How to Select a Laser

Transcription

1 How to Select a Laser We have carefully selected the lasers found to be most popular and useful over the decades for our catalog line. We have included HeNe and semiconductor diode lasers, both of which have traditionally found broad OEM use, as well as diode-pumped solid state (DPSS) lasers, a rapidly growing tool in OEM applications. Our long history in the manufacture of lasers results in consistent and dependable operation, as well as smooth transitions for custom products into manufacturing. By way of example, the optical resonator mirror quality is critical to the performance of a laser. We manufacture these in-house to high standards, and use only the most durable optical coatings. The high laser damage threshold of our cavity mirrors helps to maintain consistent power output over time. Additionally, all of our systems comply with the applicable CDRH standards and IEC directives for stand-alone laser equipment. This selection guide is intended to serve as an introduction to some of the considerations when selecting a laser, with specific comments on our off-the-shelf products. OEM versions of these lasers, designed specifically for inclusion in other systems, are available upon request. Please call a Melles Griot applications engineer to discuss. Selecting a laser is a balance between finding the right wavelength, optical, and operational parameters, including power, mode purity, divergence, stability, and lifetime. A requirement for a particular wavelength may drive the choice to a particular type of laser, but the reverse can be equally true. Types of lasers offered The HeNe laser is perhaps the most ubiquitous laser, due to a combination of performance and practical factors. Small and compact compared to other gas lasers and traditional solid state lasers, they also have the best inherent beam quality, producing a virtually pure single transverse mode beam. They are attractive to OEMs because they are extremely reliable, easily convection cooled, and are relatively low cost and efficient. Melles Griot is the worldwide leader in HeNe laser manufacturing, delivering over two million units to date with a high degree of unit-to-unit repeatability, excellent pointing stability, long coherence length, and low output noise. HeNe lasers work by using a low current dc discharge to excite a mixture of helium and neon gas. Neon is the active lasing medium, while helium acts as a buffer gas to populate the upper neon levels for lasing through collisions. The laser cavity is created by end mirrors mounted directly to a plasma tube containing the gases, mounted within a metal sleeve for protection and heat dissipation. This design is very reliable, and it is not unusual for some HeNe lasers to operate for 50,000 hours or more. The most popular HeNe laser wavelength is nm. Low power HeNe lasers (<0.95 mw, or IEC Class II) find use in FTIR spectroscopy, alignment, pointing, bar code scanning, and teaching applications. Moderate power lasers (up to 35 mw) are used in Raman spectroscopy, holography, and test and measurement. Stabilized singlefrequency lasers, with coherence lengths measured in kilometers, are ideal for wavelength calibration, precision measurements, interferometry, and ranging applications. Though much less common and lower in optical gain, the nm green line and nm yellow line in the neon spectrum have their own unique uses. Green HeNe lasers

