Fiber lasers? Now!

This back-to-basics article offers a review of the technology, components, device architectures, and applications of fiber lasers

Bryce Samson and Andy Held

In the last few years fiber lasers have made significant technological advances, including the introduction of kilowatts class CW devices,1 and are poised to become an important commercial laser technology with annual sales surpassing $100M. Arguably fiber lasers are the fastest-growing laser technology on the market today.

Basic building blocks

One advantage of fiber lasers is the monolithic (so called all-fiber) nature of the device. This refers to the fact that the laser light (and indeed the pump light) never leaves the optical fiber waveguide until the collimation lens at the output of the module. Splicing or fusing together the relevant chain of optical fiber components is the key to this monolithic architecture. In order to achieve a low loss between these spliced fiber components a family of fibers with similar optical parameters, such as diameter and numerical aperture, must be manufactured, each optimized for the relevant “laser” function. Examples are the Yb-doped active fiber (the gain medium), the photosensitive fiber (which is exposed to UV light to make the fiber Bragg grating reflectors that form the laser cavity), various coupler/combiner fibers for pump delivery, beam expanding endcap fibers, and so on. Figure 1 shows a typical fiber laser cavity.

The monolithic architecture clearly reduces the number of air/glass optical interfaces in the laser cavity, in contrast with a traditional Nd:YAG solid-state laser. This leads to improved reliability and less servicing because contamination and failure of the critical intra-cavity optical surface is essentially removed. In addition, fused fiber laser cavities eliminate the need to realign cavity mirrors during the life of the laser, and overall the reduced maintenance associated with fiber laser technology can dramatically reduce the cost of ownership compared with other solid-state lasers.

Furthermore, because the mode quality from the fiber laser is set by the fiber waveguide parameters, which in turn are defined by the glass refractive index profile, beam quality is essentially fixed for the life of the laser-it neither changes over time nor requires a warm up period. This is in contrast with most solid-state lasers where the beam quality is defined by the resonator design and will often change with time, perhaps requiring on-line monitoring and realignment. By optimization of the fiber design, single-mode, near-diffraction-limited beam quality can be delivered from fiber lasers even at powers more than 1kW continuous wave.1 Such beam quality is difficult to obtain from traditional Nd:YAG lasers and even from disc laser technology.

FIGURE 2. Typical high-power fiber laser module with coil of doped fiber and components. Because of the high conversion efficiency of the Yb-doped active fiber, passive cooling of the laser cavity is possible even at powers approaching 100W CW. The pump diodes are not shown.

Importantly, some degree of standardization is beginning to take place in the component supply chain, from the doped active fiber itself to the fiber Bragg gratings, couplers, and pump diodes (see Figure 2). In addition, most manufacturers of splicing machines are now developing and optimizing semi-automatic splice routines specifically for the fibers used in high-power fiber laser cavities, a development that will help many users unfamiliar with handling fibers and reduce the barriers to entry for the technology. This standardization (which occurred in telecom fiber amplifiers more than 10 years ago) will encourage yet more component suppliers. The increased competition will help to drive down the cost of the technology and further increase the availability of fiber laser technology. In the long run, it is this widespread adoption of the fiber technology that will enable competition to the fiber laser companies that are currently pursuing the approach of complete vertical integration.

Pump diode options

The beauty of the fiber laser is the highly efficient method by which it dramatically improves the beam quality of the highly multimode pump diode lasers (launched into the cladding) to essentially a single-mode output (from the Yb-doped core) with around 75 percent optical-to-optical conversion efficiency. The refractive index profile of the fiber is such that the pump radiation is coupled into the cladding of the optical fiber, which has a large diameter (typically 130-400 μm) and high NA (typically 0.46). By contrast the Yb-doped fiber core has a low NA and small diameter such that it supports only one transverse mode (see Figure 3). Absorption of the cladding pump light (say at 915 nm) by the rare-earth-doped core takes place over tens of meters of fiber length and provides gain in the 1060-1100nm region. Optical feedback provided by the pair of fiber Bragg gratings (high reflector and output coupler) forms the lasing cavity. The core waveguide determines the output lasing mode quality and can be single mode even at output powers >1kW.1

