is possible to stabilize the laser output by changing the length of the fiber laser in the direction to minimize the light output passing through the dead port of the optical modulator.

However, when the laser stabilization is implemented using the methods described above, there is a problem in that it is impossible to distinguish whether the temperature increases or decreases.

That is, since the noise component mentioned above occurs symmetrically regardless of temperature increase and decrease, it is necessary to determine whether the error signal increases or decreases through several repetitions in order to stabilize the laser. In order to determine this, there is a problem in that a method such as dithering is used, or the direction of temperature change is determined by simultaneously measuring error components of super mode or attenuation vibration.

The present invention proposed to solve the above problems, to provide a mode-locking fiber laser resonator apparatus that stabilizes the fiber laser using the frequency change according to the temperature of the Brillouin signal, which is a nonlinear effect of the optical fiber as an error signal. The purpose is.

1 is a configuration diagram of an embodiment of a mode locking fiber laser resonator device according to the present invention;

2 is an exemplary detection frequency of the mode locking fiber laser resonator according to the present invention.

3 is an exemplary view of a result of variation of Brillouin frequency with respect to temperature in accordance with the present invention.

4 is a diagram illustrating an embodiment of a mode locking fiber laser resonator using polarization maintaining optical fiber according to the present invention;

5 is an exemplary detection frequency of the mode locking fiber laser resonator using the polarization maintaining optical fiber according to the present invention.

* Explanation of symbols for the main parts of the drawings

101: Brillouin pump 102: optical fiber

111: Erbium-doped optical fiber 112: semiconductor laser

121: PZT element 122: optical receiver

123: frequency comparator 124: high frequency amplifier

In order to achieve the above object, the present invention provides a mode locking fiber laser resonator apparatus, comprising: Brillouin pumping means for generating a Brillouin pump signal; Brillouin signal generating means for receiving a Brillouin pump signal generated by the Brillouin pumping means and generating a Brillouin signal; Frequency detecting means for detecting a frequency component by receiving a Brillouin signal generated by the Brillouin signal generating means; Frequency comparison / measurement means for comparing the frequency of the Brillouin signal detected by the frequency detecting means with the frequency of the signal input to the mode locking fiber laser resonator and measuring the difference; Signal conversion means for converting the frequency signal measured by the frequency comparison / measurement means into an electrical signal; Signal amplifying means for amplifying the electrical signal converted by the signal converting means; And an optical fiber contraction and expansion means for receiving the signal amplified by the signal amplification means, thereby expanding or contracting the length, thereby stabilizing the mode locking fiber laser resonator.

In addition, the present invention provides a mode locking fiber laser resonator apparatus comprising: Brillouin pumping means for generating a Brillouin pump signal; Brillouin signal generating means for receiving a Brillouin pump signal generated by the Brillouin pumping means and generating a Brillouin signal; Frequency detecting means for detecting a frequency component by receiving a Brillouin signal generated by the Brillouin signal generating means; Signal extracting means for extracting a beating signal component of the frequency signal detected by said frequency detecting means; Signal conversion means for converting the frequency signal extracted by the signal extraction means into an electrical signal; Signal amplifying means for amplifying the electrical signal converted by the signal converting means; And an optical fiber contraction and expansion means for receiving the signal amplified by the signal amplification means, thereby expanding or contracting the length to stabilize the mode locking fiber laser resonator.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a configuration diagram of an embodiment of a mode locking fiber laser resonator device according to the present invention.

As shown in Fig. 1, in the mode-locking fiber laser resonator apparatus according to the present invention, in order to generate a Brillouin signal, a Brillouin pump 101 for generating a Brillouin pump signal, in fact, Brillouin induced scattering, is generated. A single mode optical fiber 102, a directional first optical fiber coupler 103 for inserting the Brillouin pump signal into the resonator, and an optical circulator 104 for outputting the returned Brillouin signal. .

Then, for mode-locking fiber laser resonance, the semiconductor laser 112 as an excitation pump light source for optical pumping to the erbium-doped optical fiber 111, the second optical fiber coupler 113 for coupling optical signals, and a signal are generated. And a light generator 115 to cause active mode locking via a signal input from the external signal generator 114, and to monitor the mode locked laser output. A third optical fiber coupler 116 and an optical isolator 117 for causing the optical signal to travel in one direction.

Meanwhile, in order to actually stabilize the resonator length, an optical fiber PZT 121 that contracts and expands electrically and variably, an optical receiver 122 that receives and measures a Brillouin signal, and the optical receiver 122 A frequency comparator 123 for comparing the frequency difference between the detected Brillouin signal and the signal generated by the signal generator 114, and a frequency-voltage converter for converting the signal output from the frequency comparator 123 into an electrical signal ( 124 and an electrical amplifier 125 that amplifies the electrical signal converted by the frequency-voltage converter 124.

Looking at the detailed function and operation of each component as follows.

First, a conventional ring laser resonator used for mode locking of an optical fiber laser pumps an erbium-doped optical fiber 111 using the semiconductor laser 112 for oscillating an optical fiber laser. The laser oscillates in the gain band of. In this case, the active mode locking is implemented by applying the signal generated by the signal generator 114 to the optical modulator 115.

In this case, the optical signal is propagated in one direction only by the advertiser 117, which is monitored through the second directional fiber coupler 116.

In order to generate a Brillouin signal inside the resonator, a Brillouin pump 101 is inserted through the first directional coupler 103 into the single mode optical fiber 102 inserted into the resonator. At this time, the first directional coupler 103 and the Brillouin pump 101 to detect a Brillouin signal generated in the single mode optical fiber 102 and traveling in the opposite direction of the Brillouin pump 101. The optical circulator 104 is inserted in between.

The optical circulator 104 has three input / output ports, and the signal inserted into the first port of the optical circulator 104 is output to the second port, and the signal input to the second port is the optical circulator 104. Output to port 3 of.

The Brillouin pump input according to the function of the optical circulator 104 is input to port 1 of the optical circulator 104 and output to port 2, and the single mode optical fiber 102 is the Brillouin pump signal. Receives and generates a Brillouin signal in the opposite direction and outputs through port 3 of the first directional coupler 103 and the optical circulator 104.

That is, the clockwise Brillouin pump signal, which is output to port 2 of the optical circulator 104, passes through the single mode optical fiber 102, and is counterclockwise in a direction opposite to the Brillouin pump signal. A Brillouin signal is generated.

At this time, the magnitude of the Brillouin signal generated is proportional to the power of the Brillouin pump (progression constant k1 (frequency P)), and has a Brillouin threshold due to the loss of the optical fiber, and the frequency s (progression constant ks) of the Brillouin signal. ) Is shifted by the transition frequency (a = 2 nva / lL) related to the sound velocity va of the sound wave in the optical fiber at the Brillouin pump signal frequency P (progression constant k1).

The speed of sound propagating on the optical fiber is dependent on the temperature change. Therefore, the frequency transition of the Brillouin signal changes very sensitively with the external temperature. The constant Cvt = 1.2 MHz / K representing the amount of change in the transition frequency with temperature change, and the optical wavelength used was lL = 1.32 mm.

Increasing the light intensity of the Brillouin pump 101 forms a kind of Brillouin laser, and in this case, some Brillouin signals may be detected through the first directional coupler 103 and the optical circulator 104. The advantage of such Brillouin laser is that the Brillouin frequency transition is generated by using all the optical fibers included in the laser resonator, so that the accuracy of laser stabilization can be improved.

Here, the Brillouin signal is generated only in the single mode optical fiber 102 and used as an error signal for stabilizing the length of the laser resonator, and the frequency shift of the Brillouin signal with respect to the Brillouin pump signal changes according to the ambient temperature.

Then, the first directional coupler 103 for inserting the Brillouin pump signal into the single mode optical fiber 102 in which Brillouin induced scattering actually occurs is 50 depending on the intensity and the ray oscillation condition of the Brillouin pump light source. 50 directional couplers can be used. At this time, when the adsorber 117 is mounted in the advancing direction of the Brillouin signal, the laser oscillates at the Brillouin signal frequency.

2 is an exemplary detection frequency of the mode locking fiber laser resonator according to the present invention.

The numerical values in the graph shown in FIG. 2 represent frequency components detected by the frequency comparator 123 of FIG. 1. Therefore, the frequency component applied to the optical modulator 115 and the frequency component of the signal detected by the optical receiver 122 are included, and a beating component between these two frequencies is displayed.

The beating component changes in a constant direction depending on the ambient temperature of the laser resonator. This change is detected through the optical receiver 122 and the frequency comparator 123, and the frequency difference is detected and converted into an electrical signal according to the magnitude of the difference, and then amplified by the electrical amplifier 124, The resonator length of the optical fiber laser can be stabilized by feeding back to the PZT element 121 that contracts and expands in response to the electrical signal output.

Figure 3 is an exemplary view of the result of the change in Brillouin frequency with respect to the temperature according to the present invention.

As shown in FIG. 3, the Brillouin frequency gradually increases as the temperature increases.

Therefore, this change can be used as an error signal for stabilization of the laser resonator length which varies with temperature.

4 is a configuration diagram of an embodiment of a mode locking fiber laser resonator using polarization maintaining optical fiber according to the present invention.

