Argon-Ion and Solid-State Cyan Lasers

Transcription

1 Optical Performance Comparison of Argon-Ion and Solid-State Cyan Lasers Edward H. Wahl, Bruce A. Richman, Chris W. Rella, Guido M. H. Knippels and Barbara A. Paldus Detailed measurements of the optical performance of three commercial 488-nm lasers were performed to evaluate two new semiconductor-based designs compared to the incumbent air-cooled argon-ion laser benchmark. Both solid-state lasers were found to have excellent optical performance which met or exceeded that of the ion laser. Since the solid-state lasers consume far less electricity and are much smaller and more reliable, it appears that for key commercial applications such as biomedical instrumentation, these new technologies will compete well with cyan air-cooled ion lasers. 36 Optics & Photonics News November /3/11/36/7-$15. Optical Society of America

2 Recent advances in biotechnology have created a need for compact, efficient and reliable 488-nm cyan lasers with good optical performance. This opportunity has hastened the development of new technologies to replace air-cooled argon-ion lasers in biomedical instrumentation applications. In this article we review a recently introduced commercial architecture, the optically pumped semiconductor laser (), and introduce a new approach, the doubled external cavity semiconductor laser (). We compare the performance of these two next generation cyan lasers with that of a traditional air-cooled argon-ion laser. In the 196s, lasers were considered a solution in search of a problem. 1 During this period, the argon-ion laser was primarily an instrument for basic scientific research. Today, however, the situation is quite different.the overall market for low-power (< mw) blue-green lasers for use in bio-instruments, semiconductor inspection and printing approaches $1 million. Cyan lasers comprise the largest portion of the blue-green laser market. The generation of blue-green laser light has a long history, dating back to the invention of the argon-ion laser in 1964 by William Bridges at Hughes, just seven years after the invention of the laser itself. Argon-ion lasers operate over a wide range of blue-green wavelengths from indigo (458 nm) to lime green (514 nm), with the 488-nm (cyan) line being the most widely used. A typical argon-ion laser has a useful life of about 5, hours. Competing solid-state technologies also appeared early on, with the development in 1965 of the first 53-nm green laser, based on intracavity second harmonic generation (SHG) of a 164-nm Nd:YAG laser. 3 The basic design was improved by use of a highly reliable and stable semiconductor diode laser to pump the gain medium. 4, 5 Owing to their higher reliability (~1, hours) and smaller footprint, these diode-pumped solid-state lasers, called DPSS, dominate the market in the green (53 nm), are strong competitors in the blue (473 nm), and are steadily eroding the market base of argon-ion lasers at these wavelengths. Because no suitable Polarization Selector 88nm Pump Laser VECSEL: 98nm Gain +98 nm HR Wavelength Selector High Reflector (HR) Intracavity Doubling Crystal 49nm OC +98 nm HR solid-state laser transitions exist at 976 nm, DPSS technology has been unable to penetrate the large and rapidly growing cyan laser market, where the argon-ion laser remains dominant. But new lasers based on SHG of next generation 976-nm semiconductor laser sources are now entering the marketplace. With the promise of higher performance and higher reliability in a smaller package, these new lasers should open up new opportunities and applications that up to now argon-ion lasers have been unable to address. In this article, we describe two of these next generation designs, the optically pumped semiconductor laser () and the doubled external cavity semiconductor laser (), and compare their optical performance to that of an argon-ion laser. Argon-ion lasers and their applications An argon-ion laser consists of an argon gas filled plasma tube, the windows of which are at Brewster s angle to the laser cavity axis (see Fig. 1). The laser cavity is formed by two mirrors: a high reflection mirror and an output coupler that transmits the laser radiation. A multi-line laser will emit all wavelengths simultaneously, while a single-line laser will have an additional prism or mirror to select a single wavelength from the multitude of lines. The laser output power is controlled by setting the current level through the plasma tube. Plasma Tube (Gain) Output Coupler (OC) 98nm ECL Gain HR Chip Wavelength Selector Wavelength Selector SHG Section Doubling Crystal Figure 1. Schematic of three blue-green laser architectures: single line argon-ion laser, optically pumped semiconductor laser and (c) doubled external cavity semiconductor laser. (c) Today, the argon-ion laser commands the lion s share of the blue-green laser market, with diverse applications including: flow cytometry and DNA sequencing; confocal microscopy; semiconductor wafer inspection and alignment systems; digital imaging and reprographics; high power laser