Optical Fibers for Industrial Laser Applications

Chapter 22

Optical Fibers for Industrial Laser Applications Adrian Carter, Kanishka Tankala, and Bryce Samson Nufern, East Granby, Connecticut

22.1 FIBER LASERS AND AMPLIFIERS: AN INTRODUCTION Both optical amplifiers and lasers are based on the process of stimulated emission; a concept first proposed by Albert Einstein in 1916 but not demonstrated until 1954 when Charles Townes used stimulated emission to produce microwave oscillation in the "maser" (microwave amplification by stimulation emission of radiation). It was another 6 years later before Theodore Maiman demonstrated the first laser (light amplification by stimulated emission of radiation). In these devices, an "optically active" species has one of its electrons "excited" into a higher energy level, a passing photon of energy equal to the difference between the electron's energy and a lower energy state "stimulates" the electron to fall into the lower energy state and thereby emit a second photon. This photon will necessarily have the same energy, optical frequency, and phase as the original photon. The waves associated with these two photons constructively interfere and the result is a more intense "amplified" optical beam. If the electrons are reexcited into the higher energy level and feedback of the amplified signal is provided via a resonance cavity, then the "amplifier" may become a "laser." The lanthanide-doped glass fiber laser was invented in the mid 1960s [1-3], making it almost as old as the laser itself However, the complications inherent to their early design have until now restricted their real-world applications. Moreover, fiber lasers remained for many years significantly inferior to their Nd/YAG and gas laser alternative technologies, rendering them little more than a scientific curiosity with but a few minor niche applications. 671

672

Optical Fibers for Industrial Laser Applications

22.2 CLADDING PUMPED FIBERS Early fiber lasers were side-pumped with a flashlamp, but in 1974, Julian Stone and Charles Burrus [4] took the technology a significant step forward when they demonstrated a neodymium-doped multimode fiber laser that was end-pumped with a laser diode. However, at that time the only available technique for achieving an acceptable optical quality of the laser output was to employ a fiber with a geometrically small core (on the order of a few microns). The need to couple excitation energy directly into this small core meant that the total achievable output power of these devices was limited to the milliWatt range. With the advent of cladding pump fiber designs in 1988 [5], the limitation to power scaling fiber devices became the availability of high brightness pump radiation rather than the fiber itself This trend continued using the available fiber technology, culminating in 1999 with the demonstration of the world's first single-mode fiber laser exhibiting a continuous-wave (CW) output power in excess of 100 W [6]. Traditional optical fiber has a core refractive index raised respective to the surrounding cladding material. The coating has a significantly higher refractive index than either the core or the clad and from an optical perspective is designed to strip out higher order cladding modes that might otherwise re-couple with the core modes. Double-clad fibers (DCFs) differ from traditional "single-clad" optical fiber in the fundamental design of a secondary external wave-guiding structure surrounding the inner core waveguide. For some appHcations, it is potentially advantageous to be able to splice the fiber directly with the all-glass pump delivery fiber, and in such cases, a triple-clad fiber incorporating a third all-glass 0.23-numerical aperture (NA) cladding between the inner cladding and the polymer coating may be used. The differences in design are shown in Fig. 22.1. By negating the requirement for excitation energy to be coupled directly into the relatively small single-mode core, DCF makes it possible to employ low-cost, large-area (multimode), high-power semiconductor pump sources. The fundamental concept of the DCF structure is that low-brightness, high-power multimode diodes yielding tens and even hundreds of Watts can be used to provide pump power for lanthanide-doped fibers that will convert that energy into highbrightness, high-power, potentially single-mode output. Fibers for high-power laser and amplifier applications require large claddings with high NAs for efficiently coupling pump energy. Such fibers are typically available with cladding diameters up to around 1 mm but more commonly around 400 fim. A fluorinated polymer optical cladding typically provides an NA of around 0.46 and that is in turn often surrounded by a more standard telecommunications type of jacket (for abrasion resistance). The choice of cladding diameter is

Large-Mode-Area Ytterbium-Doped Fibers: The Power Revolution

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dictated primarily by the brightness of available pump diodes and the total power being coupled. The geometry of the inner cladding is typically shaped to prevent the propagation of skew rays that might otherwise pass down the fiber length without traveling through and being absorbed by the doped core region.

22.3 LARGE-MODE-AREA YTTERBIUM-DOPED FIBERS: THE POWER REVOLUTION For certain applications, such as ranging and free-space communications, operating in the "eye-safe" 1.5-2.0 /mm range is preferred. Furthermore, there are a number of sensing and medical applications that require other specific wavelengths. For such applications, it becomes necessary to employ various

Optical Fibers for Industrial Laser Applications

674

Figure 22.2 Cross-section of a 20-^im core, 400-/Am inner-clad PANDA-type ytterbium-doped fiber.

optically active lanthanide ions, such as neodymium, thulium, or co-doped erbium/ytterbium. However, for non-wavelength-specific applications requiring only extremely high-output powers, a number of unique advantages have made ytterbium the dopant of choice. More specifically, ytterbium-doped fibers offer high output powers tunable over a broad range of wavelengths, from around 975 to 1120 nm (typically -1060 nm) [7]. Ytterbium also has a relatively small quantum defect—that is to say, because the pump wavelength (typically 915-975 nm) is close to the lasing wavelength, very little energy is lost to heating. Furthermore, unlike other lanthanide ions, ytterbium has only a single excited state and thereby is not subject to complications arising from excited state absorption (ESA) and is relatively immune to self-quenching processes. Consequently, high concentrations of ytterbium ions

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Figure 22.3 Absorption and emission cross-sections for an ytterbium-doped double-clad fiber.

