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Lasers & Sources

Ultrafast fiber laser delivers high energy and long-term reliability

A software-controlled high-power laser achieves superior beam quality and durability without the need for skilled technicians.
20 March 2009, SPIE Newsroom. DOI: 10.1117/2.1200902.1516

Conventional lasers used for materials processing often introduce undesirable side effects. Those operating at visible to near-IR wavelengths typically leave behind thermal damage, such as rough edges or changes to the material structure.1 UV lasers induce unwanted alterations in the basic chemistry of target materials.2 By contrast, ultrashort-pulse (USP) lasers—for which the pulse duration is shorter than the heat-diffusion time of the target material—can produce extremely fine features in almost any material without side effects.3 USP lasers that are cost-effective, reliable, and easy to use will enable commercial production of advanced medical implants (see Figure 1), low-cost solar panels,1 and many other devices.

Researchers have demonstrated compelling applications for USP lasers using large and cumbersome systems based on solid-state-crystal amplification (i.e., titanium sapphire). These systems employ many free-space optical components4 and therefore require regular maintenance by a highly skilled technician or researcher. Commercially available systems use optical-fiber-based technology to bring more compact but lower-pulse-energy USP lasers to the market place.5 Still, none of the existing laser architectures provide hands-off operation (i.e., no laser tuning or maintenance) with sufficient pulse energy and average power output.

Figure 1. Microscope image of the uniform material structure and well-defined edge of a biodegradable stent (a wire-mesh tube) made with an ultrashort-pulse laser. The device was not subjected to any post-processing.

To address this technology gap, we have combined robust, uniquely configured fiber-optic telecom technology with microprocessors and embedded stabilization software to produce high-performance, reliable, and cost-effective USP lasers. In our platform, the software operating system does all of the monitoring and adjusting functions normally performed by a person. Our use of fiber to route the beam through the system largely avoids optical misalignment associated with using free-space components like mirrors and lenses. Scaling this platform to high energy requires expanding the mode-field area in the Latter amplifier stages, reducing losses in the pulse-compression stage, and minimizing pulse-phase errors through every component.

Optical and electronic sensors and controls throughout the laser system are managed by a Linux kernel that responds to higher-level instructions sent through an application-programming interface. Component and user safety are guaranteed by limits set in hardware registers and fast circuit interlocks that prevent faulty operating conditions. Each pulse-generation stage has precise targets for signal energy and pulse quality. These are accurately maintained by exquisitely tuned software proportional-integral-derivative loops that control pump-diode power, module temperature, and other optical-signal figures of merit. These are the same parameters usually monitored and controlled by a well-trained technician or engineer.

Erbium-fiber amplifiers are a mature technology for generating very high repetition rate—yet very low energy—sequences of optical pulses at a wavelength of 1.55μm.6 These pulses make up the ones and zeros in digital-communications protocols. Unfortunately, using similar erbium-fiber amplifiers to generate high-energy pulses for laser machining is challenging due to nonlinear optical distortion inside the confined diameter of single-mode fibers. To reach beyond the usual limits, we implement large-mode-area erbium fiber, which has a mode-field area greater than 800μm2, versus an area of less than 50μm2 for regular erbium fiber.

USP lasers are typically characterized by the pulse full width at half maximum (FWHM), pulse energy, laser average power, and beam quality. Although necessary, these are not sufficient for characterizing USP lasers for practical applications. Pulse-energy confinement, beam-pointing and pulse-energy consistency, laser duty cycle, and system lifetime are also critical specifications. Our 100μJ system has excellent temporal quality with 701fs FWHM and 90% of the energy confined to a 2.10ps interval (see Figure 2). This laser emits 2.5W average power at 25kHz pulse-repetition rate, with nearly perfect laser-beam quality. The beam-propagation ratio is M2<1.1, compared to M2=1.0 for a perfect (diffraction-limited) Gaussian beam.7 We have also operated the laser at 8W average power and 400kHz repetition rate (20μJ pulse energy) with consistent, excellent beam quality. The higher repetition rate and average power are appealing for a variety of high-speed machining applications. For short- and long-term pulse energy and beam pointing, variation is less than 1%. Our lasers operate 24 hours a day, 7 days a week at full output. In addition, preliminary lifetime testing indicates that they can operate for more than 20,000 hours between service checks.8

Figure 2. Laser-pulse intensity (black) and integrated energy (red) as a function of time when operating at 100μJ per pulse. FWHM: Full width at half maximum. a.u.: Arbitrary units.

Laser processing of high-value materials demands a new level of performance: superior reliability from subpicosecond, high-energy, and high-power USP systems. We demonstrated the first ever systems that enable the potential of USP lasers to create the next generation of devices for advanced medicine, clean-energy technologies, and defense-related capabilities. We have combined rugged fiber optics with sophisticated software control to produce a 100μJ per subpicosecond pulse with a high-average-power and high-reliability platform. Our next step will be to increase the mode-field area of our fiber amplifiers through propagation in higher-order modes, which will support pulse energies close to 1mJ.

Michael Mielke, Adam Tanous 
Raydiance Inc.
Petaluma, CA

Michael Mielke is vice president of engineering.

Adam Tanous is marketing director.