projectors; and laser light shows. That this 39-year-old technology has had such staying power in a highly competitive marketplace is a testament to the extraordinary performance capabilities and elegant simplicity of the design. Argon-ion lasers long history also has made them very cost-effective. Yet many of the traditional applications of the argon-ion laser are slowly disappearing. Semiconductor applications, for example wafer inspection, require a wavelength suitable for detecting minute defects.to date, 488-nm wavelengths were sufficiently short, but next generation systems are expected to employ 355-nm or even 66-nm DPSS lasers. Digital photofinishing has also slowly shifted away from argon-ion lasers (457.9 nm and nm) to DPSS lasers (473 nm and 53 nm) because photopaper has broad color absorption features and therefore doesn t require the exact argon-ion laser wavelengths for effective exposure. Analytical instrumentation for biological applications remains the stronghold of the argon-ion laser. Low-power (1 to 3 mw), single-line, cyan argon-ion lasers are almost ubiquitous in this field because November 3 Optics & Photonics News 37

3 Power Variation (%) Deviation from Room Temperature Power (%) Temperature ( C) Figure. Comparison of power stability of three commercial blue-green lasers: over time, during a 14-hour period at room temperature and over temperature, during three ramp cycles between 1 C and 4 C. popular fluorescent reagents have their peak efficiency at or near 488 nm. Many other fluorophores, such as those used in DNA sequencing, are traditionally excited by multi-line argon-ion lasers which primarily exploit the 488-nm and nm lines. Although much work has been done to develop new fluorophores that take advantage of DPSS lasers at 53 nm and 473 nm, the bulk of the fluorophore legacy remains at 488 nm. Bio-analytical instrumentation is a demanding application that requires a high performance solution. The excellent optical performance characteristics of argon-ion lasers have been pivotal to their widespread adoption and have led to their use in thousands of commercial instruments. Specifically, argon-ion lasers provide a clean Gaussian TEM beam (M < 1.1) that is extraordinarily stable in wavelength (<.1 nm), beam pointing (< 1 rad/ C) and power (< %) over long periods of time and under harsh environmental conditions (4 C to 4 C, % to 9% humidity), all at a competitive volume price of less than $5,. But argon-ion technology is not without limitations: size (5 6 1 for head and for power supply), weight (14 pounds for head and 15 pounds for power supply), power consumption (~.5 kw) and limited operational life (~5, hours). By comparison, a typical mw DPSS laser operating at 53 nm can produce an optical beam of similar quality and stability in 1 percent of the volume while consuming less than 5 percent of the power, and has a lifetime that is twice as long. Because these characteristics are increasingly important in biomedical applications, there is a growing need for a solid-state alternative to the cyan argon-ion laser that does not entail compromises in optical performance. Next generation cyan lasers Despite the strong market demand for a solid-state cyan laser, the technical hurdles have been daunting. A lack of semiconductor materials that operate directly at 488 nm has driven laser designers to employ SHG. Although SHG technology can use many different nonlinear materials, the conversion efficiency of these materials remains limited: the generation of mw of cyan light typically requires several hundred milliwatts of 976-nm radiation. Moreover, because the acceptance bandwidth of most nonlinear materials is quite narrow (<.1 nm), wavelength stability and linewidth become critical for such a high power pump laser. Two architectures for generating cyan light using semiconductor pump lasers are shown in Fig. 1: intracavity SHG using an optically pumped semiconductor laser () and extracavity SHG using a doubled external cavity semiconductor laser (). technology uses a 88-nm pump laser to pump a vertical external cavity surface emitting laser (VECSEL) lasing at 976 nm. In the intracavity geometry, included inside the cavity are the SHG crystal and, in order to select a single longitudinal mode, a wavelength-selective element. To achieve the large intracavity power required for efficient frequency doubling, high finesse VECSEL cavities are generally used. The VECSEL s dichroic output mirror transmits the cyan radiation generated inside the 976-nm cavity. technology uses a 976-nm semiconductor gain medium inside an external cavity containing a wavelengthselective element, a configuration which forces the laser to emit only a single longitudinal mode. To generate cyan light, the radiation from the external cavity laser is doubled by an external SHG crystal. The control system for the laser can be robust because the laser gain and the SHG both nonlinear phenomena are controlled independently. technology, first proposed by Aram Mooradian in 1991, 6 was demonstrated at Micracor. Coherent, which acquired Micracor in 1997, 7 has refined and altered the technology for SHG, launching the first commercial solid-state cyan laser in 1. technology, developed at Picarro, resulted in the first commercial mw cyan in June 3. 