A new type broadband near-infrared polarizer was developed for use with near-infrared lasers and light sources. This polarizer utilizes evaporated metal thin films which can be deposited on transparent optical materials to form a sheet type polarizer. Polarizing filters with extinction ratio greater than 100:1 across the spectral band from 780 nm to 1550 nm are available as catalog items with clear apertures of 15 mm, 25 mm, 35 mm. Custom designed broadband polarizers for this spectral band can be fabricated with extinction ratios approaching 10,000:1. The sheet type polarizer material has an acceptance field of +30° about the normal to the polarizer surface.
Extending the "absolute" current continuous frequency tuning range (without any mode hopping)of a GaAlAs laser diode up to 5A (about one order higher than original) has been obtained with an external cavity constructed by a mirror very close to the facet of the diode. Oscillations of the lasing wavelength around some certain desirable line is also observed when the diode heat sink temperature is increased while keening the injected current constant. These results can be applied to laser diode optical pumning and spectroscopy and it significantly improves the absolute continuous frequency tuning ability of the laser diode.
We report here a novel new twin beam laser diode consisting of a high power laser and a low noise laser in one chip, for writing and reading an optical disk, respectively. The maximum light output of the high power laser is typically 45mW CW, and the signal to noise ratio of low noise laser is higher than 90dB under such conditions that the operating power is 3mW CW, the frequency is 20KHz, the band width is 300Hz and the range of optical feedback ratio is 0.1-5.0%. Moreover, the up-side-down mounted device shows a good performance for the thermal cross-talk between high power and low noise laser.
A laser diode collimator based on a conventional four-element design was optimized for application to free space optical communication systems. The collimators were fabricated using conventional techniques and aligned interferometrically. Test results indicate the rms wavefront errors of the assembled collimators ranged from 0.039 wave to better than 0.020 wave rms, exceeding the minimum 0.050 wave requirement. Several advanced collimator designs using aspheres and finished lens molding (FLM) technology are currently under investigation. One design has been fabricated and tested.
The design and expected performance of an all-glass, high-NA, collimator/objective lens pair are described. The numerical apertures are: 0.55 (objective) and 0.40 (collimator). Both lenses are biaspheric and both are designed to perform over relatively broad wavelength and temperature ranges. The as-fabricated, on-axis wavefront error of both lenses is expected to be less than 0.04 waves (RMS, 765-845 nanometers). This is one of the first such glass matched pairs to be designed for use with laser diodes in compact-disc, video-disc and optical-memory applications.
A series of lenses for focusing and collimating the output of semiconductor lasers is described. The description of an anamorphic prism pair to convert the beam cross-section from elliptical to circular is also given. Designs which perform to diffraction limitation, yet lend themselves to low cost manufacturing are specifically considered. Correction for the aberrative effects of diode windows, a situation commonly encountered, is analyzed in detail.
Stand alone, completely assembled high quality Diode Laser Collimator is being produced. Optical components are designed to match a particular model of single mode OW laser diode. Production level test systems are developed to ensure a high quality collimated beam.
The rapid emergence of semiconductor lasers as a viable source is discussed and the problems relating to incorporation of diode lasers into systems are addressed. The characteristics which give rise to significant engineering challenges such as collimating, focusing, temperature stabilization and its effect on wavelength, modulation and stable current requirements will be identified and solutions offered. A turnkey diode laser system, the Diolite 800, is reviewed and its applicability to R&D and prototype use is pointed out. Off the shelf laser diode drivers (LDD's), collimating optics and thermoelectric coolers are mentioned and specific examples will be given.
Free space communication systems of today are primarily based on existing microwave and millimeter wave technology. However, recent advances in diode laser technology have provided the impetus for considerable development of free space optical communication systems. A spectral multiplexing beam combiner built for use with diode lasers is described. The design emphasizes a modular approach while minimizing weight and size and demonstrates the technology required for use on free space optical communication satellites. Individual diode laser beams are collimated and aligned before being incoherently combined using narrow bandpass optical filters. The composite output beam is shaped and expanded before leaving the transmitter. Overall wavefront performance, exclusive of sources, was predicted to be 0.05 rms waves at 830 nanometers for the average of the emitted wavefronts.
