The autoionizing two electron states we have considered so far are those which can be represented sensibly by an independent electron picture. For example, an autoionizing Ba 6pnd state is predominantly 6pnd with only small admixtures of other states, and the departures from the independent electron picture can usually be described using perturbation theory or with a small number of interacting channels. In all these cases one of the electrons spends most of its time far from the core, in a coulomb potential, and the deviation of the potential from a coulomb potential occurs only within a small zone around the origin.

In contrast, in highly correlated states the noncoulomb potential seen by the outer electron is not confined to a small region. In most of its orbit the electron does not experience a coulomb potential, and an independent electron description based on nℓn′ℓ′ states becomes nearly useless. There are two ways in which this situation can arise. The first, and most obvious, is that the inner electron's wavefunction becomes nearly as large as that of the outer electron. If we assign the two electrons the quantum numbers niℓi and noℓo, this requirement is met when ni approaches no, which leads to what might be called radial correlation. The sizes of the two electron's orbits are related. The second way the potential seen by the outer electron can have a long range noncoulomb part is if the presence of the outer electron polarizes the inner electron states.

Introduction

One method by which the rate of information transmission can be increased is by the use of multiple, independently addressable sources to form parallel data channels. In the optical domain, this may be implemented by semiconductor diode lasers, which can be fabricated into monolithic arrays of emitters. The purpose of this chapter is to describe the development of linear arrays of index-guided diode lasers, and indicate directions in which the technology may proceed. Note that all the discussions in this chapter will be confined to semiconductor diode lasers in which the light propagates parallel to the epitaxial planes.

A few words are in order to explain the motivations that have driven the development of individually addressed diode laser arrays. Early work included the fabrication of both AlGaAs and InGaAsP devices for parallel data transmission through bundles of optical fibers, and arrays of lasers have often been considered in optical printing or optical data processing applications. The application that has been most responsible for the development of these devices, however, is optical data storage. This discussion will focus on the use of multielement arrays in optical recording systems, as a very useful example that has driven diode laser array technology towards certain performance capabilities.

Diode laser arrays have been studied for the last 15 years. Initially the interest was in creating phase-locked arrays of high coherent powers delivered in narrow, diffraction-limited beams for applications such as free-space optical communications. Around 1983 interest arose in spatially incoherent arrays as efficient pumps for solid-state lasers. This remains nowadays the most widespread use of diode laser arrays. In recent years there has been a demand for individually addressable one-dimensional and two-dimensional arrays to be used in parallel optical-signal processing, optical interconnects, and multichannel optical recording. We have attempted in this book to cover the development and features of all types of arrays demonstrated to date. The first five chapters treat various aspects of coherent arrays: lasers, amplifiers, external-cavity control, modeling and operational dynamics. Chapters 6–8 are dedicated primarily to spatially incoherent arrays. High-power capability, reliability, packaging and pumping schemes are discussed as they relate to the major application: solid-state-laser pumping. Individually addressable arrays of both the surface-emitting type (i.e. vertical-cavity surface emitters) and edgeemitting type are treated in Chapters 9 and 10, respectively.

Coherent arrays have proved a particularly challenging task. For the first ten years of research (1978–1988) the best that could be achieved was 50–100 mW in a diffraction-limited beam; that is, pretty much the same power as from a single-element device.

Introduction

Semiconductor optical sources exhibit several distinct advantages relative to other solid-state and gas laser systems including compact size, high efficiency, high reliability, robustness and manufacturability. As a result, many of the rapidly growing commercial and consumer markets, including telecommunications, printing and optical data storage, have only been realized through the introduction of semiconductor lasers. The next generation of high-performance electronics will require higher-power semiconductor lasers; but as recently as 1992, commercially available high-power (greater than 200 mW cw) semiconductor lasers did not operate in a single spatial mode and therefore did not radiate in a single diffraction-limited lobe. However, the latest advances in the design and fabrication of semiconductor lasers have resulted in the development of high-power diffraction-limited laser sources.

