Laser, diode

The principle of optical triangulation has been known since the time of the early Greeks, and indeed optical triangulation has been used for hundreds of years in applications such as surveying, camera auto-focus and even smart-bombs. With the advent of low-cost, compact electro-optic components such as lateral-effect photodetectors, diode lasers and micro-optics, laser-based triangulation sensors can now be employed for applications that were, heretofore, considered uninspectable. 

Matsuura S, Chen P, Blake G A, Pearson J and Pickett H M 1999 A tunable, cavity-locked diode laser system for terahertz photomixing IEEE Micro. Theory Technol. 48 380-7... 

Soper S A and Legendre B L Jr 1998 Single-molecule detection in the near-IR using continuous-wave diode laser... 

Muller R et al 1996 Time-resolved identification of single molecules in solution with a pulsed semiconductor diode laser Chem. Phys. Lett. 262 716-22... 

Figure C2.16.2 shows tire gap-lattice constant plots for tire III-V nitrides. These compounds can have eitlier tire WTirtzite or zincblende stmctures, witli tire wurtzite polytype having tire most interesting device applications. The large gaps of tliese materials make tliem particularly useful in tire preparation of LEDs and diode lasers emitting in tire blue part of tire visible spectmm. Unlike tire smaller-gap III-V compounds illustrated in figure C2.16.3 single crystals of tire nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growtli of device stmctures. Substrate materials such as sapphire and SiC are used instead.

Figure C2.16.ll. Changes in the tlireshold eurrent density of diode lasers resulting from new stRieture eoneepts. A homojunetion diode laser was first demonstrated in 1962. SH and DH stand for single and double heterostaieture, respeetively. The best laser perfonuanee is now obtained in quantum well (QW) lasers.

The history of tire diode laser illustrated in figure C2.16.11 shows tire interiDlay of basic device physics ideas and teclmology. A new idea often does not produce a better device right away. It requires a certain leap of faitli to see tire improvement potential. However, once tire belief exists, tire teclmology can be developed to demonstrate its validity. In tire case of diode lasers, tire better teclmology was invariably associated with improved epitaxial growtli. 

Coldren L A and Corzine S W 1995 Diode Lasers and Photonic integrated Circuits (New York Wiley)... 

Even more remarkable is tlie fact tliat tliese infrared diode lasers have sufficient resolution to measure tlie Doppler... 

Figure C3.3.4 shows a schematic diagram of an apparatus tliat can be used to study collisions of tlie type described above [5, 9,12,16]. Donor molecules in a 3 m long collision cell (a cylindrical tube) are excited along tlie axis of tlie cell by a short-pulse excimer laser (typically 25 ns pulse widtli operating at 248 mil), and batli molecules are probed along tliis same axis by an infrared diode laser (wavelengtli in tlie mid-infrared witli continuous light-output...

A tunable diode laser operating at 4.3 pm is used to probe the P and/or R branch bands of tire following transitions. 

The data obtained in tlie infrared-diode-laser-probe studies described above provides quenching infonnation at a given substrate donor energy E. By varying tlie laser excitation wavelengtli for production of vibrationally hot... 

Sedlacek A J, Weston R E Jr and Flynn G W 1991 Interrogating the vibrational relaxation of higly exoited polyatomios with time-resolved diode laser speotrosoopy CgHg, CgDg, and CgFg + COj J. Chem. Rhys. 94 6483-90... 

Flynn G W and Weston R E Jr 1993 Diode laser studies of oollisional energy transfer J. Phys. Chem. 97 8116-27... 

The practical use of photochromic dyes as memory layers in erasable and rewritable data storage disks fails not only because of their physical limitations (lacking sensitivity, insufficient stabiHty, low number of cycles), but also because the diode lasers required for switching in the visible range (wavelength between 450 and 600 nm) and the uv-range (around 350 nm) are not available. 

Lasers having wavelengths ranging from the deep uv to the near infrared have been used in Raman spectroscopy. In industrial laboratories, the most common laser is the Nd YAG operating at 1.06 pm. Increasingly, diode lasers or other lasers operating in the 750—785-nm region are encountered. These... 

The high performance of modem spectrographs means that low power lasers can be used as excitation sources. These are typically 10—100-mW devices which are air-cooled and can be operated from 117-V a-c lines. In the green, the Ar" (514.5-nm) laser remains the most popular but is being challenged by the smaller and more efficient frequency doubled Nd YAG (532-nm). In the nir, diode lasers (784-nm) and diode-pumped alexandrite... 

The light source for excitation of Nd YAG lasers may be a pulsed flashlamp for pulsed operation, a continuous-arc lamp for continuous operation, or a semiconductor laser diode, for either pulsed or continuous operation. The use of semiconductor laser diodes as the pump source for sohd-state lasers became common in the early 1990s. A variety of commercial diode-pumped lasers are available. One possible configuration is shown in Figure 8. The output of the diode is adjusted by composition and temperature to be near 810 nm, ie, near the peak of the neodymium absorption. The diode lasers are themselves relatively efficient and the output is absorbed better by the Nd YAG than the light from flashlamps or arc lamps. Thus diode-pumped sohd-state lasers have much higher efficiency than conventionally pumped devices. Correspondingly, there is less heat to remove. Thus diode-pumped sohd-state lasers represent a laser class that is much more compact and efficient than eadier devices. 

Fig. 11. Schematic diagram of semiconductor diode laser where the junction is ca 1 ]lni. Other dimensions are <1 mm.

Fig. 12. Details of an aluminum gallium arsenide semiconductor diode laser.

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

Fig. 13. Availability of semiconductor diode lasers where represents the Al Gaj As system, UIn..Gaj As Pj, and I AlInGaP. ...

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

R. Gdsar, H. Preier, G. Schmidtke, and G. ResteUi, eds.. Monitoring of Gaseous Pollutants by Tunable Diode Lasers, D. Reidel Publishing Co., Dordrecht, Germany, 1987. 

---