Dec. 1, 1999
High-power solid-state laser targets cable TV and dense WDM; Gratings reduce reflection from silicon substrates; Compression minimizes thermal lensing in disk laser, and more.

High-power solid-state laser targets cable TV and dense WDM

Researchers at Laser Power Corporation (San Diego, CA) have constructed a 130-mW, 1550-nm solid-state laser source intended for use in cable television and dense wavelength-division-multiplexing (WDM) applications. The device was fabricated using a 1-W, 980-nm indium gallium arsenide (InGaAs) pump diode laser and a laser cavity consisting of an erbium ytterbium codoped phosphate glass disk, a lithium niobate phase modulator, and a Polarcor Fabry-Perot etalon. The 1-mm-thick modulator, 1-mm-thick glass disk, and 100-µm-thick etalon were bonded together to form a single structure. The modulator enables tuning of the laser wavelength and chirping of the approximately 22-kHz linewidth to defeat stimulated Brillouin scattering. It also acts as a heat spreader, reducing peak temperatures and stresses in the cavity by about 50%, thereby enabling high pump- and output-power values. The etalon serves to fix the polarization state of the laser output. An optoelectronic feedback loop reduces relative intensity noise by as much as 35 dB. The size of the entire device, including additional optical elements, is 0.5 x 1.5 x 2.0 in. Contact Timothy Boyd at [email protected].

Gratings reduce reflection from silicon substrates

Thin-film coatings can suppress Fresnel reflection for devices such as solar cells and electro-optical instruments, but they can have problems with adhesion, thermal mismatch, and the stability of the thin-film stack. Researchers at Tohoku University (Sendai, Japan) have fabricated a two-dimensional subwavelength structured surface on a crystal silicon substrate that dramatically decreases the reflectivity and is more stable than a multilayered thin film.

The researchers started with a 200-µm-thick crystal silicon substrate and coated it with a 400-nm-thick electron-beam resist. They drew a pattern on the resist with an electron beam, then developed it to form a tapered resist. The pattern was then transferred through fast atom-beam etching with sulfur hexafluoride and the resist removed. That produced a conical grating with a period of 150 nm and grooves 350 nm deep. The researchers measured reflectivity of wavelengths from 190 to 2500 nm and found a marked decrease, particularly between 200 and 1000 nm, encompassing the visible region. At 400 nm, for example, reflectivity dropped from 54.7% for the silicon substrate to 0.5%. Contact Y. Kanamori at [email protected].

Compression minimizes thermal lensing in disk laser

Diode-pumped solid-state lasers operating at TEM00 are limited by degradation of beam quality brought on by pump-induced thermal lensing. This was the driving force behind the development of the disk laser, in which heat is conducted one-dimensionally through the back disk face. But there are transverse thermal gradients even in disk lasers, and the gain crystal is susceptible to deformation. By clamping a disk laser between a thick sapphire window and a copper heat sink, researchers at the University of Toronto (Toronto, Ontario, Canada) and the University of Rochester (Rochester, NY) have successfully minimized thermal lensing through compression.

Applied pressure can, in theory, exactly cancel forces generated by pump-induced thermal expansion. The researchers fabricated a 1.3-µm-emitting Nd:YVO4 (vanadate) laser having a 5 x 5 x 0.4-mm crystal and applied a pressure of just less than 700 MPa, pumping the laser with a 1.5-mm-diameter, fiber-coupled diode-laser array. The disk laser reached 19 W of TEM00 output power (M2 of 1.3)—limited by the pump, not by thermal lensing. An additional benefit is an increased thermal fracture limit. Using this technique, the researchers believe disk lasers can be scaled to greater than 100 W of TEM00 output. Contact R. J. Dwayne Miller at [email protected].

Holographic method records submicron pixels

To make a holographic memory system inexpensive enough to be commercially competitive, it is important to be able to record pixels less than a micron in size. Researchers at the California Institute of Technology (Pasadena, CA) have recorded submicron pixels in lithium niobate doped with iron. The high storage density is a result of both the wide recording bandwidth of the material and the researchers' use of the phase-conjugate readout method. They began with a phase-conjugate memory module that consisted of a spatial light modulator (SLM), a pixel-matched detector array, a photorefractive crystal, and an array of laser diodes. A signal beam passes through the SLM and interferes with a plane-wave reference beam to record the pattern. A reference beam reflecting off a mirror on the opposite side of the crystal propagates in the opposite direction of the recording reference beam to retrieve the information. By having multiple laser sources at slightly different angles, several holograms can be recorded in the same crystal. The researchers used a resolution mask with pixels ranging from 2 to 0.2 µm2 and recorded a strong image. Contact Demetri Psaltis at [email protected].

Thulium-doped gallium nitride produces blue light

A research group at the University of Cincinnati Nanoelectronics Laboratory (Cincinnati, OH) has produced blue light using an alternative to gallium nitride (GaN) based light-emitting diodes (LEDs). Although also based on GaN, these devices are not LEDs, but are electroluminescent devices (ELDs) in which thulium (Tm)—a luminescent rare-earth dopant—provides the light. To create the ELDs, a 1-µm-thick film of Tm-doped GaN was grown over a GaN buffer layer on silicon using molecular-beam epitaxy, resulting in a Tm concentration of 1019-1020/cm3. Indium tin oxide electrodes were deposited. A ring-shaped ELD with an area of 7.64 x 10-4 cm2 was constructed, along with a dot structure having a 0.125-cm2 area.

