TECHNOLOGY FIELD

The present disclosure relates to laser systems and related methods. In particular, the present disclosure relates to laser systems and related methods for use in, for example, generating high-energy, sub-nanosecond pulses.

DESCRIPTION OF RELATED ART

Lasers are used in numerous applications in industry, research and medicine. Of special interest are lasers generating high energy and short duration pulses, for example, nanosecond and sub-nanosecond pulses. Such lasers would facilitate micro-processing applications of some precision materials. Such lasers could also address a number of scientific and technological applications, as well as certain medical procedures, where the short interaction time with the work piece or tissue can reduce heating effects and improve the interaction quality.

Q-switched solid state lasers are currently used to generate short duration laser pulses. These pulses typical could have durations as short as a few nanoseconds, although reports of shorter pulses tested in laboratory conditions exist. A special type of Q-switched laser, commonly called microchip lasers, which have very short resonators (few millimeters) can generate sub-nanosecond pulses. However, such devices typically can only provide pulse energies of less than 1 millijoule (mJ).

Master Oscillator Power Amplifier (MOPA) systems are also used to generate sub-nanosecond, high-energy laser pulses. Usually, a MOPA includes two pump lasers with one pump laser pumping the master oscillator and the other pump laser pumping the power amplifier. For proper MOPA operation, the pumping sources must be closely synchronized. The synchronization, however, requires complicated electronic systems.

MOPA systems, especially ones with high amplifier gain, are sensitive to parasitic reflections, which can lead to problems of self-lasing. Self-lasing may severely limit the amplification achievable in the power amplifier. The feedback sensitivity of these parasitic reflections is usually cured only by placing a Faraday isolator between the master oscillator (MO), also sometimes termed as a seed laser, and the power amplifier (PA), also sometimes termed as an amplifier. Particularly for high-power pulsed devices, this may introduce serious limitations. Moreover, the complexity and cost of such MOPA systems are high, and the output energy is often low.

The gain achievable in a power amplifier is often severely limited by amplified spontaneous emission (ASE). Gains higher than those limited by ASE are usually achieved by use of power amplifiers having multiple stages with optical isolation between the stages. This may further complicate the design of the power amplifiers and increase its cost.

SUMMARY

Therefore, various exemplary embodiments of the present disclosure relate to improved laser systems and related methods that may overcome one or more drawbacks or problems discussed above.

To attain the advantages and in accordance with the present disclosure, as embodied and broadly described herein, one exemplary aspect of the present disclosure may provide a MOPA laser system configured to generate high-power, sub-nanosecond laser pulses with output pulse energy exceeding 100 mJ. The laser system may include a seed laser configured to output pulsed laser light with pulse energy exceeding 5 mJ, an amplifier with gain of less than 100 configured to receive the pulsed laser light emitted by the seed laser and amplify it. The use of an energetic seed laser may reduce the gain requirement to less than 100 in the amplifier such that a relatively simple amplifier could be used to generate output energies up to 5 J.

According to another exemplary aspect, careful selection of the pump source wavelength, the doping concentration and the length of the seed laser gain medium, the pump beam intensity, the cross-sectional diameter of the active pumped region, and/or the diameter of the gain medium may facilitate achievement of greater than 5 mJ output energy in the sub-nanosecond seed laser. This selection of parameters may also be of help in overcoming limitations that could be imposed by amplified spontaneous emission.

In one example, the seed laser could be pumped by diode lasers and the amplifier could be pumped with flash lamps or a solid state pump laser. Alternatively, both the seed laser and the amplifier could be pumped by a single solid state pump laser. Pumping the seed laser and the amplifier with a single laser may obviate the need of separate pump sources and the need for electronic synchronization of the separate pump sources.

In some exemplary aspects, the solid state pump laser may be configured to provide simultaneously pump laser beam to both the seed laser and the amplifier. The pump laser could be an Alexandrite laser pumped by one or more flash lamps, which may be configured to generate pulse energies greater than 1 J with pulse durations shorter than 0.2 millisecond (msec) at 755 nm and pump a Nd:YAG MOPA configured to generate an output at a wavelength of 1064 nm. The pump laser beam emitted by the single pump laser may be coupled to a multimode optical fiber and similar multimode optical fibers to deliver or conduct the pump laser beam to the seed laser and the amplifier. The length of the multimode optical fibers delivering pump energy to the seed laser and to the amplifier can be selected to mode-scramble the pumping energy such as to provide a homogenous pumping energy distribution at the output of the multimode optical fibers. This may render the seed laser and the amplifier insensitive to output beam quality fluctuations in the pump laser. Use of a single pump laser may also support automatic synchronization of the seed laser and the amplifier.