Large-Mode-Area Ytterbium-Doped Fibers: The Power Revolution

675

can be incorporated while maintaining excellent conversion efficiencies (typically >75%). For this reason, the industry has focused on the development of ytterbium-doped fibers and the following discussion deals primarily with these fiber designs. It should be noted, however, that neodymium/ytterbium-co-doped fibers have demonstrated power scaling advantages by virtue of the fact that they increase the options for wavelength multiplexing the pump diodes (the neodymium has a peak absorption ~810 nm and gain peak ~1060 nm) [8]. One of the most significant keys to ensuring broad marketability of the fiber laser is to develop a technique for producing ever increasing output powers without sacrificing beam quality. Naturally it is possible to ensure diffractionlimited beam quality from a single-mode core in a DCF geometry. Unfortunately, such a design also limits the total achievable output power and in pulsed laser devices the average power, peak power, and pulse energy. These limitations are the result of low energy storage (for pulsed applications, the energy storage capacity is determined by a combination of the number of active species present and the maximum achievable population inversion, which is in turn determined by the likelihood of amplified spontaneous emission [ASE] [9]) and the effects of parasitic nonlinear processes. More specifically conventional small-core, high NA fiber designs limit the maximum achievable output power because of their fundamental susceptibility to optical nonlinearities, including stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and self-phase modulation. To overcome the limitations imposed by these parasitic nonlinear processes, it has been necessary to develop fibers with high lanthanide concentrations in relatively large-core, low-NA fibers. By increasing the core diameter of a fiber and reducing the core NA, it is possible to maintain single-mode operation while both reducing the fraction of spontaneous emission captured by the core and decreasing the power density in the fiber, thereby increasing the threshold power for the nonlinear processes. Furthermore, the total number of active ions present and so the energy storage capacity increases as the square of the core diameter (for a given glass dopant concentration and cladding diameter). Consequently, it is possible to reduce the length of the fiber device thereby further increasing the threshold for the nonlinear processes. Of course, there is an upper limit to the core diameter beyond which singlemode operation is not guaranteed. More specifically for a step-index fiber, it is known that single-mode operation requires that the V-value remains below 2.405, where V is proportional to the core diameter (
core

{22 A)

At very low NAs (approximately <0.06), fibers begin to exhibit extremely high bend sensitivity. This imposes a practical lower limit on NA and, hence, an

676

Optical Fibers for Industrial Laser Applications

upper limit on core diameter. Fortunately, however, there are a number of techniques for the suppression of higher order lasing modes that allow us to use even larger core diameters, wherein essentially multimoded fibers can be made to operate with a diffraction-limited beam quality. These techniques include suitably manipulating the fiber index and dopant profiles [11, 12]; using special cavity configurations [13]; tapering the fiber ends [14]; adjusting the seed launch conditions [15]; and coiling the fiber to induce substantial bend loss for all transverse modes other than the fundamental [16]. Perhaps the simplest and least expensive of these is the coiling technique, which does not require careful matching of the seed mode and does not rely on complex fiber designs. It is only necessary to choose the radius of curvature (based on core diameter and NA) that will discriminate against high-order modes. This technique exploits the fact that the fundamental mode is the least sensitive to bend loss and that the attenuation due to bend loss is exponentially dependant on the bend radius. For example. Fig. 22.4 shows the bend loss as a function of bend radius for a 0.06 NA, 30-/im core diameter fiber. Such a fiber in a linear configuration can support around five modes, but with the appropriate choice of bend radius (say, ~50 mm), the LPll experiences around 50 dB/m of attenuation (and higher order modes are even more severely attenuated) while the LPOl mode experiences only around 0.01 dB/m. It is important to note that this technique does not involve the stripping of power from higher order modes, but the suppression of those modes along the entire fiber length. As such, power is not attenuated and the efficiency of the laser device is not markedly reduced. In Fig. 22.5, we show the measured near-field spatial profile of an ytterbiumdoped fiber amplifier with a core diameter of 25 /xm and an NA of 0.1 when

40

50

60

70

Bend Radius (mm)

Figure 22.4 Bend loss as a function of bend radius for a 0.06 numerical aperture, 30-/Am core diameter fiber and coil forms commonly used to induce the chosen bend diameter in large-mode area fiber.

Large-Mode-Area Ytterbium-Doped Fibers: The Power Revolution

Uncoiled

677

Coiled

Figure 22.5 Measured near-field spatial profile of an ytterbium-doped large-mode area doubleclad fiber amplifier in an uncoiled (left) and coiled configuration (right) (used, with permission, from reference [17]).

seeded with a CW laser at 1064 nm. The profile on the left shows the multimoded {^21 guided modes) output of the uncoiled fiber and on the right the diffractionlimited (measured M^ value of 1.08 + 0.03) output of the coiled fiber [17]. These so-called large-mode-area (LMA) fibers are directly responsible for the explosion in demonstrated diffraction-limited beam-quality output powers, now exceeding the JciloWatt level from a single fiber [18-20] (Fig. 22.6). With the advent of this new class of fibers, the power limitations were once again placed on the pump source rather than the fiber. The advantages of lanthanide-doped LMA fibers are realized by understanding the limiting mechanisms of output power for a typical laser or amplifier. One such mechanism is ASE, which extracts energy from the fiber in an incoherent

Optical Fibers for Industrial Laser Applications

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Figure 22.6 Since they were introduced, large-mode area (LMA) fibers have enabled a power scaling revolution and now produce near diffraction-limited beam quality at powers exceeding 1 kW and slope efficiencies of around 75%.

manner. As described earlier, LMA fibers have cores with low NAs, typically smaller than single-mode telecom fibers. This reduction in core NA reduces the amount of fluorescence captured by the core and, thus, the reduction of amplification of that fluorescence. A second mechanism, nonlinear in nature, is SBS, which results from an acoustic wave formed from the superposition of the propagating Hght wave and the counter-propagating stokes wave generated from the index modulation in the glass created by the propagating wave. The threshold power at which SBS occurs in single-mode fibers is given by Eq. (22.2):

Polarization-Maintaining LMA DCF

'Th

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(22.2)

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where Aeff is the effective area of the fiber, Leff is the effective length of the fiber given by Leff = [l-exp(-aL)]/a, and gB is the Brillouin gain coefficient. The larger core sizes (i.e., effective areas) of LMA fiber raises the SBS threshold, compared to single-mode fibers (by a few orders of magnitude) and enhances the power handling capability of the laser. For example, when pumped at 915 nm a single-mode ytterbium-doped fiber with a 0.15 NA, a core diameter of 5 /mm, and an ytterbium ion concentration of around 1 wt% has an SBS threshold of around 40 W at 3-kHz line width, while a 20-jLim core 0.06-NA fiber has a threshold of around 340 W and a 30-/>tm core 0.06-NA fiber has a threshold of around 680 W [21]. In practice, the Brillouin pump line-width can range from a few kiloHertz to several megaHertz, so the actual thresholds may be significantly higher depending on the system configuration. By pumping at 976 nm or using more highly doped fiber, even higher SBS thresholds may be achieved.

