We present coding methods for protecting against tampering of write-once optical disks, which turns them into a secure digital medium for applications where critical information must be stored in a way that prevents or allows detection of an attempt at falsification. Our method involves adding a small amount of redundancy to a modulated sector of data. This extra redundancy is not used for normal operation, but can be used for determining, say, as a testimony in court, that a disk has not been tampered with.

We report on a new method for fast amplitude modulation of blue 429-nm radiation that is produced by frequency doubling of a GaAlAs diode laser in a monolithic KNbO3 resonator. The diode-laser frequency was locked to the resonator by use of the Pound–Drever technique and FM sidebands that were generated by superimposition of an rf current on the diode-laser injection current. We modulated the blue power output by controlling the amplitude of this rf current, which controls the FM spectrum of the diode laser and thus the fraction of the diode-laser power that is coupled to the frequency-doubling resonator. Using this technique, we have produced blue pulses of 40-mW peak power with rise and fall times of 5 ns and pulse durations as short as 10 ns. Nonperiodic pulse sequences produced with this technique were used to write data on magneto-optical disks.

We present a comprehensive theory for heterodyne absorption spectroscopy with phase-modulated light. The general equations presented allow for an arbitrary modulation index and an arbitrary modulation frequency. We use this description for three purposes: First, we review the special cases of so-called frequency modulation and wavelength modulation spectroscopy. Second, we present the additional case of large-index, high-frequency modulation. Third, we present an overview of how the absorption signal depends on the experimental parameters of modulation frequency and modulation index. This overview may be helpful to experimentalists in choosing these parameters, for it provides a systematic understanding of how moving around in parameter space changes certain features of the signal, while leaving other features invariant.

Frequency doubling of diode laser radiation was achieved by operating an AR-coated diode laser in an extended laser cavity which contained a monolithic potassium niobate frequency doubling resonator as an intracavity element. Extended cavity operation ensured that the diode laser would oscillate only at frequencies that were resonant with the intracavity resonator. A diffraction grating was used to ensure single mode oscillation at the wavelength needed for noncritically phase-matched second harmonic generation. At 100 mA of injection current to the GaALAs diode laser, 14 mW of 429 nm light were produced.

There has been considerable interest recently in the application of laser arrays to optical data storage. Performance enhancements that are possible include Direct-Read-After-Write or Direct-Overwrite using a 2-element array, increased track density using parallel read with a 3-element array, and high-data-rate parallel recording with an array of n lasers. One of the major considerations is the spacing of the lasers in the array. A large spacing makes laser packaging easier but puts demands on the optical system : large flat field, control of vignetting, etc. As the spacing is reduced, however, crosstalk between the clements will increase, and a large factor in determining the optimum spacing will be the maximum allowable crosstalk.

Compact, efficient, diode-laser-based blue light sources are useful for many applications including optical storage and laser printing. Blue light can be generated by frequency doubling the output of reliable, high-power 860-nm lasers in potassium niobate. The Second Harmonic Generation (SHG) conversion efficiency can be substantially increased by placing the nonlinear crystal inside of a resonator in order to increase the infrared intensity in the crystal. 1 , 2 , 3 , 4 The build-up of a high infrared intensity and efficient generation of blue output requires precise frequency matching of the diode laser and the SHG resonator. Previously, electronic locking 1 and weak optical feedback techniques 2 , 3 , 4 have been used to maintain this frequency matching. Electronic locking has the disadvantage of requiring an optical isolator to prevent destabilization of the laser frequency by light scattered from the resonator. Weak optical feedback has the disadvantage of requiring electronic control of the phase of the optical feedback as well as precise control of the feedback amplitude. 3 , 4 We describe the operation of an antireflection-coated GaAlAs diode laser in an extended laser cavity that contains the SHG resonator as an intracavity element. The use of an antireflection coating on the laser and strong optical feedback is intended to suppress coupled-cavity effects and produce stable operation of the laser at a single frequency that is resonant with the SHG resonator. With the extended cavity we have obtained blue output powers of up to 12.5 milliwatts.

The authors report blue, green and yellow upconversion laser action in Er3+:YLiF4 following pumping at 1.500 mum below 80 K. For the blue transition the ratio of the output frequency to the input frequency is 3.3, the highest yet reported for an upconversion laser. In spite of the multiple upconversion steps, the thresholds for laser action were less than 80 mW. It is proposed that the nonlinear upconversion pumping proceeds via a sequence of several pairwise cross-relaxation steps.

We have shown that multiple-step upconversion pumping of Er:YLiF 4 at 1.50 μm into the metastable 4 I 13/12 level excites blue (469.7 nm), yellow (560.6 nm), and green (551 nm) laser emission from the 2 P 3/2 , 2 H 9/2 , and 4 S 3/2 levels, respectively. This requires the energy of at least five erbium ions pumped into 4 I 13/2 to populate the upper level for the blue transition, four for the yellow, and three for the green. We believe that the mechanism for upconversion consists of a sequence of pairwise energy transfer steps involving several metastable excited levels. Monolithic laser crystals, ~3 mm long and doped with 1% Er 3+ , were prepared with mirrors applied directly to the crystal surfaces. The thresholds for laser action were 25 mW, 50 mW, and 80 mW for the yellow, green, and blue lasers, respectively. This shows that high order excitation processes in upconversion lasers involving long lived (here ~10 ms) metastable levels can be rather efficient. Output powers of these lasers at 30 K reached 10 mW at 551 nm for 350 mW of pump light, 10 mW at 560.6 nm for 140 mW of pump light, and 0.7 mW at 469.7 nm for 350 mW of pump light. Given the quantum defect between the input and output photons, these are good overall efficiencies. Lasing was observed up to 80 K. This work extends an earlier study, 1 in which we obtained upconversion laser output at the three wavelengths by pumping in the near IR at 0.80 μm and 0.97 μm, to the observation of even higher order upconversion processes.

Blue upconversion laser operation in Tm:YLiF4 at 450.2 nm on the 1D2→3F4 transition, and at 483.0 nm on the 1G4→3H6 transition has been achieved using cw pumping with Ti Sapphire and DCM dye lasers. The 450.2-nm laser was excited by sequential two-photon absorption with 784.5-nm and 648-nm laser sources and was operated at temperatures up to 70 K. The 483.0-nm laser was pumped with a single red-dye laser using a wavelength resonant only with absorption from a metastable intermediate state but not with absorption from the ground state. With this avalanche absorption pumping scheme, 483.0-nm laser emission was observed up to 160 K.

We demonstrate a very high-power AlGaAs single quantum well GRINSCH ridge laser operating in a diffraction-limited fundamental transverse mode up to 360 mW at a wavelength of 856 nm. The maximum power output of the laser reached 425 mW and was limited by thermal saturation of the device and not by catastrophic optical mirror damage. These lasers not only exhibit very high power levels, but also show excellent reliability at high output power levels. The extremely high, CW fundamental mode power combined with very low-intensity and optical low-phase distortion as well as low astigmatism render this ridge waveguide laser very suitable for optical storage systems, printers, and direct frequency doubling. These devices have been successfully used for direct frequency doubling of their output in a resonant KNbO3 cavity yielding 41 mW of blue radiation at 428 nm.

A single-mode diode laser at 802 nm was used to pump the 4I15/2➝4I9/2 transition of YLiF4Er3+, producing up to 2.3 mW of laser output at 551 nm (4S3/2 - 4I15/2) at 50 K; 0.5 mW of green output were obtained at 77 K. The upconversion mechanisms were found to depend on the pump wavelength. Energy transfer crossrelaxation is generally the dominant mechanism, but at certain pump wavelengths two-step absorption becomes important. Lasing at 560.6 nm from the 2H9/2 level is also reported. Although the upconversion excitation of 2H9/2 requires more than two pump photons, the efficiency of this laser is comparable to that at 551 nm which uses a two-step process.

Presently, optical data storage systems are based on the use of near infrared GaAIAs diode lasers operating at wavelengths of 780 - 830 nm. Substantial advances of the areal storage density are possible by using lasers in the blue - green wavelength range. Such blue - green laser sources can be developed by making use of nonlinear optical processes for frequency upconversion of existing state-of-the-art III-V semiconductor lasers. These approaches circumvent the severe fundamental material fabrication problems associated with the potential development of blue semiconductor injection lasers.