When positively biased at 55-83 V, the devices emit light at 477 nm, with a spectral full width at half maximum of 7.5 nm, and 801-nm light with 9.9-nm spectral width. Reversing the bias lowers but does not eliminate light output. The devices exhibit a sharp threshold at 0.8 mA; at greater than 2.3 mA, the efficiency is essentially constant. Absolute efficiency has not yet been determined. The researchers also have produced red light from praseodymium-doped GaN and green light from erbium-doped GaN. Contact Andrew Steckl at [email protected].

Diffractive optics allow achromatic correlation

Researchers at the Universitat de València (Burjassot, Spain), the Universitat Jaume I (Castelló, Spain), and the Universität Erlangen-Nüremburg (Erlangen, Germany) have developed an all-incoherent optical correlator that produces a point-spread function (PSF) whose scale is independent of wavelength over much of the visible spectrum. This allows a wide variety of color optical operations to be performed using white light.

The hybrid diffractive-refractive imaging system contains two continuous-relief diffractive elements constructed using half-tone mask technology. One of the diffractive elements must be placed beyond the traditional output plane; an additional refractive element is then required to produce a real image. As in most achromatic optical systems, a low residual scaling chromatic error remains in this correlator: over a wavelength range of 480-660 nm the PSF changes in scale by 2.5%. This compares to an uncorrected scale change of at least 30%. When illuminated by white light from a xenon lamp, the device produces high autocorrelation signals for objects of varying colors. Contact Pedro Andrés at [email protected].

Interferometer with few parts measures distance

A distance-measuring interferometer developed by a group at Tohoku University (Sendai, Japan) may be the simplest ever invented. In its basic form, it consists of a laser, a detector, a mirror, and nothing else (although a lambda/4 plate and polarizing beamsplitter can be added for isolation). The trick is in the detector itself: made of silicon, it is constructed to be so thin that it is almost perfectly transparent to light. When inserted into a laser beam that is reflected back on itself to create a standing-wave pattern, the 40-nm-thick photodiode absorbs 0.6% of the light and, when either the mirror or detector is moved, maps the periodic intensity profile of the standing waves.

Appearing yellowish to the eye, the detector has interdigitated p+ and n+ regions, is 1 mm square, and has a sensitivity of 0.01 mA/W at 633 nm. Fresnel surface reflections cause the transmission to be 70% and the standing waves to appear nonsinusoidal; antireflection coatings should eliminate these effects. The researchers have mapped standing waves for a mirror movement of 20 mm. Acceptance angle of the device is ±0.3 mrad about the normal, at which point the detector begins to intersect more than one standing wave at a time. Adding a second detector with a pi/2 phase-shift plate will allow direction-sensitive distance measurement. Contact Minoru Sasaki at [email protected].

Fabrication scheme reduces PIN dark current by order of magnitude

Researchers at Bell Labs, the research arm of Lucent Technologies (Murray Hill, NJ), have developed a wafer-bonding method that has allowed them to decrease the dark current in silicon/indium gallium arsenide (Si/InGaAs) PIN photodetectors by an order of magnitude. The previous bonding method involved placing the Si and InGaAs wafers in contact under 1 to 10 MPa of pressure at a temperature of 650°C, which produced substantial stress at the heterointerface. In the new method, the two wafer surfaces were first cleaned and flattened within atomic tolerances such that x-ray photoemission spectroscopy indicated less than 0.2 monolayers of carbon, oxygen, fluorine, and chlorine on the surfaces, and atomic force microscopy measured root-mean-square roughnesses of less than 1.5 Å. The two surfaces were then placed in contact for Van der Waals bonding under a weight of about 5 lb. The 650°C temperature annealing was not applied in this case until after the weight was removed, which reduced thermal expansion stress and enabled elastic accommodation of thin device layers. The improved heterointerface resulted in a minimum dark current of 180 pA under a reverse bias of -10 V, representing an order-of-magnitude improvement over the 2-nA minimal dark current of devices fabricated to date. Contact Barry Levine at [email protected].

Organic LEDs produce saturated, bright red light-emission spectra

Researchers at the Optical Sciences Center (OSC) of the University of Arizona have fabricated efficient organic light-emitting devices with saturated red light-emission spectra. The interesting aspect of these devices is that they are hybrids and use a more stable cathode such as aluminum (Al) instead of calcium or magnesium. Current state-of-the-art devices, based on the phosphorescent dopant platinum octaethylporphyrin and magnesium silver alloy cathode, emit saturated red light with luminance of about 1 cd/m2 at an external efficiency of 5.6%. These same devices also have an external efficiency of 2.2% at a luminance level of 100 cd/m2, a level more suited for display applications. The hybrid devices with Al cathode allow for the emission of saturated red color and have external quantum efficiency approaching 9% at forward light output of about 14 cd/m2. At about 100 cd/m2, these devices have an external quantum efficiency approaching 7%. More optimization is also underway to push the performance even higher. Contact Ghassan E. Jabbour at [email protected].

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