22.4 POLARIZATION-MAINTAINING LMA DCF It is not feasible to indefinitely increase the output power capability of an LMA DCF through scaling of the core diameter. Ultimately there will be some upper limit, above which output beam quality will begin to degrade. To help overcome this hurdle, research is also underway to further refine the design of

1610.0

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10 15 Fiber Length (m)

20

Figure 22.7 SBS thresholds versus fiber length for LMA fibers with 20-, 30-, and 50-/>im core diameters and 1 wt% of ytterbium (3-kHz pump spectral line width and 36.5 MHz Brillouin line width) (used, with permission, from reference [21]).

680

Optical Fibers for Industrial Laser Applications

LMA DCFs, through optimization of the glass composition and wave-guiding structure. These include techniques for reducing the peak power density of light propagating in the core, via careful manipulation of the core refractive index profile [22, 23]. Large-flattened-mode (LFM) optical fibers have been designed and fabricated to further increase the threshold for nonlinear processes in LMA fiber by homogenizing the power-intensity profile across the core region (Fig. 22.8). At very high peak powers, optical damage of the fiber and selffocusing are limitations to power scaling. Beam-expanding endcaps reduce surface damage effects and the increased mode-field area (MFA) achieved through LFM fiber designs have enabled greater than 1.5-MW peak powers (at ~l-ns pulse duration) to be achieved. Despite the very large MFA from these fibers, good beam quality (Table 22.1) has been demonstrated in practical systems [24]. Nevertheless, the effectiveness of such techniques is somewhat limited and alternative techniques are required for significant power-scaling requirements. CW output powers exceeding 1 kW have already been demonstrated in multiplexed fiber devices with poor beam quality [25] and near diffractionlimited output powers exceeding 1 kW have also been demonstrated [19, 20]. However, with the growing need for output powers of several kiloWatts for industrial cutting and welding applications and greater than 100 kW (CW) for military and aerospace applications, the current goal of a number of research groups is to achieve diffraction-limited kilo Watt powers from a single fiber and then to combine the outputs of several such devices. A number of such powerscaling techniques have been demonstrated including coherent beam combining, spectral beam combining, and polarization beam combining. For these extremely high-power applications, operation under stable linear polarization is becoming a requirement [26, 27]. Furthermore there are a number of other applications requiring polarization-maintaining (PM) output including coherent

20

30

Figure 22.8 Normalized signal power as a function of radial dimension for a conventional stepindex and large flattened mode fiber.

Polarization-Maintaining LMA DCF

681 Table 22.1

Power amplifier performance of a largeflattenedmode (LFM) fiber with around 1.5-MW peak power [24] Parameter

Achieved

Average power Pulse energy Repetition rate Pulse duration M^ Spectral line width SNR ratio in 0.1 nm

>10W 0.75 mJ @ 10 W 12.16 Kpps @ 10 W <0.5 ns @ 10 W <1.15@10W 25 GHz @ 10 W -27 dB @ 10 W

optical communications, nonlinear frequency conversion, pumping optical parametric devices, and all manner of mode-locked, Q-switched, and narrow linewidth fiber lasers. Consequently, there has been an increasing demand for PM DCFs. In the past, different approaches have been suggested to obtain PM operation using non-PM fibers [27, 28]. Such approaches, however, have their limitations and the preferred technology is to use a truly PM DCF. Although passive PM fibers have been commercially available for many years, actively doped PM fibers have not been available until recently [29, 30]. In fact an amplifier employing Yb-doped PM DCF was first reported by Kliner et al. [30] in 2001. This fiber was of bowtie geometry, and though acceptable for proof of concept and research and development, it has substantial limitations in terms of preform manufacturability, uniformity, and scalabiUty. Furthermore, the nonideal refractive index profile inherent to such doped bowtie fibers (Fig. 22.9) makes diffraction-limited operation difficult to achieve. Figure 22.10a schematically demonstrates the steps involved in making a bowtie type of PM fiber. A high-quality synthetic quartz tube is used as a substrate and several layers of borosilicate glass are first deposited on the inner wall of the rotating substrate. Next the substrate rotation is stopped, and using a specialized ribbon burner, the boron in the glass is volatilized from a selected sector of the deposited layer. The substrate tube is then rotated by 180 degrees and a similar sector is volatilized. Special care has to be taken to ensure that the sectors of glass from which the boron has been volatilized are diametrically opposite to each other and dimensionally equal along the substrate length. Several layers of glass are further deposited before the doped core is deposited. These layers act as a buffer between the borosilicate stress members and the core and ensure that the evanescent field does not propagate in the stress elements to any significant extent. The actively doped core is typically deposited using a solution doping technology. The substrate tube with the various layers of deposited glass is then carefully

Optical Fibers for Industrial Laser Applications

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Figure 22.10 Schematic diagrams illustrating the steps involved in the fabrication of (a) bowtie and (b) PANDA-type PM fibers.

collapsed into a rod. The collapsed preform is further processed to obtain the desired inner cladding geometry and drawn into a fiber. PANDA-type PM DCFs are manufactured in two separate stages, as schematically illustrated in Fig. 22.10b. The actively doped preform is fabricated in a

