This chapter discusses compact blue–green laser sources. This chapter illustrates that there are many possible approaches to an all-solid-state blue–green laser source. Most are based on converting the output of infrared diode lasers, which are well-developed, high power, reliable, and relatively inexpensive sources, to shorter wavelengths using non-linear conversion techniques. These technologies are in various stages of development: some have been commercially available for a few years, some are in the product development stage, and some are active areas of research. A promising approach to smaller and cheaper sources is non-linear conversion of an infrared diode laser in a QPM waveguide. Active development of devices based on this technique makes it likely that a compact device producing several milliwatts in blue should not be far off. Similarly, up-conversion fiber lasers might provide tens of milliwatts of blue-green power in an inexpensive, although somewhat larger device. Moreover, research on wide-bandgap semiconductor materials and device fabrication has accelerated recently with the demonstration of direct injection lasers at room temperature. These advances make it possible that blue diode lasers might be commercialized in the not-too-distant future, which would provide the ultimate in compact and low-cost blue laser sources as required for high volume, consumer-based applications.

Advances in diode-laser-based nonlinear blue sources as well as blue injection lasers have generated interest in using these sources for optical data storage. Recording results using a 429-nm, frequency doubled-diode laser are illustrative in understanding the requirement on these laser sources. A very stringent requirement on the laser source is the low cost necessary for high market penetration of the storage device.

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.

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.

A blue output power of 1.2 mW at 425 nm was generated using a periodically poled KTP waveguide for frequency doubling of a grating tuned, extended-cavity laser containing an antireflection-coated GaAlAs SQW-GRINSCH laser diode as the gain element. A normalized conversion efficiency of 127%/W cm2 was achieved in the extended-cavity configuration.

Optical recording at 2.4 Gbit/in2 has been demonstrated at both 488 nm (argon-ion laser) and at 429 nm (doubled diode laser) using TbFeCo and CoPt multilayers on flat glass media with a soft sampled servo tracking system. The data was written using pulse width modulation (PWM), 2/3 (1, 7) run length limited (RLL) encoding, and detection accomplished using a dual clock channel. Edge shifts of 3.5% of the timing window and jitters of 7% of the timing window were observed. At a data rate of 2 MBytes/s and with data written on adjacent tracks, a bit error rate of 10-5 with noise margins of 10 dB and 8.5 dB were measured at 488 nm and 429 nm, respectively.

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.

Single-mode infrared diode lasers produce high output powers with long lifetimes, 1,2 and nonlinear optical processes can be used to convert the output of these sources to the blue. For instance, 430–nm light can be generated by frequency doubling the output from a GaAlAs diode laser in potassium niobate. To increase the efficiency of blue generation, an external enhancement resonator can be used to increase the fundamental intensity inside the nonlinear crystal.3–8 For stable operation of such a system, a match of the output frequency of the diode laser and a resonant frequency of the external cavity must be maintained, since frequency differences will result in substantial fluctuations of the infrared power inside the resonator and thus in the blue output Passive optical feedback has been used to lock the diode laser frequency to a resonator frequency,3 with the requirement that the phase of the optical feedback be electronically controlled.7 As an alternative, we have used an electronic feedback technique to frequency lock the diode laser frequency to a resonator frequency to produce 54 mW of stable blue output.8

The pump absorption of a 946 nm Nd:YAG laser was improved by resonating the GaAlAs diode-laser pump light in the 0.33 mm long, monolithic laser resonator, whose maximum single pass pump absorption was only 18%. When the diode laser frequency was tuned to the pump resonance that was properly impedance matched, the Nd:YAG laser produced 29 mW from an incident diode laser power of 61 mW for an overall efficiency of 48%. Laser oscillation was observed at both 946 nm and 938 nm.

Efficient generation of blue light is demonstrated by using resonantly enhanced sum-frequency mixing in a monolithic KTP resonator. Two infrared sources, a diode-pumped Nd:YAG laser and a GaAlAs laser diode, were simultaneously coupled into the monolithic cavity and tuned to resonant frequencies, so that high-intensity intracavity fields were built up at both wavelengths. A blue power of 4 mW in a TEM00 mode was generated with 55% of the 30 mW of 809-nm power and 45% of the 33 mW of 1064-nm power coupled into the standing-wave nonlinear resonator. By using a novel monolithic ring resonator, 2 mW of power was generated in a TEM00 mode at 462 nm. Electric-field-induced tuning of the resonant frequencies was also demonstrated.