2 exploit the sensitivity of the human eye to green light, and can be used effectively at a fraction of the output power of a red laser for visual applications like aiming and pointing. Many fluorescent dyes, reagents, and films absorb strongly at 543 nm, making these lasers ideal for biotechnology, cytometry, microscopy, and laser printing. Yellow HeNe lasers are close enough in wavelength to the sodium D lines to act as a spectral reference in the optical industry. They are also used to excite several popular fluorescent dyes. Diode lasers convert electricity directly to light. They consist of a p-n diode junction with an active region where electrons and holes recombine to emit light (photons), together with an optical cavity where stimulated emission and light amplification take place. The high efficiency of this process (nearly 50%), low power consumption, compact size, and the variety of emission wavelengths available makes them a very attractive light source. Available in UV through NIR wavelengths, they have increasingly replaced other laser types in many research and industrial applications. The output power of a diode laser is heavily dependent on the current supplied to the diode junction and its temperature. Single mode operation occurs only above a threshold current, but output power is then directly proportional to that current. This allows a diode laser s output to be quickly and easily controlled by an analog or digital signal. It also enables modulation of the output, reducing system cost by eliminating acousto-optic modulators, and allows synchronous detection schemes. Temperature variations, however, affect both output power and wavelength stability. Both effects underscore the importance of either maintaining a constant junction temperature through thermoelectric cooling or maintaining output power using automatic power control (APC). Cooling can also extend a diode laser s lifetime, as operating life doubles for every 10 C reduction in operating temperature. At room temperature, some diode lasers can be expected to have a lifetime of 50,000 hours or more. Diode lasers are fabricated using photolithographic patterning in a stripe format on a single crystal wafer, which is then cleaved normal to the stripe direction to create a pair of planar, aligned crystal surfaces that act as end mirrors (sometimes dielectrically coated for enhanced reflectivity or power handling). This unique geometry typically results in TEM 00 (Gaussian) emission, with a 10 emission angle parallel to, and a 30 emission angle perpendicular to, the laser junction. Variability from component to component can be as high as 25%, so it is extremely important to match collimating optics to each individual diode to ensure a circular beam in far field operation. The stripe structure of a diode laser also results in linearly polarized output; ours have a nominal 100:1 extinction ratio. Diode lasers with free-space collimating optics benefit from high optical efficiency and output power, as well as good beam quality, compact size, and low cost. Fiber coupling, however, generates optical beam quality that is far superior to free-space coupling, albeit at the cost of output power. Output coupling via single-mode fiber eliminates many of the high-order spatial modes and creates a diffraction-limited, astigmatism-free Gaussian (circular) output. This enables focusing to the theoretically smallest spot size and/or propagation over hundreds of meters. Efficiency for infrared wavelengths is up to 75%, but drops to less than 50% for visible wavelengths. Multimode fiber bundles achieve more than 70% efficiency, but with a reduction in mode quality. GaN diode lasers emit at blue and near-uv wavelengths, and are often used as excitation sources for biomedical fluorescence studies, DNA sequencing, and confocal microscopy, as well as DVD mastering and HeCd laser replacement. GaAs diode lasers emit at red and nearinfrared wavelengths, and are used extensively for HeNe laser replacement in applications like pointers, alignment, biomedical fluorescence, barcode scanners, and surgical applications. Near-infrared GaAs diode lasers find use in audio CD readouts, Raman spectroscopy, thermal printing, and as optical pumps for Nd:YAG lasers. InP diode lasers emit at infrared wavelengths, useful for pumping of Yb:YAG and erbium fiber lasers, as well as OCT. They are used widely in telecom applications to pump erbium fiber amplifiers and generate Raman gain, and are an input source for short- and long-wavelength channels. Diode-pumped solid state (DPSS) lasers use diode lasers in place of discharge lamps to pump solid-state laser crystals, resulting in a compact and efficient alternative to air-cooled ion lasers and green HeNe lasers. Many proprietary designs exist for DPSS lasers, but Melles 2 IDEX Optics & Photonics Marketplace