In another exemplary aspect, the seed laser may include a gain medium configured to output pulsed laser light and a passive Q-switch. The seed laser gain medium can be a short Nd:YAG rod or plate. The length of the seed laser Nd:YAG rod could be selected to be long enough to reduce gain in transverse dimension and to reduce amplified spontaneous emission. Additionally, the length of the seed laser Nd:YAG rod could be selected to be short enough to support generation of sub-nanosecond pulses. The Nd concentration could be chosen so that significant portion of the pump laser beam can be absorbed in the rod. The cross-sectional diameter of the pumped region in the seed laser and the pump beam intensity could be chosen to facilitate the generation of the required output energy while minimizing the effects of ASE.

According to one exemplary aspect, use of an unstable resonator may prevent occurrence of hot spots in a seed laser beam. A saturable absorber or a Faraday rotator optical isolator placed between the seed laser and the amplifier may prevent self-lasing,

In some exemplary aspects, the amplifier may include a Nd:YAG rod, a coupling lens that couples the seed laser beam to the Nd:YAG rod, and a dichroic mirror. The dichroic mirror may be configured to reflect the laser beam amplified by the amplifier and transmit a pumping beam generated by the single pump laser. To minimize amplified spontaneous emission, the gain per unit amplifier rod length may be kept small. In one exemplary embodiment, the Nd:YAG rod length may be greater than 2 cm, preferably greater than 4 cm. Also to minimize amplified spontaneous emission, the Nd:YAG rod could have a diameter significantly larger than the diameter of the pumped region within constraints imposed by the cost of the rod. The diameter could preferably be greater than 1 cm. Such relation between the Nd:YAG rod diameter and the diameter of the pumped region may minimize amplified spontaneous emission that could be caused by reflections from a barrel surface of the Nd:YAG rod.

In another exemplary aspect, the seed laser beam may be amplified in the amplifier in a single pass. The seed laser emitted beam and the pump beam may be counter-propagating in the Nd:YAG rod of the amplifier. The amplifier may output laser beam in a pulse mode at a wavelength of 1064 nm with energy of up to 5 J per pulse. The pulse duration may be from about 10 psec to about 900 psec. The gain in the amplifier is about 100 or less.

Alternatively, one can use a single optical fiber to deliver both the pump laser beam and the seed laser beam to the amplifier. Use of a single optical fiber may enable optical alignment and spatial overlap between the seed laser beam and the pumped gain region in the amplifying device. In this case, the seed laser beam and the amplifier pump beam may be co-propagating in the amplifier. This arrangement may also facilitate the placement of the amplifying device remotely from the seed laser, for example, within the beam delivery system for bringing the output beam to the work material or target tissue. This may eliminate the need for a maneuverable free space beam delivery system, such as an articulated arm with multiple mirrors in order to flexibly bring the beam to the work material.

According to one exemplary aspect, the MOPA may include at least one energy sensor configured to detect laser energy radiated by the seed laser and/or by the amplifier and an electronic mechanism. The electronic mechanism may be configured to ensure a single output laser pulse by terminating the flash lamp drive current in the pump laser upon optical detection of an output pulse from at least one of a group of the seed laser and the amplifier.

Pump Alexandrite energy for the seed laser may be actively controlled through a variable attenuator. The variable attenuator could be controlled by rotational polarization based elements (i.e., a polarizer cube or a polarizer pair) or attenuation variable filter. The attenuation may be such adjusted that time delay of Q-switched pulse relative to the start of pump laser maintains at a certain time window. For Nd:YAG, the time delay may be kept within a range from 100 .mu.s to 170 .mu.s. It can be monitored with a photodetector for the seed laser. Active control of time delay of Q-switched pulse helps optimize energy extraction from the amplifier as well as prevent missing pulse.

The MOPA system may also include a MOPA output laser beam shaping and frequency multiplying segments. The MOPA output laser beam shaping optics could be, for example, a reverse Galilean optical system. Other optical system configured to facilitate formation on the work material of a laser spot of about 3 mm to 6 mm could be used. A saturable absorber, such as a Cr:YAG absorber, could be used to prevent self-lasing mode of operation and minimize ASE that could be caused by reflections from the work material. An off-axis reflective optical element such as a 90 degree parabolic mirror could also be used in the MOPA output beam shaping optics or beam delivery system to minimize reflected light from going back into the amplifier.

Optionally, the MOPA output laser beam shaping optics could include at least one laser beam frequency doubling device configured to receive the output laser beam with wavelength of 1064 nm and double the frequency such as to produce an output laser beam with wavelength of 532 nm.

An optical fiber could be used to couple the seed laser beam to the amplifier. The fiber optics coupling may support remote location of the amplifier together with the output laser beam shaping and frequency multiplying components.