8 Alternative commercial variations of and technologies are currently under development at other companies. Both and technologies satisfy next generation requirements for size (15 mm 7 mm 34 mm), power consumption (< 6 W), and operational lifetime (1, hours). But any successful cyan laser product must also meet a host of critical optical performance parameters: power stability over temperature; pointing stability over time and temperature; intensity noise; wavelength stability; spectral purity; beam quality; and beam polarization. Semiconductor diode lasers, upon which both and architectures rely, display poorer performance than traditional gas lasers in these categories. 9 We present here a detailed optical performance comparison of 38 Optics & Photonics News November 3

4 representatives from each laser technology: a traditional mw argon-ion laser (JDSU, Model 19-SLS), which also served as the standard in this study; the only commercially available (Coherent Sapphire 488-); and the only commercially available mw DECSEL (Picarro Cyan Laser). Optical performance of cyan lasers Modern cyan laser applications demand not only that the laser supply mw of optical power at start of life but that the power remain fixed over time and temperature. In an instrument designed for cytometry, for example, tight limits on power stability are critical to the task of counting cells since instability in laser power can translate directly into less accurate readings. In such real world applications, the laser must provide good power stability not only at startup in a controlled laboratory environment but day after day, under harsh and varying environmental conditions. The argon-ion laser performs well in this regard, thus setting a high standard for new technologies. We measured the optical power stability of each of the three lasers as a function of time at room temperature and over a broad range of temperatures in an environmentally controlled chamber. Both sets of measurements were made using a NIST-traceable laser power head (Ophir PD-3-SH). Figure shows the power stability at room temperature, normalized to the output power at time zero. Figure shows the power stability over three 1 C to 4 C temperature cycles, normalized to the output power at room temperature (5 C). Care was taken to eliminate any measurement artifacts caused by temperature and spatial non-uniformity of the detector. The data from these measurements are summarized in Table 1. The data essentially show equivalence between the argon-ion laser and the two semiconductor-based lasers on the basis of most of the parameters considered. Directionality, or beam pointing, is an important parameter for many cyan laser applications. In flow cytometry, the cyan laser is directed through several additional optics before being focused on a micrometer-sized flow cell. The pointing Power Stability Room Temperature (14 hrs) Mean (mw) Standard Deviation (%) Range (%) Temperature Ramping Temp range ( C) Standard Deviation (%) Range (%) Table 1. Output power characteristics. Horizontal Beam Displacement ( m/ C) Vertical Beam Displacement ( m/ C) Horizontal Beam Angular Change ( rad/ C) Vertical Beam Angular Change ( rad/ C) Table. Beam stability characteristics. stability of the laser beam is essential to the stability of the measurements; in the extreme, the laser beam could entirely miss the flow cell or cause severe fluctuations in the observed scattered power distribution. Argon-ion lasers, because of their relatively long cavity, generally offer excellent pointing stability. Solid-state designs, because of their smaller-scale cavities, often exhibit poorer performance. We measured the pointing stability of the three cyan lasers as a function of time in a small range of temperatures around 5 C. Two quadrant detectors (UDT Sensors Inc., Model SPOT-9DMI) were used to determine the angle and displacement of the beam. The horizontal and vertical angular shifts over a 14-hour period are shown in Fig. 3. All three devices under test showed excellent pointing stability characteristics, which tested the limits of the experimental setup. A summary of both the positional and angular data is given in Table. In the table, the data have been normalized by the temperature range experienced by each laser (generally 1 C to C). The positional and angular displacements of the and DESCL were found to be comparable to those of the argon-ion laser, within the uncertainty of this experiment. In a second set of experiments, the reproducibility of beam pointing was measured as a function of an on-off-on power cycle. The lasers were turned on for minutes, shut off for 1 minutes to cool down from their normal operating temperature, then restarted. The resulting data are illustrated in Fig. 3. The and lasers show very similar behavior, converging quickly to their initial steady-state angles within several minutes. The argon-ion laser stabilized to its original angular position much more slowly; it required about 5 minutes, probably owing to its larger thermal mass. The optical noise spectrum, defined as fast (>3 Hz) fluctuations of laser light intensity, is important to maintaining good signal-to-noise in DNA sequencing and cytometry applications. In the experiment, the system used to measure intensity noise was based on direct measurement of the optical power using a fast photodiode and a digital sampling scope. The noise measurement was made by computing the root mean square (RMS) November 3 Optics & Photonics News 39

