Yushi Takenaka

High power and high focusing CWCO/sub 2/ laser using an unstable resonator with a phase-unifying output coupler

A high-power and high-focusing continuous-wave (CW) CO/sub 2/ laser with a novel unstable resonator was developed. This resonator contains a step-wise variable reflecting output coupler named the phase-unifying output coupler and a total reflector. The laser was excited by a capacitive-barriered AC discharge called a silent discharge. A linearly polarized laser beam with a diffraction-limited divergence angle of 0.55 mrad was obtained. Very stable CW laser operation at an output power of 5 kW was achieved with a threshold discharge power of 10 kW and a slope efficiency of 18%. >

A 5 kW CW CO/sub 2/ laser using a novel negative-branch unstable resonator with a phase-unifying output coupler

A 5 kW CW CO/sub 2/ laser using a negative-branch unstable resonator (M=-1.5) is proposed and characterized experimentally. The resonator consists of a stepwise variable reflecting output coupler, called the phase-unifying output coupler, and a total reflector. A 5 kW CW laser beam with diffraction limited quality (2 theta =0.45 mrad) is obtained and affected by the focal point in the resonator. The misalignment angle to reduce the laser power to 95% is improved by a factor of 19 compared with a positive-branch unstable resonator (M=1.5) with a phase-unifying output coupler at the same resonator length. >

Advantages of negative-branch compared with positive-branch one-dimensional unstable resonators.

Effects of small misalignments in positive- and negative-branch strip confocal unstable resonators have been compared at the same absolute values of collimated Fresnel numbers and at the same Fresnel numbers. We show that positions of beam modes in negative-branch unstable resonators are far less sensitive to misalignment because of both geometric features and diffraction effects of beam flipping in the resonators.

Gauss-core resonator: a novel stable resonator with a large-diameter diffraction-limited output beam.

A novel cavity design based on a stable resonator configuration is presented. The output coupler of the new stable resonator has a circular partial-reflection region in the center that is surrounded by an antireflection region. This resonator operates in a large-volume TEM(00) mode that is mainly determined by the partial-reflection region of the output coupler and is characterized by the use of an additional laser beam that is spread from the optical path in the cavity because of diffraction and is amplified by a laser medium. By combining part of a beam transmitted from the partial-reflection region with the amplified diffraction beam, a laser beam having a large diameter is obtained. A diffraction-limited output beam with the M(2) factor of 1.3 is obtained, as predicted theoretically when the new stable resonator is applied to a high-power cw CO(2) laser.

Novel stable resonator for large-volume TEM00 mode operation

A novel stable resonator is developed to realize stable operation at a TEM00 mode with a large beam diameter. The output coupler has a circular partial‐reflection region in the center surrounded by an antireflection region. The transverse mode is mainly selected by the partial‐reflection region. An amplified diffraction beam is extracted from the outer antireflection region. By combining part of the ‘‘core’’ beam transmitted from the partial‐reflection region with the amplified diffraction beam, a diffraction‐limited beam with the M2 factor of 1.3 is obtained from a high power continuous wave CO2 laser as predicted theoretically.

High-power CO 2 laser with a Gauss-core resonator for high-speed cutting of thin metal sheets

A novel resonator, the Gauss-core resonator, based on a stable resonator configuration designed to yield a highly focusing beam operating in a large-volume TEM(00) mode, is presented. A 6.2 kW linearly polarized output beam with an M(2) factor of 1.7 is obtained experimentally for a high-power cw CO(2) laser. The capability of the Gauss-core resonator to process laser materials is also studied. We can cut 1-mm-thick mild (soft) steel with a maximum cutting speed of 58 m/min at 5.6 kW and 0.2-mm-thick steel 145 m/min at 2.8 kW.

High-power CO2 laser using a new negative-branch unstable resonator with a phase-unifying output coupler

A high power CO2 laser using a new negative-branch unstable resonator (M equals -1.5) is proposed and investigated both theoretically and experimentally. The resonator consists of a stepwise variable reflecting output coupler termed phase-unifying output coupler and a total reflector. A 5 kW CW laser beam with diffraction limited quality (2(theta) equals 0.45 mrad) is obtained, not affected by the focal point in the resonator. The misalignment angle to reduce the laser power to 95% is improved by a factor of 19 compared with a positive-branch unstable resonator (M equals 1.5) with a phase-unifying output coupler at the same resonator length.

High-power high-brightness diode-pumped solid state laser for precise laser processing

We have proposed a highly efficient and high-brightness quasi-cw Nd:YAG rod laser with a novel-side-pumping design using micro-lens free diode-stacks. We achieved 320W output power with 28-% electrical-to-optical efficiency, which is, to our knowledge, the highest efficiency reported for diode- pumped solid-state lasers. We generated the high quality beam of M2equals4 with the output power of 500W while maintaining the electrical-to-optical efficiency of 20%. We also demonstrated through-hole formation of 1mm thick copper plate using the high brightness laser beam and successfully obtained round holes with the diameter of less than 40 micrometers .

Novel circular-beam equalizing techniques that use graded-index fiber optics for a high-power laser diode.

We demonstrate a novel method of equalizing a laser diode (LD) beam into a circular beam. This method uses the twist effect of graded index (GI) fiber optics. An asymmetric LD beam with beam qualities of M2 = 500 in the slow axis and M2 = 4 in the fast axis is converted successfully into a symmetric circular beam with a beam quality of M2 = 175. The circular-output beam with 92% coupling efficiency from the fiber input to the fiber output is obtained with a 5-m-long GI1200 (1200 means a core diameter of 1200 microm) fiber for a 2-W LD array. We extend the experiments to a higher-power source with higher asymmetric beam qualities of M2 = 3000 and M2 = 4. By slightly bending the GI10000 (10000 means a core diameter of 10,000 microm) fiber, we have succeeded in generating a symmetric beam with a improved beam quality of M2 = 2000. The average beam quality is preserved when the asymmetric ratio is not high, and the beam quality degradation ratio is investigated up to asymmetric ratios of 750.

Novel circular-beam equalizing techniques using graded-index (GI) fiber optics for high-power laser diode

We demonstrate a novel method of equalizing laser diode beam into circular beam. The method uses the twist effect of graded index(GI) fiber optics. An asymmetric laser diode beam with the beam qualities of M2=500 in the slow axis and M2=4 in the fist axis is successfully converted into a symmetric circular beam with the beam quality of M2=175. The circular output beam with 92% coupling efficiency is obtained by using a 5m long GI1200 fiber for 2W laser diode array. We have Ibund that the required minimum length of G11200 fiber is 550mm for circular beam equalizing. We extend the experiments to higher power source with higher asymmetric beam qualities of M2=3000/M2=4. By using a large core diameter ofGI10000 fiber, the higher asymmetric beam is not converted into a perct symmetric beam. We consider that the length is too short Ibr this large core fiber. Since the GI1200 fiber required 550mm, the GI10000 fiber should require at least 4583mm, however, the fiber length is limited to 500mm because ofthe production matter. By slightly bending the fiber, however, we have succeeded in generating symmetric beam with improved beam quality of M2=2000. The average beam quality is preserved when the asymmetric ratio is not high and the beam quality degradation ratio is investigated up to asymmetric ratios of 750.