The frequency-tuning and -control properties of monolithic doubly resonant optical parametric oscillators are analyzed for stable single-mode pump radiation. Single-axial-mode operation is observed on the idler and the signal for both pulsed and continuous pumping. Projections are made for tuning-parameter tolerances that are required for maintenance of stable single-frequency oscillation. Continuous frequency tuning is possible through the simultaneous adjustment of two or three parameters; thus the synthesis of specific frequencies within the broad tuning range of the doubly resonant optical parametric oscillator is permitted.

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.

If the pump radiation is strongly absorbed in the laser material, end-pumping of solid-state lasers can lead to high efficiency as a result of the good mode overlap between the solid-state laser and the diode-laser pump. Weak absorption of the diode-laser radiation can result from the use of a very short solid-state laser cavity for single-mode operation,1 inherently weak absorption2 in the laser material at the wavelengths of available pump lasers, or low laser ion concentration in the host material. Weak absorption in quasi-three-level lasers can also result if short laser crystals or low-concentration material are used to reduce the reabsorption loss.3,4 Some method of increasing the total absorption of pump light is desired to increase the overall efficiency of laser systems with weak single-pass absorption.

A monolithic ring resonator of KNbO3 was used for efficient frequency doubling of a 856 nm GaAlAs diode laser. A special electronic servo technique was devised to lock the diode laser frequency to the KNbO3 cavity so that stable generation of blue output was obtained. With 105 mW of incident near-infrared power, 41 mW of 428 nm radiation were produced. The conversion efficiency from electrical input power into the diode laser to blue output was ∼10%.

Single-axial-mode lasers pump very-low-loss doubling crystals. Important advance in making resonant generation of second harmonics possible for diode-laser-pumped solid-state lasers is recent development of monolithic nonplanar ring geometries in neodymium:yttrium aluminum garnet (Nd:YAG) lasers that produce frequency-stable single-mode outputs. Other advance is development of high-quality MgO:LiNbO3 as electro-optically nonlinear material. Series of experiments devised to improve doubling efficiency of low-power lasers, and particularly of diode-laser-pumped continuous-wave Nd:YAG lasers.

Compact blue laser sources with output powers of 10 and 40 mW at 473 and 428 nm have been developed using nonlinear frequency upconversion of GaAlAs diode lasers. These devices are attractive for application to high-density optical data storage.

A number of different techniques have been used for nonlinear frequency upconversion of near-infrared semiconductor diode lasers. Intracavity frequency doubling of a diode-laser-pumped 946-nm Nd:YAG laser led to the generation of 9.5 mW of blue 473-nm power. A special electronic servo technique was devised to lock the output frequency of a single-mode GaAlAs diode laser to a monolithic KNbO3 resonator. Using the approach, 41 mW of 428-nm output were obtained with an electrical-to-optical conversion efficiency of about 10 percent. Strained-layer InGaAs/GaAs diode laser operating at about 1 micron have been fabricated for noncritically phase-matched frequency doubling in KTiOPO4. A 802-nm GaAlAs diode laser was used for upconversion pumping of a YLiF4:Er(3+) laser which operates at 551 nm with a threshold power of 50 mW at 77 K.

A monolithic doubly resonant optical parametric oscillator (OPO) fabricated from MgO:LiNbO(3) was pumped by a cw, frequency-doubled, diode-laser-pumped Nd:YAG laser. The threshold for cw operation was 12 mW, and pump depletions of up to 78% were observed two times above threshold. The total OPO output power was 8.15 mW, with a conversion efficiency for the incident pump of 34% and combined conversion efficiency for the 1064-nm laser light of 14%. The OPO was temperature tuned from 1007 to 1129 nm, operated on a single-axial-mode pair over most of the range, and could be electric field tuned by as much as 38 nm near degeneracy.