3 Griot uses an infrared laser crystal pumped by nearinfrared diode lasers followed by intracavity doubling to generate visible light. Coatings on the extreme sides of the two crystals form the laser cavity and keep the design compact, while thermoelectric coolers tightly control the temperature of the laser cavity and diode pumps. Output optics collimate the beam and filter any remaining pump laser or infrared light. The resulting beam quality is comparable to that of a gas laser, but with efficiency on the order of a few percent (as compared to 0.1% for gas lasers). DPSS lasers also offer excellent stability and exceptional mode purity, ideal for laboratory and OEM applications. Designs vary significantly from one manufacturer to another due to the unique and often proprietary methods used to optimize performance; it is therefore best to select a manufacturer with a solid manufacturing track record to ensure reliable operation for 10,000 hours or more. Green DPSS lasers emitting at 532 nm are used in place of traditional flashlamp-pumped doubled Nd:YAG lasers in spectroscopy, laser-induced fluorescence, medical diagnostics, alignment, and a wide variety of testing applications. Yellow DPSS lasers emitting at 561 nm are ideal for excitation of biomedical fluorescence, including microscopy, flow cytometry and DNA analysis. Directly doubled diode (DDD) lasers couple a 976 nm telecom diode laser to a second harmonic generator to directly convert its output to 488 nm light, followed by collimating optics at the output. The resulting design is highly efficient, reliable, and easily manufactured in volume. They can be used as a compact, low-cost replacement for air-cooled argon-ion lasers, reducing power consumption dramatically, simplifying thermal handling, and increasing system life. In this capacity, they find use in many bioanalytical and fluorescence applications, as well as in spectroscopy and semiconductor inspection. Beam Characteristics: A large part of selecting the right laser is gaining an understanding of the beam characteristics, stability and operating parameters. This section will briefly review the major specifications given for HeNe, diode, DPSS, and DDD lasers available for online purchase from Melles Griot, with notes on their relative performance. Output wavelength & stability Our catalog lasers are available in discrete wavelengths from 408 nm to 640 nm, with many other wavelengths available. The wavelength or frequency stability of a laser will depend on its type and the specific design. Fluctuations of < 0.1 Hz are often referred to as drift, while faster fluctuations are considered noise, or for sudden frequency shifts, jitter. DPSS laser drift is due primarily to temperature variations, which we control to within ±0.5 nm using thermoelectric cooling (drift is ±1.0 nm for our 532 nm DPSS laser). HeNe lasers exhibit much less drift due the use of a gas as the gain medium, but can vary in frequency by hundreds of MHz due to small variations in cavity length with environmental conditions. Our frequency stabilized HeNe lasers limit this variation in frequency to less than a few MHz, even over 8 hours. Monitoring of the longitudinal modes within the cavity combined with a feedback loop to adjust the cavity length give our stabilized HeNe lasers their rock-solid frequency and power stability, with a coherence length measured in kilometers. They are ideal for calibration, fiber optic and diode testing, interferometry, and surface defect inspection. Laser modes and M 2 Laser resonators have both transverse and longitudinal modes. Longitudinal modes correspond to different resonances along the length of the laser cavity, and occur at different frequencies or wavelengths within the gain bandwidth of the laser. When there are multiple longitudinal modes, a power fluctuation phenomenon called mode sweeping occurs. All unstabilized HeNe lasers exhibit this effect, which occurs when temperature variations cause changes in cavity length, creating a small change in mode spacing and therefore the absolute wavelength of each cavity mode. In effect, the comb of longitudinal modes drifts relative to the Doppler broadened line center, causing amplitude fluctuations of a few percent. Transverse modes can be seen in the intensity pattern of the beam in cross-section, and are defined by the number of minima in the x and y directions. The fundamental mode of a laser, TEM 00, is Gaussian in shape, with peak intensity at the center. A mode with a single minimum along one axis and no minimum in the perpendicular IDEX Optics & Photonics Marketplace 3