Polarization-Maintaining LMA DCF

683

separate process and may employ a manufacturing technology more suitable for yielding highly uniform lanthanide and co-dopant distributions. A high-quality synthetic quartz tube is used to deposit the lanthanide-doped glass. The tube is then collapsed into a rod and further processed so when drawn, the fiber will have the desired core and inner cladding dimensions. In a separate step, two circular stress elements of desired composition are fabricated. Two holes of the desired dimension are drilled, either side of the core, in the lanthanide-doped preform. The circular stress members are inserted into the holes and incorporated into the preform. The preform with the stress members is then drawn into a fiber of desired size and geometry. The bowtie technology offers the advantage of fabricating the stress members and the lanthanide-doped core in a one-step process. In addition, the distance of the stress members from the core can be precisely controlled by the number of buffer layers deposited between the stress layers and the core. The stress elements can be brought very close to the core, and hence, for a given size and composition of the stress element, a relatively high birefringence may be achieved. However, this technology has several significant disadvantages. The need to deposit stress elements and a lanthanide-doped core within the same substrate tube limits the ability to independently control the polarization and lasing properties of the fiber. Furthermore, although the stress elements can be brought close to the core, the size of the stress elements that may be deposited is restricted and thereby limits the size of the preform that can be made with a desired birefringence. In other words, the technology does not lend itself to volume production. Finally, most DCFs require a noncircular geometry of the inner cladding, which calls for some processing step such as grinding or thermal processing to obtain a desired geometry. In the case of a bowtie type of preform, the grinding (or thermal processing) operation has to be conducted with the stress members in place. PM preforms are relatively fragile because of the large amount of stress incorporated in the preform and are, therefore, prone to fracture on exposure to mechanical (or thermal) shock during a grinding (or thermal processing) operation. The bowtie preform technology is, therefore, not preferred for making volume production of PM DCF. The technology used to make PANDA-type PM DCF not only offers several advantages but addresses the limitations of the bowtie technology. In this process, both the lanthanide-doped preform and stress member fabrication steps are effectively decoupled, providing independent and highly effective control of the polarization properties and composition of the lanthanide-doped glass. Furthermore, relatively large stress-inducing members may be fabricated, which substantially increases the limit of preform size and makes the process more suitable for preform scale up. Finally, all processing required to achieve a noncircular geometry may be accomplished before incorporating the stress members and, hence, improving production yields. The PANDA-type PM

684

Optical Fibers for Industrial Laser Applications

technology is, therefore, amenable to fabricating PM DCF and is the technology of choice for reproducible and uniform volume production. The PM ability of all PM fibers relies on residual stress anisotropy across the core, which in turn arises from differences in thermal expansion coefficient between the stress members, core and cladding. The composition, location, and geometry of the stress members determine the birefringence in the fiber. In PM DCFs, the core and cladding geometries are very different to standard telecommunications type of PM fibers; more specifically in LMA DCFs, the large diameter of the core negatively affects the achievable birefringence. Before the feasibihty of PANDA-type PM-LMA DCFs could first be demonstrated, considerable research had to be performed to optimize the compositional and the geometrical design of the stress members, and in 2003, the results of such detailed experimental and theoretical analyses were reported [31, 32]. Figure 22.11 shows the key dimensional parameters that determine the birefringence that can be obtained in a PM DCF. These include the size of the stress member (ds) and the position of the stress member (dp) relative to the inner cladding diameter (df) and the core diameter (dc). In addition to the geometric factors, the composition of the stress rod determines the birefringence that is achieved in the fiber. Figure 22.12 shows the effect of stress rod size and location on the birefringence (and beat length) of the fiber. As can be seen, the birefringence may be increased (or the beat length reduced) by increasing the size of the stress members (ds) and keeping all other parameters constant. Similarly, the birefringence may be increased by moving the stress rods closer to the core. Although it is theoretically possible to use these two geometric parameters to achieve very large values of birefringence, a limiting criterion imposed on ds and dp is the distance of the stress members from the core. This limiting distance is

Stress member

Outer Cladding (Polymer) Inner Cladding (Glass)

Core

Figure 22.11 Geometric considerations in a PM double-clad fiber (used, with permission, from reference [31]).

Polarization-Maintaining LMA DCF

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0.95

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1.15

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indicated by the distance between the inside edges of the stress members (df). If 4 becomes very small, the probability of overlap between the mode field and the stress members increases, resulting in increased attenuation and bend loss of the laser or amplifier signal wavelength. To provide a safety margin for avoiding any overlap between the modal power profile in the fiber and the stress members, it is necessary to determine a critical ratio dt/MFD so that losses are minimized. For small-core single-mode fibers used in low to medium power applications, it is possible to achieve sufficient birefringence using standard stress member compositions and operate well within the limiting ratio. However, for large-core fibers as needed for high-power applications, achieving sufficient birefringence

686

Optical Fibers for Industrial Laser Applications

while operating within the Hmiting ratio is more challenging. In such cases, a higher coefficient of thermal expansion difference and, hence, higher birefringence can be achieved by adjusting the composition of stress members so they are similar to those used for gyroscope fibers. Indeed a broad range of ytterbiumdoped LMA DCFs, whose characteristics are optimized for various output powers, are now commercially available [33]. An optical image showing the cross-section of such a fiber, with a lO-fxtn core and 400-/>im inner-cladding diameter and a birefringence exceeding 3.5 x 10"^, is presented in Fig. 22.2.

22.5 FIBER LASERS: STATE OF THE ART LMA fibers with core diameters of 20-30 /xm and NAs of around 0.06 have become the industry standard for high-power laser and amplifier devices because of their ability to deliver good beam quality through preferential modal excitation [15] or coiling induced higher order mode losses [16]. The addition of PANDA-type stress elements to make PM-LMA fibers has added to the application space for the fiber technology and enabled high-power linearly polarized fiber amplifiers both in the CW [34] and pulsed regimens [35]. The availability of fibers with large claddings (400 ^tm) and high cladding NAs (0.46) in conjunction with high brightness pump sources has featured in many of the high-power results. More particularly, they have facilitated the amplification of single-frequency sources into the high-power regimen (hundreds of Watts [36]) and as such are potential building blocks for coherent beam-combining [37] and fiber array phase-locking [38] experiments. An indicator of maturity in the LMA fiber technology is the availability of standard support components with LMA-compatible fiber pigtails, including the multimode pump combiners, which also serve as signal multiplexers. These components are available with input fibers compatible with industry-standard pigtail fibers on commercial high-power diodes. For example, the (6+ l)-to-l design consisting of LMA-compatible 20/400 DCF on the output of the combiner with six 200/220 0.22-NA pump delivery fibers on the input side are commercially available. Furthermore high-brightness, fiber-coupled pump diodes compatible with pump combiners are now commercially available with industrial-grade reliability. These advancements in high-brightness pumps and highpower pump combiners have enabled the high-power, monolithic design shown in Fig. 22.13. Experimental results for the system are presented in Fig. 22.14 and demonstrate the applicability of these high-power LMA monolithic amplifiers to output powers greater than 200 W CW. Although the power level is well below that demonstrated with broad line-width fiber lasers and amplifiers [18-20], these LMA devices are applicable to amplifying single-frequency input signals with