Optical power combining with laser diodes is receiving considerable attention. The objective is to noncoherently combine the diffraction limited beams of N diodes into a single beam, thereby increasing the field intensity by N. To accomplish this it is necessary to operate the diodes at different wavelengths in order to achieve the required combining. A question then arises as to the most efficient way to encode the diodes for maximum digital performance. In this paper, three different system architectures are considered, with combining achieved by dichroic mirrors operating in conjunction with a pulse position modulated (PPM) format. The basic criterion is maximization of data rate with increasing number of diodes, while maintaining a fixed decoding bit error probability. The three systems are: 1) power combining into a single pulse, followed by PPM encoding; 2) parallel channels, in which each diode is separately PPM encoded; and 3) color-coding, in which diodes are encoded over a common wavelength-time slot alphabet. All systems use beam combining, but the latter two require wavelength splitting as well. Data rate performance is evaluated as a function of the number of diodes, mirror combining losses, PPM alphabet size, and the operating optical SNR.
A high resolution distance meter was built using a laser diode(LD) and an avalanche photodiode(APD). The laser diode LD was modulated directly by an RF current whose frequency f1 is 300 - 500 MHz, in order to obtain high resolution. The modulated light beam emitted from LD was sent to the target, returned, and detected by APD. The heterodyne detection was done in APD without any mixer in the RF region to measure the phase shift in the low frequency region. In APD, another RF current whose frequency f2 is f1 + δf was supplied together with an ordinary DC bias. The beat signal δf of these two RF signals was generated by modulating the amplification of APD. The phase shift between this beat signal and the reference signal of frequency Of was measured. The distance between the apparatus and the target was calculated from the phase shift, the modulation frequency of LD and the time delay in the apparatus. Although the phase shift was measured in the low frequency region, that is, the beat frequency region, a high resolution of about 0.1 mm was obtained easily, because of the short modulated wave-length in the RF region. The heterodyne detection in APD brings us the following merit. First, the apparatus is simplified, since the output signal of APD can be handled without any particular consideration made for high frequency region, for example, the impeadance matching and so on. Second, a high sensitivity is obtained without any high performance mixer. It is possible to measure a distance even when the intensity of the reflected light is below 1 nW. It means that a distance up to about 10 - 20 m can be measured without any reflector, with high resolution.
Single laser elements will always have an upper limit of power. Progress in technology continuously demands higher power. This could be satisfied by developing a parallel but generic technology of laser beam combiner. Current state of technology is summarized from this "generic" point of view, categorizing the specific approaches that exploit the various parameters (or properties) of light beam and/or laser cavities.
Free-space optical communication systems using laser diode transmitters exhibit data rate limitations imposed by the diode's relatively low average output power. Power summing of multiple laser diodes through polarization beam combining is an effective method for increasing the on-axis far-field intensity of a laser communication transmitter. When using such a combining scheme, laser diodes having the highest degree of polarization purity should be used in order to obtain the maximum output (combined) power. Therefore to examine the applicability of polarization combining systems incorporating laser diodes the polarization characteristics of the devices must be determined. Polarization purity - total output power relationships for two types of single-stripe laser diodes are presented as are similar measurement results for two types of phase locked laser diode arrays (each of these devices has been individually analyzed to determine its candidacy as a free-space communication system transmitter). A simple combining system using the single-stripe diodes or the phase locked laser diode arrays is described with respect to its use in a free-space optical communication system. A polarization beam combiner is described and analytical expressions for the total transmitted power and signal extinction ratio are developed.
The phase front deviations present in the output radiation patterns of a variety of AlGaAs diode lasers has been experimentally determined in order to assess their utility as optical sources in diffraction-limited applications. Experimental interferometric characterizations have shown that the output of index-guided devices have smaller aberrations than do gain-guided laser structures.