One popular method of pumping the fiber laser involves the use of telecom-like single-emitter diodes based on a single broad strip diode delivering say 5-7 W coupled into 105μm/0.22NA delivery fiber.2 The advantages of this choice of pump technology are the long MTBF for each individual diode, the flexibility in cooling the whole ensemble of diodes, and the so-called “granularity” allowing fiber lasers in ~3W increments to be readily fabricated simply by adding an additional pump diode, an important detail for manufacturing low-power fiber lasers cost effectively. By contrast pump sources based on diode bars typically come in ~30W building blocks and are more often coupled into larger fibers, typically 400μm/0.22NA. Increased pump power, for say a 100W fiber laser, is obtained by either coupling multiple pump diodes though a fiber-based fused fiber pump combiner (Figure 1) or by using free-space micro-optics in combination with beam shaping techniques to multiplex many bars or stack of bars into one fiber. Some relevant options for making a 100W fiber laser are detailed in the table.

Improvements in packaging of diode bars over the last few years3 now means that 20,000-hr lifetimes and longer are commercially available. The suitability of single emitters or bars for pumping fiber lasers currently depends on the application (the power requirement) but in most cases will be eventually settled by the price of the pump source, dollars/watt. In terms of packaging and manufacturing, dealing with the 40 or so individually pigtailed 5-7W single emitters needed to make a 100W laser is less attractive than three or even seven fiber coupled bars.

FIGURE 4. Fiber-coupled diode bars can be used to remotely pump the fiber laser through the use of industry standard connectorized delivery fibers/cables (green cable) allowing the power supplies and cooling requirements to be removed completely from the proximity of the actual laser cavity. The use of connectorized pump modules facilitates the rapid removal and replacement in case of failure with no realignment of the laser cavity required.

Because of their excellent beam quality, fiber lasers in the 100W and higher regime are finding new applications not possible with traditional solid-state lasers. Micro-machining and printing have been early adopters of the technology5 and at lower powers 10W fiber lasers are finding applications in marking of non-metals and the semiconductor industry. However one of the major markets for fiber lasers at the moment is marking systems using Q-switched fiber lasers with peak powers in the 5-10kW regime.

Pulsed fiber lasers

In the case of pulsed fiber lasers, the configuration is slightly more complicated than that shown in Figure 1, most often formed from so-called MOPA designs (mater oscillator power amplifier). The oscillator may be a Q-switched fiber laser that is subsequently amplified to the 10-20W level typically producing pulses in the 0.5-1mJ pulse energy or a variation on this design replaces the Q-switched oscillator with a seed diode laser and pre-amplifier stages. One advantage of these pulsed fiber lasers is the high flexibility of the temporal characteristics, including repetition rate, pulse duration, and pulse shape without affecting beam quality from the laser.6

Conclusion

Fibers lasers have become a commercially significant laser technology with annual sales in excess of $100M and an expanding application space making use of the key attributes of the technology-high efficiency and low cooling requirements, excellent reliability, good beam quality, and highly flexible system performance. The fiber technology and assorted components for building fiber lasers, which have traditionally been in the hands of a few vertically integrated fiber laser companies, are now becoming standard and reaching a maturity where more suppliers are encouraged to enter the market and help drive down the price of making a fiber laser compared with several years ago. An excellent example is the development of high-brightness diode pumps specially tailored to pumping high-power fiber lasers and the growing number of diode suppliers for this critical technology. This is evidenced by the reduction in dollars/watt of high-brightness pump diodes over the last few years.

Bryce Samson is vice president of development and Andy Held is vice president of sales and markeing for Nufern, E. Granby, CT. Visit www.nufern.com.

References

1. From www.ipg.com.
2. From www.jdsu.com and www.bookham.com.
3. From www.jold.com.
4. From www.limo-microoptic.com.
5. Laser Focus World magazine market data from 2004/2205.
6. T. Lauterborn et al, Proc. SSDLTR 2005, paper Fiber-8.