Fiber Lasers: State of the Art

687

--15171 of LMA-YDF-20/400 in heat-dissipating coil

(6 + 1)to1 Combiner Splice to • input signal

High-power end-cap assembly

Fiber coupled 975 nm 50 W pump diodes

Figure 22.13 Schematic of the monoUthic 200-W large-mode area amplifier.

coherence lengths suitable for further beam combining into the multi-kiloWatt regimen [36]. PM versions of these LMA fibers have also been demonstrated to exhibit excellent slope efficiencies and operate at high powers, greater than 400 W pump power limited [34]. Indeed, Nufern scientists have combined the PM-LMA fiber concept with an optimized coil form to deliver a unique linearly polarized fiber laser, as shown schematically in Fig. 22.15. Alternative methods for delivering polarized fiber lasers inevitably include external or free space components, or at the very least extra polarizing components within the cavity. By simply optimizing the fiber and coil combination, it is possible to make a high-power polarized fiber laser by taking advantage of the difference in bend-induced attenuation for each of the two polarization states. The excellent polarization/extinction ratio, greater than 95%, was obtained with diffraction-limited beam quahty, and a grating stabilized line width of around 0.1 nm. Importantly, the output power

Amplified Signal at 1064 nm 250

. ^ ^

g^200 75% Slope Efficiency

I 150 O

^

o.

1^0 0

50

100 150 200 250 Coupled Pump Power (W)

300

Figure 22.14 Amplified signal power as a function of coupled pump power (left) from the 19-in. rack mounted large mode area monolithic amplifier (right).

688

Optical Fibers for Industrial Laser Applications £ c §

1.E+03 1.E+01

SS 1.E-03 Punnip diodes

PZ-fiber coil Fiber Bragg grating § ^ 1 .E-05 -| ^g^ based cavity ^ 1.E-07 g 1.E-09 DO 50 100 150 200 Coil Diameter (mm)

Figure 22.15 Nufern's proprietary technique for making a linearly polarized fiber laser using the coiling induced polarizing effect removes the need for external polarizing elements or extra elements in the cavity.

was limited only by the available pump power, and in fact it is anticipated that the maximum CW output power for this fiber design will be around 2 kW [34]. LMA fibers have also had a dramatic impact on pulsed fiber laser technology where pulse energies are now approaching 100 mJ with multimode beam quality and around 4 mJ single mode [39]. High peak powers, in combination with the short pulse durations (nanoseconds) and diffraction-limited beam quality, are desired in many applications including marking, micromachining, and drilling anything from fuel injectors through to turbine blades. Mihtary applications involve target designation and laser radar, which is also a growing commercial market with applications such as vehicle guidance, robotics/vision systems, metrological, and surveying. The combination of high average power (over hundreds of Watts), excellent beam quahty, and a polarized output has caught the attention of the DPSSL community. Such performance specifications are extremely difficult to achieve in either CW or pulsed YAGA^anadate lasers where fundamental problems such as thermal lensing push resonator design to the limits. Furthermore, the flexibihty of monolithic components make concatenating amplifier stages relatively easy in fiber devices, opening the potential for highly flexible devices generating pulse durations from sub-nanosecond to CW and pulse energies in the milliJoule range.

22.6 LARGE-MODE-AREA EYE-SAFE FIBERS As discussed earlier, new developments in LMA fibers have led to demonstrations of kiloWatt-level outputs in CW lasers and megaWatt-level peak powers in pulsed amplifiers (with sub-nanosecond pulses). However, development of LMA fibers has largely been restricted to ytterbium-based fibers for use at around 1.0 /xm, because of the relative ease of manufacturing LMA ytterbium-doped

Large-Mode-Area Eye-Safe Fibers

689

fibers and high optical efficiencies (^80%), associated with this laser system. The ability to achieve relatively high concentrations of ytterbium with relatively low levels of matrix-modifying co-dopants lends itself to fabricating low-NA, largecore fibers. The low-NA core supports only a few modes and the higher order modes can be easily discriminated against by preferential seeding [15] or bending [16] to achieve diffraction-limited operation. In spite of the numerous advantages, a significant drawback of the ytterbium-based system is the relatively high sensitivity of the human eye to wavelengths in the 1.0-jiim region. The retinal absorption of radiation in the 1.5- and 2-fim wavelength regions is substantially lower than that at 1.0 fim. High-power lasers and amplifiers operating in these wavelength bands are, therefore, of interest in both military and commercial applications such as free-space and satellite optical communications and LIDAR. In military applications, these wavelengths are also of interest because they minimize collateral damage. Similarly, eye-safe lasers are welcome in commercial application because they greatly reduce the challenges associated with laser safety. In addition to eye safety, the 2.0-/xm lasers may also find applications as pump source for holmium-doped lasers or nonlinear conversion to longer wavelengths. Although there has been significant interest in eye-safe lasers, progress in developing high-power CW and pulsed lasers at these wavelengths has been limited by the availability of LMA erbium/ytterbium- and thulium-doped fibers. This limitation is itself due to the difficulties in manufacturing optically efficient low-NA optical fibers containing these lanthanide dopants. Despite the lack of LMA fibers, important work has been conducted in developing high-power erbium/ytterbium lasers using multimode fibers. Koroshetz et al. [40] demonstrated a 40 W, 10-Gbps amplifier for free-space communication and Shen et al. [41] reported 188 W of CW output with an M^ of 1.9 using a 30-/xm, 0.22-NA erbium/ytterbium-co-doped fiber. Furthermore, Yusim et al. [42] have been able to take a lO-jnm core erbium/ytterbium fiber, which supports 30 modes, and achieve 100 W output with a near diffractionlimited beam by employing single-mode fiber-based components in the cavity and making careful splices between the single-mode fibers and the multimode active fiber. Although a high-power diffraction-limited output was achieved, such methods are cumbersome and emphasize the need for LMA fibers. Similarly, noteworthy work has been conducted in developing highly efficient thulium-doped fibers [43, 44] by exploiting the two-for-one cross-relaxation process between thulium ions and output powers as high as 85 W have been demonstrated albeit with a multimode output [44]. Unlike ytterbium-doped fibers, the compositional requirements for erbium/ ytterbium-co-doped and thulium-doped fibers make fabrication of LMA variants particularly challenging. More specifically, it is well known that sensitizing erbium-doped fibers with ytterbium enhances pump absorption and hence increases the efficiency at the erbium lasing wavelength. Erbium has a very narrow

Optical Fibers for Industrial Laser Applications

690

absorption peak and, like any lanthanide ion, cannot be incorporated into silica glass at extremely high concentrations without clustering. Sensitization is accomplished by taking advantage of the broad absorption band and the high crosssection of ytterbium as compared with erbium [45], with the net result that erbium/ ytterbium-co-doped fibers have a very broad absorption band with a peak absorption that is more than two orders of magnitude greater than conventional erbium-doped fibers [46]. For efficient energy transfer from the ytterbium to erbium ions, the Raman shift of the base glass is increased by doping it with phosphorus. The presence of P = O bonds increases the phonon energy of the glass host, and the Raman spectrum has a peak at 1330cm"^ as compared to 1190 cm~^ for pure silica [47]. This helps in rapid depopulation of the erbium ^Iii/2, energy level, limiting the back-transfer of energy from erbium to ytterbium ions (Fig. 22.16). Thus, efficient erbium/ytterbium fibers contain phosphorus in the core, which also aids in minimizing the clustering of the lanthanide ions [48]. A substantially high level of phosphorus is required to achieve both of the

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Figure 22.16 Energy transfer processes in erbium/ytterbium- (left) and thulium-doped fibers (right).

Large-Mode-Area Eye-Safe Fibers

691

aforementioned goals. However, phosphorus also increases the refractive index of the base glass, resulting in relatively high core NAs of around 0.17-0.20 or higher. Thus, the difficulty in producing an LMA fiber, which requires a low NA, becomes apparent. Thulium-doped fibers have a number of potential pump bands (Fig. 22.17). However, pumping at 793 nm is preferred because of the availability of highpower semiconductor diodes in the 800-nm spectral region. In conventional thulium-doped fibers, the maximum conversion efficiency of a 793-nm pumped laser is about 40%. However, compositionally engineered thulium-doped silica fibers can achieve substantially higher efficiencies (approaching 80%). Important energy transfer processes relevant to the performance of thulium-doped silica fibers have been identified [44, 49, 50]. Figure 22.16 shows the relevant crossrelaxation mechanism observed in thulium-doped silica fibers. Two cross-relaxation processes, namely ^H4, ^He -^^ F4, ^F4 and ^H4, ^H6 -^^ H5, ^F4, have been identified with the ^H4, ^He ^ ^ F4, ^F4 being particularly efficient because of the large degree of spectral overlap between the ^H4, -^^ F4 emission and the ^H6 ^ ^ F4 absorption. This cross-relaxation process results in the generation of two signal photons for every pump (793-nm) photon and may be promoted by using high thulium concentrations. However, energy transfer up-conversion processes, namely ^F4, ^F4 -^^ H5, ^H6 and •^F4, ^F4 -^^ H4, ^H6, have to be kept in check to prevent quenching of the ^F4 energy multiple. This can be minimized by using very high alumina:thulium concentrations and thereby

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692

Optical Fibers for Industrial Laser Applications

preventing clustering of the thulium ions [48]. Therefore, both high thulium and high alumina concentrations are required to achieve greater than 100% quantum efficiencies in thulium-doped fibers. In fact, careful compositional engineering can yield power conversion efficiencies similar to that of ytterbium-doped fibers, making such fibers extremely desirable for eye-safe laser system. However, these high dopant concentrations substantially increase the refractive index of the core as compared with the pure silica substrate, and typical NAs are in the range 0.18-0.24. Hence, as in the case of erbium/ytterbium-co-doped fibers, the compositional requirements for an efficient thulium-doped fiber also limits the ability to manufacture low NA LMA fibers. In a typical step-index fiber, the NA of the core is defined by the index of the core and that of the cladding surrounding it. However, if an appropriate pedestal is designed, the core may have an effective index as defined by the index of the core and that of the pedestal, as shown in Fig. 22.18. The pedestal index may be chosen so as to define an effective core NA of less than 0.1. Similarly the pedestal size may be chosen so that further increasing the pedestal diameter has no significant effect on the core modes, particularly the fundamental mode. At this diameter, the pedestal behaves as a "true" cladding to the core, rather than an extended core feature. If the pedestal is made any smaller, the modefield diameter of the core fundamental mode starts decreasing, and core light may be coupled to the pedestal modes. At the same time, the pedestal should not be made much larger than required, because of increased manufacturing cost, as well as the possibility of trapping pump light in the helical modes in the pedestal. The first demonstration of highly efficient large core (25 fim) LMA ytterbium/erbium-co-doped and thulium-doped DCFs was recently reported [51]. In both cases, the core NAs were chosen to be around 0.1 and the fibers were drawn to 300 and 250 jmrn cladding diameters, respectively, with a cladding NA of 0.46. Figure 22.19 shows the absorption and about 30% slope efficiency for the erbium/ytterbium 25/300 pedestal fiber. The key benefit of the pedestal fiber design is clearly the lower number of modes supported by the doped core of the fiber. Consequently, it should be easier to excite and maintain the fundamental mode in this fiber than in a fiber with a similar diameter high NA (~0.17) core. The V-number of a IS-jnm core fiber at an NA of 0.17 is 8.616, while at an NA of 0.10, it is 5.067. Figure 22.20 shows that incorporation of the pedestal is expected to reduce the core modes from 11 to 4, making it possible to achieve near diffraction-limited beam quality. Importantly, both exciting the fundamental mode (at the expense of higher order modes) and maintaining the power within this mode through the appropriate fiber length should be significantly easier in the few-moded fiber as compared with the highly multimode fiber, because of reduced mode coupling. Experimentally, good beam quality from the pedestal fiber has been verified in a seeded amplifier configuration, in which care was taken to excite the

693

Large-Mode-Area Eye-Safe Fibers

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Figure 22.18 Schematic diagram of a large mode area fiber using a pedestal design (top) and modeling data for the determination of minimum pedestal diameter (bottom).

