KR20040089772A - Pulse-amplitude equalization method using polarization-maintaining laser resonator - Google Patents

Abstract

PURPOSE: A method for equalizing pulse amplitude using a polarization maintaining laser resonator is provided to equalize the pulse amplitude without using an additional device by removing unevenness of the pulse amplitude. CONSTITUTION: An optical intensity modulator(12) is used for modulating intensity of beam according to an electric modulation signal. An optical isolator(14) is used for transmitting the forward beam and reflecting the backward beam. A tunable filter(16) is used for tuning an oscillating wavelength of the laser. An Er-doped fiber(18) is used as a gain medium of the laser. A WDM coupler(20) is used for coupling optical fibers. A laser diode(22) is used for supplying energy by pumping the Er-doped fiber. A dispersion shifted fiber(24) is used for optimizing dispersion values of the laser resonator. An m:n coupler(26) is used for outputting partially the laser beams. An optical delay line(28) is used for changing the length of the laser resonator.

Description

Pulse-amplitude equalization method using polarization-maintaining laser resonator}

The present invention relates to a pulse amplitude equalization method using a polarization sustaining laser resonator, and more particularly, to glass pulses having a repetition rate higher than the optical modulator bandwidth using a polarization sustaining laser resonator without instability due to the polarization mode. The present invention relates to a method for realizing pulse amplitude uniformity by optimizing the modulation index of an existing optical modulator without an additional pulse amplitude equalization device by removing the non-uniformity of the pulse amplitude generated in the mode locking.

As a light source for ultra high speed optical communication or ultra high speed full light signal processing, a high repetition rate light source is required, and in order to increase the repetition rate, a wide bandwidth optical modulator is required and the repetition rate is limited by the bandwidth of the optical modulator.

In general, laser pulses having the same repetition rate as the modulation frequency are generated through harmonic mode locking so that the modulation frequency applied to the optical modulator is an integer multiple of the fundamental resonance frequency of the laser.

However, when the modulation frequency is different from the laser's fundamental resonance frequency by an integral multiple of the ratio, the repetition rate can be increased by an integral multiple of the modulation frequency, which is called rational harmonic mode locking.

By using this technology, it is possible to increase the repetition rate while using an existing optical modulator as it is, and a high repetition rate laser pulse can be obtained easily and inexpensively, while the amplitude of the laser pulse is not uniform.

Therefore, pulse-amplitude equalization is required for the practical use of the glass harmonic mode locked pulses. Various methods have been developed for this purpose.

In other words, CG Lee, YJ Kim, HK Choi, Chang-Soo Park, Pulse-amplitude equalization in a rational harmonically mode-locked semiconductor ring raser using optical feedback, Opt, commun., Vol. 209, pp 417-425, 2002), How to inject Farby-Perot etalon into a laser resonator (KK Gupta, N. Onodera, M. Hyodo, Technique to generate equal amplitude higher-order optical pulses in rational harmonically modelocked fiber ring laser, Electron. Lett., vol. 37, pp, 948-950, 2001), How to install a polarizer in a resonator (Zhihong Li, Caiyun Lou. Kam Tai Chan, Yuhua Li, and Yizhi Gao, Theoretical and Experimental Study of Pulse-Amplitude-Equalization in a Rational Harmonic Mode-Locked Fiber Ring Laser, IEEE J. Quantum Elecron., Vol. 37, pp. 33-37, 2001), and many other methods have been developed: .

Hyuek Jae Lee, Kwangjoon Kim, Hae Geun Kim, Pulse-amplitude equalization of rational harmonically mode-locked fiber ring laser using a semiconductor optical amplifier loop mirror, Opt. Commun., Vol. 160, pp. 51-56, 1999.

2. M.Y. Jeon, et al., External fiber laser based pulse amplitude equalization scheme for rational harmonic modelocking in a ring type fiber laser, Electron. Lett., Vol. 34, pp. 182-184, 1998.

3. M.Y. Jeon, et al., Pulse-amplitude-equalized output from a rational harmonically mode-locked fiber laser, Opt. lett., vol. 23, pp. 855-857, 1998.

However, the above methods require a separate additional device for realizing pulse amplitude uniformity in addition to the laser resonator for laser oscillation, thereby making the device complicated and difficult to operate and expensive.

In more detail, in the commonly used harmonic mode locking, the modulation frequency F of the optical modulator is nf, which is n integer multiples of the fundamental resonance frequency f of the laser, and the repetition rate of the output pulse is also given by F = nf.

In glass-high mode locking, F = (n + 1 / m) × f, which differs by 1 / m glass magnification, giving the output pulse repetition rate mF = (mn + 1) f, resulting in a modulation frequency F The repetition rate can be as high as m integer times.

However, the glass harmonic mode locking has a lower stability than the general harmonic mode locking, so that the output pulse amplitude is not uniform, so that a pulse amplitude equalization device is required to make it practical, thereby making the device complicated, difficult to operate, and expensive. .

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a polarization mode for removing the nonuniformity of the pulse amplitude generated in the glass harmonic mode locking which can obtain a laser pulse with a repetition rate higher than the optical modulator bandwidth. It is an object of the present invention to provide a method for realizing pulse amplitude uniformity by optimizing a modulation index of an existing optical modulator without an additional pulse amplitude equalization device by using a polarization maintaining laser resonator having no instability due to instability.

In order to achieve the above object, the present invention uses a laser resonator composed of an optical component and an optical fiber having a polarization maintaining characteristic to obtain a laser pulse train that is an integer multiple of the modulation frequency (n) by glass-high mode locking, An object of the present invention is to provide a pulse amplitude uniformity method using a polarization maintaining laser resonator, characterized by obtaining a glass harmonic mode locked pulse having a uniform pulse amplitude by optimizing an output of an electrical modulation signal applied to an optical modulator.

1 is a configuration diagram of a polarization maintaining laser resonator applied to the present invention.

2 is an oscilloscope photograph of a glass pulsed modelocked laser pulse at 40 GHz in accordance with the present invention.

3 is a light spectrum graph of a glass pulsed modelocked laser pulse at 40 GHz in accordance with the present invention.

4 is an RF spectrum graph of a glass pulsed modelocked laser pulse at 40 GHz, in which FIG. 4A is uneven and FIG. 4B is uniform.

<Description of the symbols for the main parts of the drawings>

1: laser resonator 10: optical fiber

12: light intensity modulator 14: optical isolator

16: variable filter 18: Erbium-doped optical fiber

20: wavelength multiple coupler 22: laser diode

24: dispersion transition optical fiber 26: 90:10 combiner

28: optical delay device 30: signal generator

Hereinafter, the configuration and operation of the present invention will be described with reference to the accompanying drawings.

1 shows a configuration of a laser resonator according to the present invention.

The optical fiber 10 constituting the laser resonator 1 is a polarization maintaining optical fiber so as to oscillate with only a single linear polarization, and each optical component having a function as described below is manufactured to have polarization maintaining characteristics.

The optical intensity modulator 12 receives a signal from the signal generator 30 and modulates the light intensity according to an input electrical signal.

The optical isolator 14 allows light to pass only in the forward direction and blocks travel in the reverse direction.

The tunable filter 16 changes the oscillation wavelength of the laser.

Er-Doped Fiber 18 is the gain medium of the laser.

The wavelength division multiplexing coupler 20 couples optical fibers passing different wavelengths. For reference, the laser diode wavelength is 980 nm and the fiber laser wavelength is 1550 nm.

The laser diode 22 optically pumps the erbium-doped optical fiber 18 to supply energy.

The dispersion shifted fiber 24 optimizes the dispersion value of the laser resonator 1.

A 90:10 coupler 26 outputs a portion (10%) of the laser light in the laser resonator 1 as an output.

An optical delay line 28 changes the resonator length of the laser.

As described above, when the laser resonator is constructed using polarization-maintaining optical fibers and optical components, the output fluctuations caused by the change in polarization can be eliminated to obtain stable laser output.

Taking advantage of these characteristics, the present patent applies polarization-maintaining laser resonator technology to glass harmonic mode locking, and when the glass harmonic mode locking is performed, optimizing the intensity of the electric signal applied to the optical modulator, the pulse which was impossible in the general laser resonator is impossible. Uniformity of amplitude can be realized.

Adjust the resonator length or modulation frequency to meet the desired modelocking conditions.

First, when the high mode lock is performed using a 10 GHz optical modulator, an output pulse having a repetition rate of 10 GHz can be obtained.