5 Horizontal Angular Displacement ( rad) Vertical Angular Displacement ( rad) Horizontal Angular Displacement ( rad) Vertical Angular Displacement ( rad) Time (min) Time (min) Figure 3. Comparison of pointing stability of three commercial bluegreen lasers: over time, during a 14-hour period at room temperature and during an on-off-on power cycle. fluctuation of the optical power in the 3 Hz to 3 MHz frequency band. Additional digital filtering was performed on the low frequency data to assess the impact of ultralow frequency (3 Hz 7 Hz) components of the noise spectrum. The total measurement time was several seconds. RMS noise values were then recorded over a 3 minute period. The resulting data are shown in Fig. 4; detailed analysis of the data appears in Table 3. The noise performance of both the and are clearly superior to that of the argon-ion laser. This is perhaps not surprising: noise has traditionally been a weak point for argon-ion lasers; plasma instability, drive current fluctuations and longitudinal mode partition noise all contribute to fast instabilities of the power. In contrast, both the and are substantially quieter lasers, primarily because they are true singlemode lasers that operate on both a single longitudinal and transverse mode. A series of measurements was performed to investigate the wavelength stability of the two semiconductor lasers compared to the well known properties of the argon-ion laser. The wavelength stability of the cyan lasers over time was RMS Noise Integrated (1s) over: 3 Hz to 3 MHz Mean of RMS value Std Dev of RMS value Hz to 3 MHz Mean of RMS value Std Dev of RMS value Table 3. Noise characteristics. measured using a Burleigh WA-1 wavelength meter. The wavelength performance for the three lasers is summarized in Table 4 and shown as a function of time over a 1.4 hour period in Fig. 5. All three lasers exhibit excellent wavelength stability, although the shows slightly poorer performance than the other two. The operating wavelength is centered closer to the argon-ion laser wavelength. The spectral purity of cyan lasers is important in many applications. A common measure of spectral purity is the power in band, defined as the ratio of the power in a 1 nm (or ±.5 nm) band around the center wavelength to the power outside this band. The power in band was measured using an optical spectrum analyzer (Ando, Model AQ- 6315A). All measurements were performed with the same sensitivity level and a resolution of.5 nm. The spectra of the laser outputs are shown in Fig. 5. All three lasers produce a very clean, narrowband spectrum, although the longitudinal mode structure is too fine to be resolved in these results. The power out- 4 Optics & Photonics News November 3

6 side the 1 nm band is given in Table 4. The performed comparably to the argon-ion laser, while the performed marginally worse; this was primarily caused by the fact that compared to the, the laser wavelength of the is centered a bit further from that of the argon-ion laser. For the biomedical applications considered in this article, the performance of both next generation cyan lasers is entirely adequate. Like most gas lasers, argon-ion lasers produce extraordinarily clean transverse beams because of their stable cavity design and highly uniform gain medium. Most modern uses of cyan lasers either explicitly or implicitly require such high transverse beam quality. For this reason, the compatibility of next generation lasers with argon-ion laser beam parameters is critical if these technologies are to have access to replacement opportunities for argon-ion lasers. The near- and farfield transverse beam profile of each laser was measured using a commercially available transverse beam profiler.the resulting measurements of transverse beam quality are displayed in Table 5. Again, the three lasers are quite comparable and in all cases fall within the applicable specification for argon-ion lasers. Given the poor beam quality of many semiconductor lasers, it is somewhat surprising that the two semiconductor-based lasers produce such a high quality beam. Their performance speaks to the transverse filtering aspects of the SHG process and to the robustness of the optical designs of both lasers. Finally, we investigated polarization purity. Most applications for cyan lasers demand a highly linearly polarized source having a specific orientation. Using a high quality commercial polarizer, we evaluated the polarization extinction