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Figure 22.19 Absorption spectra (left) and slope efficiency for the large mode area EYDF-25/ 300 (right).

694

Optical Fibers for Industrial Laser Applications

Figure 22.20 The influence of numerical aperture (NA) on the number of transverse modes supported by a 25-/im core fiber.

fundamental mode of the doped core using standard mode matching techniques. In fact it was found that beam quality was easier to maintain in this fiber than in a fiber with a high 0.17-NA core, even though the core was substantially smaller (18 fxm). Single-mode beam profiles and an example M^ measurement are shown in Fig. 22.21 for a fiber length appropriate for making efficient ampHfiers. Although good beam quality has been demonstrated in laboratory

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based experiments with high NA large core fibers [42], developing components and designing systems to guarantee a beam quality specification would be particularly challenging. It is believed that the adoption of a pedestal fiber design will make the challenges associated with delivering good quality beam profiles at eye-safe wavelengths manageable. Compositional development of thulium-doped fiber has also been performed to optimize the thulium and alumina concentrations required to promote the two-for-one cross-relaxation process [51], and slope efficiencies as high as 68% (170% quantum efficiency) have been reported [52] (Fig. 22.22). An index profile of the LIMA thulium-doped fiber (LlVlA-TDF-25/250) showing the refractive indices of the core and the pedestal relative to the cladding is also shown in Fig. 22.22. The absorption of the fiber at 790 nm was measured to be around 4.5 dB/m and the absorption spectrum is shown in Fig. 22.17. The fiber was demonstrated to have a near-diffraction-limited beam quality and an M^ of less than 1.3 was reported [52].

22.7 CONCLUSIONS Today both CO2 and YAG lasers are routinely capable of producing CW output powers in the range of sub-Watt to multi-kiloWatt and as such have become the mainstay of the laser cutting and welding industry. Despite numerous advancements in their design, these lasers are still typified by poor wall-plug efficiencies (typically 1-10%) and/or relatively poor optical beam qualities. On the other hand, developments in laser diode technology, fiber design, and beam-combining techniques have meant that cladding pumped ytterbium-doped fiber lasers have attracted growing interest as a route to highly efficient (20-40%

696

Optical Fibers for Industrial Laser Applications

wall-plug efficiencies), high output power, high beam quality (near-diffractionlimited) lasers for a vast array of material processing applications. More specifically, fiber lasers have a number of distinct advantages over their more conventional alternatives including size, reliability, wavelength selectivity, heat dissipation, wall-plug efficiency, and operational cost. Furthermore, they may be operated without the need for active cooling or optical alignment. LMA fiber technology has become an established means of power scaling fiber lasers and amplifiers, and as the basic fiber design has become standardized, this has encouraged component manufacturers to design and build highpower LMA-compatible devices. Consequently, the ytterbium-doped fiber laser is now challenging more traditional bulk solid-state and gas laser systems in a range of both industrial and military, sensing and material processing applications. Similarly, the recent evolution of LMA erbium/ytterbium and LMA thulium-doped fibers may be expected to spur the development of high-power lasers and amplifiers in the eye-safe regions of 1.5 and 2.0 /xm, respectively.

REFERENCES [1] Snitzer, E. 1961. Optical maser action of Nd3+ in a barium crown glass. Phys. Rev. Lett. 7:444-446. [2] Snitzer, E. 1963. Neodymium glass laser. In: Proc. 3rd Int. Conference of Quantum Electronics, pp. 999-1019, Paris, 1963.{Au: Pis provide full book title and the publisher name and place where published.} [3] Koester, C. J. and E. Snitzer. 1964. Amplification in a fiber laser. Applied Opt. 3:1182-1186. [4] Stone, J. and C. Burrus. 1974. Neodymium-doped fiber lasers: Room temperature CW operation with an injection laser pump. IEEE J. Quant. Electr. 10:794. [5] Snitzer, E. et al. 1988. Double-clad offset core Nd fiber laser. In: Conference Proc. of Optical Fiber Sensors, PD5, New Orleans. [6] Dominic, V. et al. 1999. 110 W fiber laser. In: Conference Proceedings of OS A—Optical Soc. of Am. CLEO, CPD-11, Washington, DC, 1999. [7] Paschotta, R. et al. 1997. Ytterbium doped fiber amplifiers. IEEE J. Quant. Electr. 33(7): 1049-1056. [8] Limpert, J. et al. 2003. 500 W continuous-wave fiber laser with excellent beam quality. Electr. Lett. 39(8):645. [9] Nilsson, J. and B. Jaskorzynska. 1993. Modeling and optimization of low-repetition-rate high-energy pulse amplification in CW-pumped erbium-doped fiber amplifiers. Opt. Lett. 18(24):2099. [10] Gloge, D. 1971. Weakly guiding fibers. Appl. Opt. 10:2252-2258. [11] Offerhaus, H. L. et al. 1998. High energy single-transverse-mode Q-switched fiber laser based on a multimode large-mode area erbium-doped fiber. Opt. Lett. 23(21): 1683-1685. [12] Nilsson, J. et al. 1997. Ytterbium^"^-ring-doped fiber for high-energy pulse amplification. Opt. Lett. 22(14): 1092. [13] Griebner, U. et al. 1996. Efficient laser operation with nearly diffraction limited output from a diode-pumped heavily Nd-doped multi-mode fiber. Opt. Lett. 21:266.