4 direction is denoted TEM 01 or TEM 10, depending on orientation. The beam quality factor, M 2, arises from the propagation factor k, which describes the relationship of a non- Gaussian beam to a Gaussian beam as it passes through an optical system. Equation 1 Where λ is the wavelength of the beam, w 0 is the beam waist, and v is the far-field divergence of the beam. If k{ M{1, the beam is Gaussian. If M 2 >1, then the beam is not Gaussian, but all of the standard Gaussian propagating formulas may be used with appropriate modifications. M 2 for the TEM 01 mode is 2.3, while M 2 for the TEM 10 mode is 3.6. Our HeNe lasers are available with M 2 < 1.1, and our DPSS lasers are available with M 2 < 1.2. Diode lasers possess a natural asymmetry, which we are able to correct such that M 2 does not exceed 1.4. Beam diameter (1/e 2 ) Beam diameter or beam width is most often defined by the point at which the beam intensity has fallen to 1/ e 2, or 13.5% of its peak value when measured normal to the optical axis of the beam. This assumes propagation of a Gaussian beam, and therefore works well for lasers operating in TEM 00 mode. Beam diameters for our lasers are in the range 1.0 ± 0.5 mm, depending on the specific laser. Our low power nm HeNe lasers offer the smallest beam diameter. Far-field divergence Gaussian beams do not diverge linearly. Close to the laser, the divergence angle is very small, while far from the laser, the divergence angle approaches a limit defined by the beam waist and its wavelength. The far-field divergence of a laser is typically measured at a distance 10x the Rayleigh range away from the laser. Far-field divergence is an important parameter when calculating spot size and other parameters in an optical train. Far-field divergence for our lasers varies, but is no more than 2 mrad, with the exception of our high power blue and green DPSS lasers. Polarization Most of our HeNe lasers are available in unpolarized or linearly polarized (extinction ratio >500:1) formats. Our frequency stabilized HeNe operates with a single longitudinal mode, and is thus highly linearly polarized (>5000:1 extinction ratio). Diode lasers are linearly polarized due to the geometry and nature of their laser cavity, but offer lower extinction ratio (>100:1). DPSS lasers are also linearly polarized with >100:1 extinction ratio. Power Stability Frequency and amplitude fluctuations are closely tied in lasers, due to the heavy dependence of both factors on the cavity length and conditions. Many of the causes of wavelength/frequency stability already discussed result simultaneously in power fluctuations. Additionally, optical noise often arises due to mode beating, caused by interference between multiple transverse or longitudinal modes within the cavity. This can result in peak-to-peak fluctuations of a few percent, and can be eliminated only by limiting the laser output to a single transverse and single longitudinal mode. Amplitude fluctuations can be stabilized in diode and DPSS lasers using ACC (automatic current control) or APC (automatic power control). ACC monitors the current 4 IDEX Optics & Photonics Marketplace

5 driving the pumping process, providing feedback and correction to minimize fluctuations. The output of HeNe lasers cannot be stabilized via APC, as their output is highly insensitive to the discharge current. APC monitors light output from the laser via a sampled beam to a photodetector, upon which feedback into the system stimulates adjustments to regulate power output. ACC does not correct for fluctuations caused by vibration or misalignment, while APC does. Neither control mechanism has a large impact on frequency stability. Melles Griot specifies amplitude fluctuations (noise) for a range of frequencies starting at Hz and going up to 1 10 MHz, using root mean square (rms) and sometimes peak-to-peak to properly characterize the lasers performance. While most of our lasers specify noise over a broad frequency range, some, like the 85-GCB series, are optimized for reduced noise over lower frequency range. Long term power drift for our lasers is typically < 2%, measured over a period of 2 8 hours. Warm-up time and pointing stability Our HeNe lasers require minutes of warm-up time to come to thermal equilibrium, after which pointing stability can measured in hundredths of a mrad. Our DPSS lasers require less than 5 minutes of warm-up time to achieve the same pointing stability. Diode lasers do not have a required warm-up time, but their pointing stability tends to vary with temperature, and can be as low as < 5 μrad/ C for our diode lasers. Operating considerations In addition to optical performance, practical factors like operating temperature, electrical requirements, and computer control should be considered. Our diode and DPSS lasers are designed for operation over a wide range of temperatures, though they can withstand a much wider range when not in operation. Our HeNe lasers are most robust with temperature, operating easily between -20 C and +40 C, and withstanding non-operating temperatures of -40 C to +80 C. All of our lasers are available for use at either 115 or 230 V, though this must often be specified prior to purchase. Only the diode and DPSS lasers offer RS-232 control. Our HeNe product line includes both class II and class IIIa/IIIb lasers, while our diode and DPSS lasers are up to class IV, and therefore pose an immediate skin and eye hazard upon direct contact. We recommend that appropriate safety precautions be taken. Making the final decision Though all of the parameters discussed above are important in selecting a laser, the choice often comes down to wavelength, coupling method, polarization, and power. The selection flowcharts below are intended to speed you in that process, with an additional chart of power vs. wavelength for our off-the-shelf lasers operating at up to 100 mw. If you do not see the product you require, or would like to discuss an OEM version of a product, please contact us to discuss. Selection Guide IDEX Optics & Photonics Marketplace 5

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