However, when the glass harmonic mode locking is performed, pulse trains of 20, 30, and 40 GHz, which are integer multiples of 10 GHz, can be obtained. In this case, the pulse amplitude becomes uneven.

However, by optimizing the output of the electrical modulation signal applied to the optical modulator, it is possible to obtain a uniform pulse.

FIG. 2 is an oscilloscope photograph of a laser output according to an output of a modulated signal applied to an optical modulator for a case where the repetition rate of the glass harmonic mode locked laser pulse is 40 GHz, (a) is 24 dbm, and (b) is 23 dbm, (c) is 22 dbm, (d) is 21 dbm, (e) is 20 dbm, and (f) is 19 dbm.

As shown, pulses of the laser generally exhibit non-uniform amplitudes, but a uniform pulse amplitude can be obtained by optimizing the output of the modulated signal (here 22 dbm in (c)).

Figure 3 shows the light spectrum when the pulse amplitude is uniform.

When the fundamental resonant frequency of the laser resonator is 0.08 nm corresponding to 10 GHz, if the repetition rate is adjusted to 40 GHz through glass harmonic mode locking, only the laser mode of 0.32 nm interval, which is four times 0.08 nm, is oscillated. The modes will disappear.

This characteristic can be usefully used to find the glass harmonic modelocking optimum condition to obtain a uniform pulse amplitude.

In other words, if the glass harmonic mode locking is optimized in the light spectrum, only the mode is oscillated and the intermediate modes are minimized.

FIG. 4A shows an RF spectrum in which the optical signal of the laser output is converted into a photoelectric converter when it is not homogenized, and FIG. 4B.

In FIG. 4A, when 10, 20, and 30 GHz components appear simultaneously in addition to the 40 GHz component when the uniformity is not shown in FIG. 4B, the remaining components 10, 20, and 30 GHz, in addition to the 40 GHz component, are lost. have.

By using these characteristics, it can be used as a method to find the optimum condition of glass harmonic mode locking to obtain a uniform pulse amplitude.

In other words, if the free harmonic mode locking is optimized in the RF spectrum, the optimization condition can be found by only oscillating the corresponding frequency components and minimizing the intermediate frequency components.

As described above, according to the present invention, the polarized light maintains the glass harmonic mode locking technology, which can obtain a laser pulse having a repetition rate higher than the bandwidth of the optical modulator, so that the pulse amplitude can be uniformized without any additional device for high practicality. By realizing through laser resonator, it can be utilized for ultra high speed light source that can be applied to ultra high speed optical communication or ultra high speed optical signal processing.

Claims (4)

By using the laser resonator composed of optical component and optical fiber having polarization sustaining characteristics, glass harmonic mode locking is used to obtain a laser pulse train that is an integer multiple of the modulation frequency, and optimizes the output of the electrical modulation signal applied to the optical modulator among the optical components. A pulse amplitude equalization method using a polarization maintaining laser resonator, characterized by obtaining a glass-high mode-locked pulse with a uniform pulse amplitude. The method according to claim 1, wherein the optical component, An optical modulator for modulating the intensity of light according to the electric signal; An optical isolator that passes light only in the forward direction and blocks propagation in the reverse direction, A variable filter for changing the oscillation wavelength of the laser, Erbium-doped optical fiber which is a gain medium of laser, A wavelength multiplexing combiner for coupling optical fibers passing through different laser diode wavelengths and optical fiber laser wavelengths, A laser diode for optically pumping the erbium-doped optical fiber to supply energy; A dispersion transition optical fiber for optimizing a dispersion value of the laser resonator; An m: n coupler for outputting a part of the laser light in the laser resonator as an output; Pulse amplitude uniformity method using a polarization maintaining laser resonator, characterized in that consisting of an optical retarder for changing the resonator length of the laser. The glass harmonic mode locking condition of claim 2, wherein in the optical spectrum output from the combiner, when the glass harmonic mode locking is optimized, only the mode is oscillated and the intermediate modes are minimized. Pulse amplitude equalization method using a polarization maintaining laser resonator, characterized in that found. The RF spectrum according to claim 2, wherein in the RF spectrum in which the optical signal output from the coupler is converted into a photoelectric converter, the glass harmonic mode locking is optimized so that only the frequency is oscillated and the intermediate frequency components are minimized. Pulse amplitude uniformity method using a polarization maintaining laser resonator characterized by finding a glass harmonic mode locking condition that can be obtained.