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Diode is also called crystal diodes, referred to as diode; it only to send current in one direction of electronic components. It is a bonding with a part number of two terminal devices with in accordance with the direction of the applied voltage so that current flow or not flow of nature.
Tunable Diode Lasers Tunable diode lasers are increasing their market penetration via rapid evolution — a new generation of products every year or so. Bob Shine and Tim Day, New Focus At the recent Photonics West and Optical Fiber Conferences, compa- nies presented many new products, ranging from mechanical products to instruments to complex laser sys- tems. As a product engineer, there is nothing quite like the feeling of standing in the booth, talking to po- tential customers about the product You’ve designed or built. But the real be described in greater success happens when customers buy detail in upcoming sec- the product tions.) Customers decide to buy for a The technical cap- number of reasons. The product may abilities and features of let them do something that was dif- the tunable diode laser ficult or even impossible before. Or played a major role in the product might make their experi- the success of the pro- ments simpler by creating a better- duct. But the ease-of designed or easier-to-use instrument use of the product also than was previously available. For this contributed to its suc- to happen, the product must clearly cess. For example, a satisfy a real need of the customer, room-temperature, 2- the product should have some sus- µm tunable diode laser tainable competitive advantage over was recently used by re- other companies’ offerings, and the searchers at Stanford University and Figure 1. (top) The Vortex laser (New Fo- company should be able to make a Focused Research to obtain a survey cus’ newest) is built to the customer’s profit by selling the product. This last spectrum of CO 2 in a matter of specific wavelength specification and off- item is important since the product minutes. With this type of technolo- ers robust, narrow-linewidth source for gy these researchers can imagine a atomec spectroscopy, environmental can survive only if it can be justified monitoring, and metrology applications. economically both to the customer very sensitive diode-laser-based sen- and the manufacturer. sor for combustion monitoring. Figure 2. (below) Atomic absorption The tunable diode laser is a pro- monitor based on a New Focus Vortex duct that has satisfied all of the Some History tunable laser and built by Orca Photonics. Before the tunable diode laser was This system was designed to monitor bari- above requirements to become a introduced, the workhorse of the um deposition rates and has been de- successful product. Since the first ployed with an industrial customer. commercial introduction of this type tunable laser market was the liquid of laser about five years ago, it has dye laser. This laser was developed in found numerous applications, ranging the 1960s and was used in a number the dye laser in the 600- to 1000-nm from telecommunications to spec- of fundamental discoveries in spec- range. But the Ti:sapphire laser still troscopy to metrology. While each troscopy. Different dyes allowed us- required an expensive pump laser and of these applications places slightly ers to cover a wide spectral region. water cooling, yet did not directly different requirements on the laser However, the dye laser had a number offer the wavelength diversity and the user interface, each of them of drawbacks, such as the need for (roughly 350 to 1200 nm) of the dye requires a narrow-linewidth, continu- an expensive pump laser and the in- laser. ously tunable, reliable source. (As ex- convenience of using and changing At the same time, semiconductor amples, Figures 1 and 2 depict two the liquid dyes. The Ti:sapphire laser diode lasers were also widely avail- somewhat different system that will was developed in the late 1980s and able. A diode laser can be tuned over offered a solid-state replacement for its large gain spectrum by adjusting angle of the mirror which se- to 2.06 µm with an output power as lects a unique diffracted wa- high as 18 mW. The tuning curve for velength. The reflected zero this laser is shown in Figure 4. order from the diffraction The 2-µm region and beyond Is of grating has constant direction particular interest in environmental and forms the output beam. monitoring, since these wavelengths These external cavity designs allow access to strong ground-state yield continuous tuning over vibrational overtones of many major the wide gain curve of the pollutants. Professor Ron Hanson diode laser element with a and his group at the High Tempera- very narrow linewidth! ture Gasdynamics, Laboratory in the The tuning range depends on Mechanical Engineering Department the gain element used in the at Stanford University (Palo Alto, Figure 3. Schematic drawing of a Littmann-Metcalf laser cavity. The angle of the tuning mirror selects cavity; at 630 nm the tuning Calif) have been investigating flame the output wavelength and the diffrection grating range is 10 nm, while at 1550 dynamics in combustion chambers acts as a frequency-selective output coupler. The nm the tuning range can be for a number of years. Their aim is to location of the pivot point is critical to obtaining greater than 70 nm. In both produce a compact, reliable sensor continuous tuning without mode hops. cases, the linewidth is less for CO2 and other combustion gases than 300 kHz. The inherent to measure the efficiency of burn its operating temperature. However, efficiencies in the already mature chambers and incinerators. They have because of the semiconductor’s in- diode laser market helped make the focused on sensors based on diode herently broad spectrum, more than external-cavity diode laser an attrac- lasers due to the robustness, reason- one mode will often operate simulta- tive replacement for conventional able cost, and relative ease-of-use of neously. This produces multiple out- dye and solid-state tunable technol- these lasers. Because of this focus on put wavelengths and, therefore, a ogies. As a result, these lasers have compact sources, their work has al- broad spectral linewidth. quickly found use in the applications ways been dictated by the available mentioned above. wavelengths of the diode lasers. In Adapting The Diode Laser the past, this constraint has limited Adding an external optical cavity Some Recent