Volume holographic gratings (VHG) provide the capability of narrowing and stabilizing the wavelength of
semiconductor lasers by forming an external cavity laser (ECL). The standard configuration of these ECL's is to use a
collimating lens followed by the VHG to provide feedback to the resonator and lock the wavelength. In this
configuration both elements have to be carefully aligned with tolerances in the sub-µm and mrad range. The present
paper presents a fast-axis collimation lens (FAC) with integrated VHG for locking a laser diode bar. Besides the
advantage of having only a single element, the integrated element is also less sensitive to alignment tolerances with
respect to the locking due to the large divergence angle of the uncollimated array compared to a collimated array. Using
a standard AR coated array with 19 emitters an output power of 67.4 W was achieved. The spectral bandwidth was
within 1 nm over the whole power range. Due to high stability requirements in this application, glass was chosen as the
VHG material. Though the refractive index is low compared to standard FAC lenses, the design and manufacturing
process of the lens still guarantees a diffraction limited collimated beam.
The conventional approach for measuring volume holographic gratings typically requires measuring the transmitted
and diffracted beams simultaneously while varying the angle of incidence. To obtain the spectral response a tunable
laser is used with a fixed angle of incidence. In the former case, the motion of the diffracted beam from reflection
gratings requires the detector to move with it, or otherwise the range of angles must be limited. Furthermore it is
often difficult to separate the diffraction from the specular surface reflections, increasing the error of the
measurement. In the latter case, a high cost tunable laser is required.
We describe methods for the measurement of volume holographic gratings with high spatial resolution. A fixed
wavelength laser in conjunction with a high-resolution digital camera is used to measure the angle selectivity of the
transmitted beam only. The measured data are fit to a model of the grating diffraction combined with the cyclical
interference from the surface reflections in order to increase the accuracy when measuring uncoated gratings. The
system is capable of simultaneously measuring diffraction efficiency, loss, surface reflectivity, Bragg angle, and
grating tilt in one plane, with a resolution of better than 250 micrometers over the area of a 45 mm by 35 mm wafer.
Through a transformation utilizing the de-phasing term of the coupled wave analysis of thick hologram gratings, the
wavelength selectivity is also obtained.
The use of LiNbO3 based Volume Holographic Gratings (VHGs) to provide spectrally filtered feedback to a semiconductor laser diode was documented in the mid 1980s1, however issues with long term stability had left this technology on the sidelines. Photo-sensitive glass based VHGs do not exhibit long term aging or thermal/photo bleaching effects, and therefore have enabled a new type of External Cavity Laser (ECL). This highly manufacturable "hybrid ECL/DBR" (HECL) laser utilizes precision VHGs and has been used to create high performance lasers with spectrally tailored output. Lasers with fiber coupled output powers in excess of 4.2 W and spectral line widths of less than 0.15 nm have been demonstrated. Additionally, multi-mode lasers have been developed for High Resolution Raman Spectroscopy that exhibit spectral line widths below 0.06 nm (i.e. < 1 wavenumber) with fiber coupled output power in excess of 350 mW. The use of glass based VHGs provides HECL laser wavelength stabilization of better than 0.01 nm/oC, and allows the production of lasers at virtually any wavelength between 650 nm - 2400 nm.
Near-infrared emission from atmospheric OH radicals is known to severely affect astronomical observations. Until now, only complex dispersive instruments were partially capable of removing this unwanted background, which is composed of hundreds of narrow emission lines. Recent development in photosensitive glass and holographic recording technologies now allow the elaboration of filters with a large number of narrow reflecting bands well matched to OH lines. This technology shows promise for removing many tens of lines in the J, H, and K bands. That would result in a many fold increase in imaging and low resolution signal-to-noise ratio. Filters with 10 lines have been tested and show the appealing possibilities of these new devices.
We propose and demonstrate a widely tunable optical filter, realized by angle tuning a volume holographic grating. The volume holographic grating selectively drops a narrow portion of the signal bandwidth into a fiber while passing through the rest of the signals. The demonstrated 1510- to 1590-nm tuning range covers the entire erbium-doped fiber amplifier (EDFA) C band, with small bandwidth variation and low insertion loss (<1 dB). Group delay, polarization-dependent loss, and polarization mode dispersion are measured and investigated for optimizing the filter characteristics.
In this paper we report on a holographic method used to record fast events in the nanosecond time scale. Several frames of the expansion of shock waves in air and in a polymer sample are recorded holographically in a single shot experiment, using a pulse train generated with a single pulse from a Q-switched Nd:YAG laser. The time resolution is limited by the laser pulse width, which is 5.9 ns. The different frames are recorded on the holographic material using angle multiplexing. Two cavities are used to generate the signal and reference pulses at different angles. We also present a method in which the recording material is replaced by a CCD camera. In this method the holograms are recorded directly on the CCD and digitally reconstructed. The holograms are recorded on a single frame of the CCD camera and then digitally separated and reconstructed.
KEYWORDS: Holograms, Holography, Diffraction, Signal to noise ratio, Polymethylmethacrylate, Data storage, Multiplexing, Molecules, Holographic materials, Binary data
Phenanthrenequinone (PQ)-doped poly(methyl methacrylate) (PMMA) is a photopolymer holographic recording material that can be made in large thicknesses and does not exhibit any shrinkage. Hologram are made permanent by a post-recording diffusion amplification process. PQ-doped PMMA thus exhibits may properties which make it an ideal candidate as a high- density read-only holographic data storage medium. Recently we reported the holographic recording characteristics of phenanthrenequinone (PQ) doped PMMA. We now demonstrate the holographic storage of binary data in this material. Shift multiplexing is used to store multiple holographic data pages in millimeter thick material samples. Each data page consists of a random array of binary 40 micron pixels. Recording is performed near the Fourier transform plane of the data mask, allowing high bit-densities to be achieve.d The signal to noise ratio of stored data pages is used as an indication of hologram quality and data integrity. Bit density limitations are discussed with respect to the current performance of the recording material.
Optical holographic correlators can perform many correlations simultaneously. Because the output plane must be divided among the individual templates in the system, for many systems shift-invariance limits the number of correlation templates than can be stored in one correlator. When the system is completely shift-invariant, the correlation peak from one correlator can shift into an area that has been reserved for a different template; in this case, a shifted version of one object might be mistaken for a well-centered version of a different object. This paper describes a technique to control the shift-invariance of a correlator system by moving the holographic material away from the Fourier plane.
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