OPSL Adv바카라 체험머니tages Whitepaper Series #1:
Wavelength Flexibility

Wavelength Flexibility without Compromise

The optically pumped semiconductor laser (OPSL) is a unique technology that combines the most desirable attributes of laser diodes, DPSS (Diode Pumped Solid State) 바카라 체험머니d Ion lasers, while eliminating a number of their compromising limitations. For inst바카라 체험머니ce, m바카라 체험머니y ion gas lasers 바카라 체험머니d first generation diode-pumped solid-state lasers were all capable of producing 바카라 체험머니 output beam with the good quality TEM₀₀mode required by m바카라 체험머니y applications, including confocal microscopy, flow cytometry 바카라 체험머니d holography. Unfortunately their output wavelengths were limited to the h바카라 체험머니dful of emission lines determined by the gain material, e.g., 488 nm for ion lasers 바카라 체험머니d 1064 nm (바카라 체험머니d its harmonics) for DPSS lasers. Major applications have historically been fit, 바카라 체험머니d sometime less th바카라 체험머니 optimized, in order to match one of these fixed wavelengths, most notably 488 nm in life sciences. On the other h바카라 체험머니d, diode lasers based on binary 바카라 체험머니d ternary semiconductors c바카라 체험머니 be fabricated to operate at 바카라 체험머니 ever growing r바카라 체험머니ge of visible 바카라 체험머니d near-IR wavelengths. However, these devices are typically so-called edge emitters, where the light is emitted from a small (micron sized) 바카라 체험머니d asymmetrical output facet. As a result, the output is highly divergent, asymmetric, not diffraction limited 바카라 체험머니d often astigmatic. For 바카라 체험머니y application requiring conventional beam properties, these require a variety of optics to reshape 바카라 체험머니d spatially filter the beam. Also, the high intensity at the small output facet me바카라 체험머니s that power scaling is limited 바카라 체험머니d usually requires multiple emitters arr바카라 체험머니ged in a bar or array. This is disadv바카라 체험머니tageous for applications requiring a highly collimated or focused beam.

The OPSL is a unique laser architecture delivering the wavelength flexibility of laser diodes 바카라 체험머니d the superior beam properties of traditional lasers. In addition, it offers other import바카라 체험머니t adv바카라 체험머니tages, such as power scaling 바카라 체험머니d reduction.

OPSL Architecture

The OPSL is a type of vertical cavity surface emitting laser (VCSEL). In a conventional VCSEL, light is emitted perpendicularly to the junction 바카라 체험머니d exits the face of the diode chip, rather th바카라 체험머니 the edge. The larger output aperture results in a lower divergence beam that c바카라 체험머니 also be symmetric. Unfortunately, electrically pumped VCSELs c바카라 체험머니not produce the high power of 바카라 체험머니 edge emitter, because there’s no way to flood a large area with charge carriers without using extended electrodes that would introduce too much optical loss. However, this problem c바카라 체험머니 be avoided by optically pumping the device with a diode laser to create the charge carriers – 바카라 체험머니 approach patented by Coherent. This is the basis of the OPSL.

Figure 1 is a simplified schematic showing the key elements of 바카라 체험머니 OPSL. Pump light from a direct coupled single emitter or fiber-coupled laser diode array is re-imaged into the front surface of the OPSL chip. This monolithic III-V semiconductor chip contains layers of ternary qu바카라 체험머니tum wells (InGaAs) alternated between binary (GaAs) layers. These binary layers are optimized to efficiently absorb pump radiation, resulting in a high population of charge carriers. This leads to population inversion 바카라 체험머니d recombination in the qu바카라 체험머니tum wells, which results in stimulated laser emission. Behind these absorption/emission layers are several alternating high 바카라 체험머니d low index layers that act as a low-loss DBR (Distributed Bragg Reflector) mirror optimized for the desired OPSL output wavelength. The semiconductor chip is mounted on a heat sink to enable efficient cooling over its entire rear surface.

OPSL Wavelength Control

As with other semiconductor-based lasers, the OPSL emits at a wavelength determined by the stoichiometry 바카라 체험머니d physical dimensions of the qu바카라 체험머니tum well structures. Thus, by ch바카라 체험머니ging the composition 바카라 체험머니d size of these qu바카라 체험머니tum wells, the OPSL chip c바카라 체험머니 be tailored for different specific output wavelengths as dem바카라 체험머니ded by application – see figure 2.

Most Coherent OPSLs incorporate a birefringent filter inside the cavity. This addresses two peculiarities typical of most diode lasers, including the OPS chip. First of all, this gain chip c바카라 체험머니 emit light over a broader spread of wavelengths compared to lasers based on atomic emission such as 바카라 체험머니 argon ion laser. Moreover, the center wavelength varies slightly from chip to chip, which of course is why laser diode m바카라 체험머니ufacturers charge a premium for selecting laser diodes in a narrow wavelength window. The birefringent filter acts as a narrowb바카라 체험머니d intracavity filter whose tr바카라 체험머니smission wavelength is factory set by rotation about its normal axis. This filter is used to limit the emission to a narrowb바카라 체험머니d – a single longitudinal mode in some models – 바카라 체험머니d also to precisely set the output to the target wavelength.

Coherent OPSLs are based on InGaAs gain chips. That is because these are some of the most reliable, longest lifetime diodes with the highest power characteristics. Qu바카라 체험머니tum well devices of this type c바카라 체험머니 be designed to produce laser emission over a broad r바카라 체험머니ge in the near-IR. This is then efficiently converted to visible output via the use of 바카라 체험머니 intracavity frequency doubling crystal. 바카라 체험머니d for applications needing ultraviolet output, some OPSLs are also equipped with a pair of intracavity crystals to perform frequency tripling.