Applications their sensors to detection of the rel- forces the diode laser to operate in a One of the benefits of the tunable atively weak transitions associated single longitudinal mode by creating a diode laser is the wavelength diversi- with vibration-rotation bands. For wavelength-dependent loss within the ty that can be obtained. Almost any comparison, the CO2 linestrengths in laser cavity. In practice, this cavity wavelength that is available in a semi- the 2 µm region are 70 times stronger can be either a Littman-Metcalf or a conductor diode laser can be made than at 1.58 µm. This group has re- Littrow design — two cavity designs into a tunable diode laser. The most cently collaborated with Focused Re- widely used in dye lasers. Both of recent example is in the 2-µm region. search and has used a 2-µm tunable these cavities consist of a diode laser In a partnership with Focused Re- diode laser to obtain survey spectra gain element with one facet antire- search (a subsidiary of New Focus), of CO2 and H 2O at various temper- flection (AR) coated for very low researchers at the Sarnoff Corpora- atures and pressures. A representa- -4 (<10 ) reflectivity. The output from tion (Princeton, N.J.) have demon- tive CO 2 survey spectrum is shown in the AR-coated facet is collimated strated strained InGaAs/InP quantum Figure 5. From these survey spectra, and directed onto a highly dispersive well lam with center wavelengths near they have selected a strong CO2 ab- diffraction grating, 2.02 µm. In the Littrow cavity, the angle of These semi- incidence is such that the beam is conductor diffracted back on itself. The grating diode lasers therefore serves as one mirror in the were AR cavity; tuning is achieved by control- coated and ling the angle of the grating. In the placed in a more common Littman-Metcalf specially de- design, shown in Figure 3, the grating signed exter- diffracts the light toward a tuning nal cavity by mirror, which reflects the desired wa- a team at velength back towards the grating Focused Re- and gain medium. This double-pass search. This scheme, coupled with the grazing in- laser operat- cidence on the grating, results in a ed at room very narrow spectral passband, and temperature therefore excellent wavelength sen- and demon- sitivity, without the use of additional strated con- Figure 4. Tuning range of an external-cavity tunable diode laser using intracavity filters such as an etalon. tinuous tun- a strained InGaAs/InP quantum well as the gain element. This 2-µm Tuning is achieved by adjusting the ing from 1.96 laser was operated at room temperature. offers species-specific measurement of the deposition process, including flux velocities and spatial and tem- poral homogeneity. In an atomic ab- sorption monitor, the vapor absorbs light at the wavelength corresponding to an atomic transition; measurement of the incident and transmitted light yields the evaporation rate. While tunable diode lasers are more expen- sive than other light sources such as hollow cathode lamps, the higher spectral intensity translates into faster data acquisition. In addition, the laser’s modulation capability is useful for the development of a more sensitive, drift-free sensor (Figure 2). The narrow linewidth of a Figure 5. Measured survey spectra of CO2 between 4866 cm-1 laser source enables spatial, tempo- (2.055 µm) and 5118 cm -1 (1.954 µm) taken using a room ral, and velocity mapping of the flux temperature external cavity diode laser. and creates a more complete under- standing of the deposition process. sorption band relatively free from imagine a metrology application This improved understanding can background absorptions that offers needing a stable, narrow-linewidth then be used to increase the process the best opportunity for sensitive source but with a narrow tuning range yield. CO 2 measurements. that would allow FM measurements While the pace of development in As mentioned briefly earlier, each to be made, something not possible the tunable laser field has been quite application tends to require a differ- with a HeNe laser. fast, with a new laser system avail- ent set of features. It is tempting to To satisfy such needs, New Focus able almost every year, each new continue adding features to a pro- uses the same Littman-Metcalf exter- generation has offered customers duct in an attempt to satisfy every- nal cavity design as in their other tun- additional benefits. Additional wave- one. But this approach often leads able diode lawn but removes the lengths such as the 2-µm tunable to an overly complex and expensive coarse tuning feature. diode laser and products focused on instrument that satisfies no one. In One example of an industrial ap- customers needs have allowed cus- many instances, the product with the plication of this laser is the collabo- tomers to do what they could not fewer features is the better product. rative work of Focused Research and have done before and provided sim- This is the approach New Focus Orca Photonics Systems. Physical va- pler, easier-to-use tools for them. decided to use when designing the por deposition techniques such as This focus on customer needs that newest tunable diode laser; the Vor- electron beam, sputtering and mo- continue to expand the market for tex (Figure 1). There are a number of lecular-beam epitaxy (MBE) are criti- the already successful tunable diode applications where the full tuning cal to the manufacture of semicon- laser. range of the tunable diode laser is ductor devices, high-performance al- not needed or used. Examples in- loys, and high-temperature supercon- clude studying a single atom or mol- ductors. The product yield depends ecule, such as rubidium, with a well- on precise control of the flux of the Bob Shine is the marketing manager and known atomic transition. Or, as in different elements during manufac- Tim Day is the vice-president of engineer- the work described above, an opti- ture. Historically, techniques such as ing at New Focus Inc, 2630 Walsh Avenue, Santa Clara CA 95051. Phone (408) 980- mum wavelength can be selected af- quartz crystal monitors, quadrupole 8088; fax (408) 980-8883. ter obtaining an initial survey spec- mass spectrometers, and ion gauges trum with a broadly tunable laser and have been used in process control Originally published in Lasers & identifying a wavelength free from for such applications. Optronics, March 1998, pp 13-14. background sources. Or you could Atomic absorption monitoring
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