ratio, defined as the ratio of the minimum and maximum power transmitted through the polarizer upon rotation through 36 degrees. While all units performed substantially better than the typical requirement of db, the performed best in this regard at 36 db. The and the argon-ion laser achieved 5 db and 3 db, respectively. Overall, the performance of the next generation and technologies appears to compare favorably with Wavelength (nm) RMS Noise (%) Noise is integrated from 3Hz to 3MHz Time (min) (Normalized to 488.1nm) Intensity (db) Wavelength (nm) Figure 4. Comparison of RMS intensity noise, integrated from 3 Hz to 3 MHz, of three commercial bluegreen lasers over a 3-minute period at room temperature. Figure 5. Comparison of three commercial blue-green lasers in: wavelength stability over time, during a 1.4-hour period measured using a wavelength meter and spectral purity and in-band power using an optical spectrum analyzer. Wavelength Peak (nm) Stability - Std Dev (nm) Power not in 1 nm Band (%) Table 4. Wavelength characteristics. November 3 Optics & Photonics News 41

7 Figure 6. Ed Wahl running the beam measurement setup for three-dimensional profile acquisition and blue-green laser prototyping clean room area. (Back row, left to right) Ed Wahl, Guido Knippels and (front row, left to right) Bruce Richman, Barb Paldus, Chris Rella. that of argon-ion lasers in all major optical parameters. The next generation designs do not appear to suffer from poor power stability, beam quality or beam pointing, all problems which have plagued previous semiconductor laser based designs. When combined with the higher reliability and smaller footprint enabled by semiconductor pump lasers and micro-optics, the excellent optical performance of s and s will allow them to compete successfully with argon-ion lasers, provided that they can also match the argon-ion in one other critical parameter: cost. Argon-ion laser manufacturing is highly labor intensive, relying on time consuming steps such as precision brazing and high finesse cavity alignment. In contrast, both the Coherent Sapphire and the Picarro Cyan are assembled using semi-automated processes in clean room facilities. These techniques, adopted from the telecommunications component market, usher in the promise of decreased manufacturing costs. It is, however, a tribute to the experience of gas laser manufacturers that, despite a labor intensive manufacturing process, argonion lasers currently set the standard not only for optical performance but for price. Conclusions Over their long history, argon-ion lasers have provided the solution to many different problems and have enabled numerous applications in analytical instrumentation, reprographics and displays. Although conventional frequencydoubled DPSS lasers have made inroads and gained market share, the venerable single-mode, air-cooled, -mw, cyan argon-ion laser remains largely unchallenged in the realm of biotechnology applications. But two new types of semiconductor-based lasers that offer comparable optical performance along with reduced size, lower power consumption and higher reliability have recently become commercially available.these lasers, based on or technologies, offer instrumentation designers the benefits of increased portability, decreased footprint, wallplug compatibility and lower maintenance. The value of these advantages to the biomedical industries will determine the adoption rate of these next generation cyan laser technologies over the next few years. Meanwhile, the argon-ion laser will soon celebrate its 4 th birthday. Acknowledgments The authors thank Serguei Koulikov, Giacomo Vacca, Boris Kharlamov, Lewis Book, Dmitri Permogorov, Hoa Pham, Eric Crosson and the Picarro engineering team. They also acknowledge the NSF Phase I SBIR program, under which a portion of this work was supported. The authors also thank R. N. Zare, M. D. Levenson, K. Vodopyanov, R. L. Byer, R. Lang and P. Anthony for their guidance over the past year. Parameter M x M y Horizontal Diameter (mm) Vertical Diameter (mm) Horizontal Divergence (mrad) Vertical Divergence (mrad) Horizontal Waist Position (mm) Vertical Waist Position (mm) Beam Asymmetry (X/Y:1) Astigmatism (mm) Table 5. Optical beam characteristics. Edward H. Wahl, Bruce A. Richman, Chris W. Rella, Guido M. H. Knippels and Barbara A. Paldus (bpaldus@picarro.com) are all with Picarro Inc. References 1. A. E. Siegman, Lasers, University Science Books, Sausalito, Calif., W. B. Bridges, Two Revolutions made possible by light, My point of View, NEC Publications, 1/4/. 3. W. P. Risk et al., Compact Blue-Green Lasers, Cambridge University Press, K. Kubodera, K. Otsuka, Appl. Opt. 16 (1) 747 (1977). 5. B. Zhou et al., Opt. Lett. 1, () 6 (1985). 6. H. Q. Le, S. di Secca, A. Mooradian, Appl. Phys. Lett., 58 (18) (1991). 7. R. Winn Hardin, Coherent Grabs at Telecom Pie, Photonics Spectra, Feb Stephen G. Anderson, Conard Holton, Laser Focus World, August 3, p Mitsuo Fukada, Optical Semiconductor Devices, John Wiley & Sons, 1999, p Optics & Photonics News November 3 Tell us what you think: