We have demonstrated high-precision cutting and drilling of CFRP using a 3-kW CW single-mode fiber laser and a galvanometer scanner. A 3-mm-thick thermoset CFRP was successfully cut with 100 scans at the scanning speed of 13 m/s, which corresponds to effective cutting speed of 7.8 m/min. We employed the multi-pass method, in which a laser is scanned on the same position at high speed, and the optimized scanning position shift to realize short-time cutting. The time interval between scans was less than 20 ms, which was much shorter than general time interval in multi-pass method. Even with such short interval, the width of heat-affected zone (HAZ) was controlled to 97 μm on average. Drilling demonstration was carried out by using the same setup as the cutting experiment. Holes with a diameter of 6.4 mm have been successfully drilled on a 2-mm-thick thermoset CFRP. The processing time was 2.7 seconds, which was equivalent to that in mechanical processing. Since a HAZ tends to expand in the direction of the carbon fibers due to their high heat conductivity, it is difficult to reduce the width of HAZ in all directions around the processed area. To overcome this challenge, we optimized the gas injection condition as well as the scanning condition. As a result, the widths of HAZs were successfully controlled to about 100 μm in all directions.
A high power single-mode fiber laser has received a lot of attention in various materials processing fields. In order to carry out processing under proper conditions with single-mode fiber laser, the dynamic flows of fluids inside the materials should be precisely understood. We carried out the numerical analysis of materials processing using a 5-kW single-mode fiber laser. The dynamic flows of the copper fluid during the bead-on-plate tests are calculated and the results agree well with the experimental results. The processing with the different scan speed and spot size, and at the output power of 10 kW are numerically calculated as well. The proper processing conditions with less spatters and dross for each materials can be determined by the dynamic analysis.
Stimulated Raman scattering gives back reflection sensitivity to a high power fiber laser. Therefor SRS suppression is necessary in order to realize stable laser processing by a high power fiber laser. A 5-kW single-mode ytterbium doped fiber laser with a 20-m long delivery fiber has been realized. The fiber laser is an all-fiber single-stage Fabry-Perot system in a co-pumping configuration. The optical to optical efficiency was 80% at the output power of 5.0 kW. And the M-squared figure of 1.3 was obtained. The Stokes light by SRS is suppressed to 45 dB below the laser output by using fibers with the effective mode area of 600 μm2 . While SRS was well suppressed, four wave mixing was observed with the frequency shift of ~6 THz. Four wave mixing between the fundamental mode and the secondary modes is believed to take place. Four wave mixing is believed not to give back reflection sensitivity to the fiber laser. The 5-kW single-mode fiber laser was applied to laser processing. Bead-on-plate tests were carried out with a galvanometer scanner. The laser ran without stopping nor damaging the laser system even during processing highly reflective material. This implies that our SRS suppressed single-mode fiber laser can be used practically in most of processing systems.
The broadband source with the arbitrary spectrum based on dispersion tuning technique was demonstrated. While a dispersion-tuned wavelength-swept laser is capable of fast and widely wavelength sweep, it can also be used as a broadband spectrally-flexible source by controlling the sweep waveform. It is the first demonstration of spectrallytunable source to the best of our knowledge. We used the dispersion-tuned laser as a broadband source for SD-OCT system by synchronizing sweep rate of a laser and exposure time of a CCD camera in SD-OCT system. We successfully obtained the images of an adhesive tape and a human finger.
Swept-Source Optical coherence tomography (SS-OCT) is a powerful tool for fast medical imaging. For the real-time 3D imaging, the wide tuning range over 100 nm and fast sweep rate over 100 kHz are typically required. We recently proposed a new wavelength-swept laser for SS-OCT. It is based on a principle called dispersion tuning. Since the cavity contains no mechanical components, such as tunable filters, we could achieve the very high sweep rate. In this review paper, we describe the principle of the dispersion-tuned swept lasers in detail and present our recent works on the application to the SS-OCT system.
We have demonstrated a wavelength-swept fiber laser based on dispersion tuning method. In this method, the light in a
dispersive laser cavity is intensity modulated and actively mode-locked, and the lasing wavelength can be changed by
controlling the modulation frequency. As the dispersion-tuned laser does not include any tunable filters, the sweep rate
and range are not limited by mechanical moving parts. We have reported the wavelength-swept laser which has the
tuning range of over 100nm with the sweep rate of 200kHz, and we have applied the laser to the swept-source optical
coherence tomography (SS-OCT) system. Although we have successfully obtained the OCT image of the human finger
at 1kHz sweep rate, we could not obtain OCT images at higher sweep rate because of the performance degradation of the
laser. As this laser cavity included 100m long dispersion compensating fiber (DCF), the long laser cavity increased the
photon lifetime and resulted in the output power decrease and the linewidth broadening at higher sweep rate. In order to
solve these problems, we inserted a reflective semiconductor optical amplifier (RSOA) and a chirped fiber Bragg grating
(CFBG) into the laser cavity. Use of these devices made it possible to shorten the cavity length drastically and the laser
performance at high sweep rate is significantly improved. We could achieve that the sweep range of 60nm and the output
power of 8.4mW at 100kHz sweep. We applied the laser to swept-source OCT system and we successfully obtained
images of an adhesive tape at up to 250kHz sweep.
We demonstrate a wavelength-swept fiber laser based on the dispersion tuning method using a chirped fiber Bragg
grating (CFBG) and a reflective semiconductor optical amplifier (RSOA). The laser cavity length is reduced to as short
as 2m owing to the short and highly-dispersive CFBG, which is much shorter than the one using dispersion
compensating fiber (DCF). The short cavity enables higher sweep speed. We successfully obtain the OCT images of a
rolled adhesive tape at the sweep rate of 50kHz.
We proposed a wide and fast wavelength-swept fiber lasers based on the dispersion tuning for the optical coherence
tomography (OCT) applications. So far, we have achieved the sweep rate of ~200kHz at the sweep bandwidth of
~180nm. The sweep rate is only limited by the photon lifetime, which is proportional to the cavity length. Since we used
a dispersion compensating fiber (DCF) as the dispersive medium, the long cavity length (~100m) was the limit of the
sweep rate. In this paper, we demonstrate faster sweep rate up to ~500kHz by using a wideband chirped fiber Bragg
grating (CFBG).
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