Pulse Laser Principle of MOPA Structure


The cavity of a fiber laser is itself a length of fiber, so it can't insert a Q switch into the cavity like a conventional laser to achieve pulse output, because the fiber cavity (that is, the fiber) is cut off into the Q crystal, which will increase the insertion loss. Second, it will affect the compactness of the entire laser and will not achieve fiber integration. So how to realize the pulse output of fiber laser is a new research topic. The more mature technology is currently implemented using MOPA (Primary Vibration Power Amplification) technology. MO (Oscillator) is the main vibrator. It is actually a laser with a small power (10-20mw). Generally, an LD with a suitable wavelength (such as 1064nm) can be selected. The small power LD can easily modulate the output parameters directly by driving current, such as repetition frequency, pulse width, pulse waveform and power level. The optical pulse signal is connected to the PA (PowerAmplifier) through the pigtail fiber-optic power amplifier for optical pulse amplification. . Optical fiber amplifiers are generally only used for fiber-optic communication. The principle is very similar to that of fiber-optic lasers, except that the fiber gratings at both ends of the fiber are removed and laser vibration cannot be formed, which only serves as a signal amplification. The fiber amplifier can perform the original amplification in strict accordance with the MO coupled recent seed source light without changing the laser wavelength, repetition frequency, pulse width and pulse waveform. Therefore, the pulsed laser adopts the MOPA method, which can obtain excellent laser characteristics and greatly improve the brightness of the output laser. This is a comprehensive effect that cannot be achieved by traditional methods. When the light pulse from the MO passes through the PA amplifier, the gain obtained by each part of the pulse will be different: the gain of the pulse front increases exponentially, the gain of the back of the pulse gradually decreases, and the gain of the trailing edge of the pulse is the smallest, especially if the pulse signal light Very strong, or when the pulse width is wide, the trailing edge of the pulse is not amplified at all. Therefore, a gain flattener is generally added to the PA so that each part of the pulse is uniformly amplified (if the energy of the incident signal is small or the pulse is short, the entire pulse can be uniformly amplified, and the pulse shape remains unchanged). After the laser pulse passes through the amplifier, the change in its waveform is directly related to the rising speed of the leading edge of the incident signal pulse. If the signal light is a Gaussian pulse, since the pulse front rises faster than the exponent, the pulse width can be compressed after amplification; if it is an exponential pulse, the shape and pulse width hardly change; if the input pulse front edge rise time ratio The exponential function is slower, and its pulse width becomes wider after amplification. Generally, in order to obtain a high power, narrow pulse width laser pulse, the slow rising portion of the pulse can be cut off by the clipping technique before the signal enters the amplifier, so that the pulse front edge becomes steep, that is, the compression pulse width can be achieved.



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