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[14] Renaud, C. C. et al. 1999. Compact high-energy Q-switched cladding-pumped fiber laser with a tuning range over 40nm. IEEE Photon. Technol Lett. ll(8):976-978. [15] Fermann, M. E. 1998. Single-mode excitation of multimode fibers with ultrashort pulses. Opt. Lett. 23:52-54. [16] Koplow, J. P. et al. 2000. Single-mode operation of a coiled multimode fiber amplifier. Opt. Lett. 25:442. [17] Kliner, D. et al. 2004. Fiber laser technology reels in high power results. OE Magazine 4(1): 32-35. [18] Liu, C.-H. et al. 2004. 810 W continuous-wave and single-transverse-mode fiber laser using 20 mm core Yb-doped double-clad fiber. Electr. Lett. 40:1471-1472. [19] Jeong, Y. et al. 2004. Ytterbium doped large-core fiber laser with 1.36kW continuous-wave output power. Opt. Exp. 25:6088-6092. [20] Gapontsev, V. P. 2005. Recent progress in beam quality improvement of multi-kW fiber lasers. Photon. West 5709-11, San Jose, January 2005. See also www.ipgphotonics.com. [21] Tankala, K. et al. 2004. Large mode area double clad fibers for pulsed and CW lasers and ampHfiers. Photonics West, 5335-22, San Jose. [22] Ghatak, A. K. et al. 1998. Design of waveguide refractive index profile to obtain flat modal field. SPIE Proc. 3666:40. [23] Dawson, J. W. et al. 2003. Large flattened mode optical fiber for high output energy pulsed fiber lasers. In: Conference on Lasers and Electro-Optics, CWD5, Washington, DC. [24] Torruellas, W. et al. 2005. High peak power Ytterbium doped fiber amplifiers. In: Tech. Digest Solid State and Diode Laser Technology Review (SSDLTR), Fiber-7, Los Angeles. [25] Ueda, K. et al. 2002. 1 kW CW output from fiber embedded lasers. In: Proceedings Conference on Lasers and Electro-Optics, CPDC4, Long Beach. OSA—Opt. Soc. Am. Washington, DC. [26] Noda, J. et al. 1986. Polarization maintaining fibers and their applications. /. Lightwave rec/?A2o/. 4(8):1071-1089. [27] Koplow, J. P. et al. 2000. Polarization-maintaining, double-clad fiber amplifier employing externally applied stress-induced birefringence. Opt. Lett. 25(6):387-389. [28] Duling, I. N., Ill, and R. D. Esman. 1992. Single-polarisation fiber amplifier. Electr. Lett. 28(12):1126-1128. [29] Tajima, K. 1990. Er3+-doped single-polarisafion optical fibers. Electr. Lett. 26(18): 1498-1499. [30] Kliner, D. A. V. et al. 2001. Polarization-maintaining amplifier employing double-clad bow-tie fiber. Opt. Lett. 26(4): 18^186. [31] Tankala, K. et al. 2003. PM-double clad fibers for high-power lasers and amplifiers. Paper presented at Photonics West, 4974-40, San Jose. [32] Machewirth, D. P. et al. 2003. Polarizadon-maintaining double-clad optical fibers for coherent beam combining. Presented at Solid State Laser Conference, New Mexico, 2003. [33] Available at www.nufern.com. [34] Khitrov, V. et al. 2005. Linearly polarized high power fiber laser with monoHthic PM-LMA fiber and LMA-based grating cavities. Presented at Photonics West, 5709-10, San Jose. [35] Liu, A. et al. 2005. 60-W green output by frequency doubling of a polarized Yb-doped fiber laser. Opt. Lett. 30:67-69. [36] Khitrov, V. et al. 2006. 242W single-mode CW fiber laser operating at 1030nm lasing wavelength and with 0.35nm spectral width. Presented at ASSP, WD5, Lake Tahoe. [37] Wickham, M. et al. 2003. High power fiber array coherent combination demonstration. In: Proceedings 16th SSDLTR, HPFib-5.

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Optical Fibers for Industrial Laser

Applications

[38] Minden, M. et al. 2004. Self organized coherence in fiber laser arrays. Proc. SPIE 533514:89-97. [39] Cheng, M.-Y. et al. 2005. High-energy and high-peak-power nanosecond pulse generation with beam quality control in 200 ^tm core highly multimode Yb-doped fiber amplifiers. Opt. Lett. 30(4):358-360. [40] Koroshetz, J. et al. 2004. High power eye-safe fiber transmitter for free space optical communications. Presented at Advanced Solid-State Photonics, MA2, Santa Fe. [41] Shen, D. Y. et al. 2005. Highly efficient Er,Yb-doped fiber laser with 188W free-running and >100W tunable output power. Opt. Exp. 13(13):4916-4921. [42] Yusim, A. et al. 2005. 100 Watt single-mode CW linearly polarized all-fiber format 1.56-/xm laser with suppression of parasitic lasing effects. Proceedings of SPIE Vol. 5709:69-77, 2005. [43] Jackson, S. D. and S. Mossman. 2003. Efficiency dependence on the Tm^"^ and Al^"^ concentrations for Tm^^-doped silica double-clad fiber lasers. Appl. Opt. 42(15):2702-2707. [44] Frith, G. et al. 2005. 85W Tm^+-doped silica fiber laser. Electr. Lett. 41(12): 1207-1208. [45] Valley, G. C. 2001. Modeling cladding-pumped Er/Yb fiber amplifiers. Opt. Fiber Technol. 7:21-44. [46] Zhou, X. and H. Toratani. 1995. Evaluation of spectroscopic properties of Yb^^-doped glasses. Phys. Rev. B 52(22): 15889-15897. [47] Grubb, S. G. et al. 1991. High-power sensitised erbium optical fibre amplifier. Proc. OFC PD7:31-33. [48] Arai, K. et al. 1986. Aluminium or phosphorus co-doping effects on the fluorescence and structural properties of neodymium-doped silica glass. /. Appl. Phys. 59:3430-3436. [49] Jackson, S. D. 2004. Cross relaxation and energy transfer upconversion processes relevant to the functioning of 2 /im Tm^^-doped silica fibre lasers. Opt. Commun. 230:197-203. [50] Shen, D. Y. et al. 2005. High-power and ultra-efficient operation of a Tm^^-doped silica fiber laser. Presented at Advanced Solid State Photonics, MC-6. [51] Tankala, K. et al. 2006. New developments in high power eye-safe LMA fibers. Presented at Photonics West, 6102-06, San Jose. [52] Frith, G. P. and D. G. Lancaster. 2006. Power scalable and efficient 790 nm pumped Tm^^-doped fibre lasers. Presented at Photonics West, 6102-08, San Jose.