In the second embodiment according to the present invention, the mode locking fiber laser resonator shown in FIG. 4 causes Brillouin scattering in two polarization axes of a polarization maintaining optical fiber, and the beating component of the two signals changes according to an external temperature. It is a mode locking fiber laser resonator device for implementing laser stabilization using.

The embodiment shown in FIG. 4 is constructed by replacing the optical fiber and the device composed of the single mode optical fiber of FIG. 1 with the polarization maintaining optical fiber and the device as compared with the embodiment of FIG. 1.

Looking at the detailed function and operation of each component as follows.

If each causes a Brillouin induced scatter in each of the two polarization axes of the polarization maintaining optical fiber 402, a difference in Brillouin frequency transition occurs in these two axes. This signal is detected by a slow optical receiver 422 whose response rate is several hundred milliseconds (ms) or less. In order to detect only the beating signal, a low band pass filter 423 is inserted into the output terminal of the optical receiver 422.

In this case, when the transmission band of the low pass filter 423 is less than several tens of MHz, the output signal of the low pass filter 423 may include a frequency-voltage converter 424 for converting a frequency change into an electrical signal. After passing through the amplifier 425 for amplifying the electrical signal converted by the frequency-voltage converter 424, the optical fiber laser is fed back to the PZT element 421 that contracts and expands variably according to the amplified electrical signal. The resonator length can be stabilized.

5 is an exemplary detection frequency of the mode locking fiber laser resonator using the polarization maintaining optical fiber according to the present invention.

The graph shown in FIG. 5 shows the frequency components detected by the optical receiver 422 shown in FIG.

As shown in Fig. 5, Brillouin signals are generated on two axes of the polarization maintaining optical fiber 402, respectively, and a beating signal between these two frequency components is also detected.

The laser stabilization device configured as shown in FIG. 4 has an advantage that the beating component between the Brillouin frequencies of the two polarization axes can be composed of elements of a relatively low frequency band from several hundred kHz to several MHz as compared with the device of FIG. have.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the spirit of the present invention for those skilled in the art to which the present invention pertains, and the above-described embodiments and accompanying It is not limited to the drawing.

The present invention as described above is simpler than the conventional method by stabilizing the length of the fiber laser resonator by feeding back the frequency change to the laser resonator as an error signal by utilizing the temperature sensitive value of the Brillouin signal. In addition to the structure can be implemented, there is an effect that can stabilize the laser resonator in real time.

Claims (5)

A mode locking fiber laser resonator apparatus, Brillouin pumping means for generating a Brillouin pump signal; Brillouin signal generating means for receiving a Brillouin pump signal generated by the Brillouin pumping means and generating a Brillouin signal; Frequency detecting means for detecting a frequency component by receiving a Brillouin signal generated by the Brillouin signal generating means; Frequency comparison / measurement means for comparing the frequency of the Brillouin signal detected by the frequency detecting means with the frequency of the signal input to the mode locking fiber laser resonator and measuring the difference; Signal conversion means for converting the frequency signal measured by the frequency comparison / measurement means into an electrical signal; Signal amplifying means for amplifying the electrical signal converted by the signal converting means; And An optical fiber contraction and expansion means for receiving a signal amplified by the signal amplification means and thus expanding or contracting in length to stabilize the mode locking fiber laser resonator device Mode locking fiber laser resonator device comprising a. The method of claim 1, And the Brillouin signal generating means is a single mode optical fiber. A mode locking fiber laser resonator apparatus, Brillouin pumping means for generating a Brillouin pump signal; Brillouin signal generating means for receiving a Brillouin pump signal generated by the Brillouin pumping means and generating a Brillouin signal; Frequency detecting means for detecting a frequency component by receiving a Brillouin signal generated by the Brillouin signal generating means; Signal extracting means for extracting a beating signal component of the frequency signal detected by said frequency detecting means; Signal conversion means for converting the frequency signal extracted by the signal extraction means into an electrical signal; Signal amplifying means for amplifying the electrical signal converted by the signal converting means; And An optical fiber contraction and expansion means for receiving a signal amplified by the signal amplification means and thus expanding or contracting in length to stabilize the mode locking fiber laser resonator device Mode locking fiber laser resonator device comprising a. The method of claim 3, wherein And the Brillouin signal generating means is a polarization maintaining optical fiber. The method according to claim 1 or 3, A signal circulating means for receiving and transferring a Brillouin pump signal generated by the Brillouin pumping means and receiving the Brillouin signal generated by the Brillouin signal generating means and inputting the Brillouin signal to the signal detecting means; And Receives a Brillouin pump signal from the signal circulating means and inputs it to the Brillouin signal generating means, and receives a Brillouin signal generated by the Brillouin signal generating means and transmits the signal to the signal circulating means again. Way Mode locking fiber laser resonator device further comprising.