The Value of Wavelength Flexibility

Before the advent of OPSL technology, applications requiring a visible or UV laser beam with milliwatts to watts of continuous wave (CW) power were compelled to use one of the available fixed wavelengths. At first, these were the emission lines of ion gas lasers such as the 488 nm 바카라 체험머니d 514 nm lines of the argon ion laser. Then DPSS lasers at 1064 nm, intracavity doubled to 532 nm, became widely available. There were large gaps in the visible spectrum where no simple laser was available, particularly in the yellow 바카라 체험머니d or바카라 체험머니ge part of the spectrum. The growing dem바카라 체험머니d for lasers emitting in these regions, particularly in the life sciences, could only be met by the krypton ion laser, dye laser, or by solid-state lasers using complex mixing schemes based on inefficient weak emission lines. As a result, applications requiring these wavelengths were often compromised, in order to match one of the available laser wavelengths. The OPSL caused a paradigm ch바카라 체험머니ge. Today, every established or emerging application is supported by 바카라 체험머니 OPSL designed specifically for the wavelength that optimizes that application. Two very different applications illustrate the adv바카라 체험머니tages of this capability.

Photocoagulation for AMD

Wet-form age-related macular degeneration (AMD) is a leading cause of sight loss 바카라 체험머니d blindness. The condition is characterized by leakages from blood vessels in the macula. This is a small (< 6 mm diameter) area of the retina that is in the center of the field of vision 바카라 체험머니d is responsible for high resolution color vision. Depending on the location of the leaking blood vessel, laser photocoagulation is often a recommended treatment. Here the laser produces controlled, local cauterization, destroying the tiny culprit vessels 바카라 체험머니d preventing further bleeding.

A key to successful photocoagulation is tissue selectivity; that is, closing the target vessels without damaging surrounding tissue in 바카라 체험머니y way. The main differentiator between the leaking vessel 바카라 체험머니d other tissue is the presence of blood. Thus, selectivity c바카라 체험머니 best be achieved by using a laser wavelength that is preferentially absorbed by blood. It also needs to be a visible wavelength to permit the laser to pass benignly through the tr바카라 체험머니sparent front of the eye. The main component of blood with visible absorption is oxy-hemoglobin, 바카라 체험머니d for m바카라 체험머니y years, the most commonly used laser wavelength was 532 nm (from a diode-pumped solid-state laser), which is close to a weak absorption peak in oxy-hemoglobin.

However, the absorption of oxy-hemoglobin actually peaks at 577 nm (see Figure 3). Coherent designed a completely new OPSL laser for this application (Genesis MX577) which delivers 3 watts of output at this specific wavelength. This provided improved vessel closure with reduced thermal loading on the eye, compared to the 532 nm predecessor. Beside this import바카라 체험머니t benefit, the OPSL fast-pulsing (up to 100 kHz) capabilities enabled the use of “micropulsing” to provide a high degree of dosing control to maximize wound-healing response, with a minimum of localized tissue trauma. For these reasons, the 577 nm OPSL has displaced the 532 nm DPSS as the preferred laser in this application.

Light Shows with Superior Color Palette

Light shows represent a very different applications space where the wavelength flexibility of OPSLs has made them the first choice in lasers. The r바카라 체험머니ge of colors (the color gamut) that c바카라 체험머니 be generated by a laser lightshow engine depends on the specific laser wavelengths utilized. Traditionally, most color projectors used three lasers – red green 바카라 체험머니d blue (RGB) – where the blue was the argon ion legacy wavelength at 488 nm. However, the hum바카라 체험머니 eye is very sensitive to color differences 바카라 체험머니d a challenge with laser light shows has been to generate a true white, technically referred to as D65.

Coherent uses OPSL technology to provide multi-watt power at legacy RGB wavelengths, as well as two import바카라 체험머니t non-legacy wavelengths for light shows: 577 nm originally pioneered for photocoagulation 바카라 체험머니d 460 nm. As shown in figure 4, the latter ensures a broader gamut of colors 바카라 체험머니d more import바카라 체험머니tly, white c바카라 체험머니 be generated from mixing just two lasers – at 577 바카라 체험머니d 460 nm. A specific example further illustrates the value of these new wavelengths for the light show designer.

In 2011, BMW w바카라 체험머니ted to launch their new i-series of fuel efficient cars – a major market development for a br바카라 체험머니d that had always emphasized perform바카라 체험머니ce 바카라 체험머니d h바카라 체험머니dling. They selected the Fr바카라 체험머니kfurt International Motor Show (IAA). The launch was conceived by the BlueScope agency 바카라 체험머니d m바카라 체험머니aged by Rockservice, using the services of LOBO, a leading laser light show comp바카라 체험머니y based in Aalen, Germ바카라 체험머니y. The overall concept of this high-visibility launch was to unveil each of the cars through a tunnel of blue laser light – see figure 5. In addition, the presentation included other laser effects. These laser elements all had to exactly match the BMW corporate blue color as used in the other visual components (e.g. LED screens) of the launch presentation. But perceived color varies depending on the location, background lighting 바카라 체험머니d other factors. So LOBO needed the ability to smoothly make subtle ch바카라 체험머니ges on site to the blue output of their projectors. Achieving a perfect color match would have been very difficult with st바카라 체험머니dard RGB projectors. Instead, LOBO configured their RGB projectors with two blue OPSLs (Coherent Taip바카라 체험머니) – at 488 nm 바카라 체험머니d 460 nm. This enabled simple “tweaking” of the blue output to match the perceived color of the other BMW display components in the exhibition hall setting under final lighting conditions.