The architectures that have been studied for high-power diffraction-limited semiconductor lasers can be divided into three categories: oscillators, injection-locked oscillators, and master oscillator power amplifiers (MOPAs). The highest-power diffraction-limited operation from oscillators has been demonstrated from antiguided laser arrays. These devices have demonstrated up to 0.5 W cw and 1.5 to 2.0 W pulsed in a diffraction-limited radiation pattern. Disadvantages of antiguide oscillators are their multi-longitudinal-mode spectra and multi-lobed far-field patterns. Another promising oscillator configuration is the broad-area ring oscillator, which has demonstrated single-frequency operation to greater than 1 W pulsed, and diffraction-limited single-lobed operation to approximately 0.5 W pulsed.

Introduction

In this chapter we review our recent work on the dynamics of coherent semiconductor laser arrays. While most of the published literature on laser arrays has focused on the spatial properties of these sources, it is becoming increasingly apparent that their temporal behavior can be marvelously rich and complex. This complexity should not be surprising. A laser is, after all, a nonlinear oscillator. When one creates an array of coupled nonlinear oscillators the resulting dynamical behavior of the system will range from synchronization and phase locking to instabilities and chaos. An understanding of the complex dynamics is essential for the design of stable, compact, high-power sources for applications such as intersatellite optical communications. Because the characteristic time scale of semiconductor laser array dynamics is less than a nanosecond, it took the pioneering streak camera measurements of DeFreez et al., to provide the first indication of temporal instabilities in these lasers. Since then, complex dynamical behavior has been observed in most types of semiconductor laser arrays. Theoretical work has proceeded apace, from the early time-dependent coupled-mode theories to more recent partial differential equation models which treat the array as a single entity. The models predict both coupling induced instabilities and saturable-absorber induced instabilities, depending on array geometry and material parameters.

Introduction

Semiconductor diode lasers emitting normal to the substrate plane, known as surface-emitting lasers, are extremely promising for addressing a range of applications from optical interconnects, optical communications and optical recording to remote sensing. The most promising aspect perhaps lies in the prospect of eliminating low yield laser fabrication steps, i.e. laser packaging processing including wafer lapping, cleaving and dicing, facet coatings and diode bonding. The possibility of being able to make any number of lasers anywhere on a wafer is also an increasingly important factor for applications such as optical interconnects. At present two completely different approaches are aimed at realizing surface-emitting lasers. The first represents an extension of the existing technology for semiconductor edge-emitting lasers that uses a 45° slanted mirror or a second-order grating to vertically couple the light out (Figure 9.1(1). (2)). The second, pioneered by K. Iga in 1979, uses highly reflective mirrors to clad the active region, resulting in a vertical cavity that produces an output beam propagating normal to the substrate surface (Figure 9.1(3)).

The vertical cavity design offers important advantages over other surface-emitting laser designs. The unique topology of a vertical cavity facilitates large-scale processing, on-wafer testing and pre-process screening. The small lateral dimensions allow for fabrication of large 2-D arrays with high packing density and integration with other optical and electronic devices.

The preceding chapters of this book have described diode laser arrays with a variety of geometries and coherence properties. Edge-emitting geometries were shown to be capable of high power and high efficiency, whereas surface-emitting geometries permitted fabrication of large two-dimensional arrays. In addition, many of these arrays were designed to contain various degrees of mutual coherence between lasing apertures. In the current chapter, we consider several of the systems aspects of these laser arrays. In particular, we will consider external optical components and systems that manipulate the laser output light to satisfy the requirements of specific applications. We will describe optical systems that act as interfaces between the laser array and other electrooptical components, as well as external laser cavities that improve the performance of the laser array itself.

The recent advances in microoptical components permit unprecedented control of light from laser arrays. Many optical functions are now possible that were previously difficult or impossible to perform with conventional optical components. Microlenses and microlens arrays can be used to collimate and expand individual lasers in an array, making it possible to change the fill factor of a given array. Astigmatism, spherical aberration, and other optical aberrations, can be removed with aspheric and anamorphic microlenses.

Introduction

In recent years there has been an evolution in interest in semiconductor laser technology from exploring the performance limits of single devices of inherently low power to exploiting the potential of large arrays of devices of much higher power. While the low-power semiconductor devices are a proven component in many commercial systems, it is clear that a number of applications also exist for high-power arrays and that semiconductor-laser researchers are now poised to tackle them.

The manufacture of large volumes of semiconductor laser arrays with high yield has enabled the fabrication of larger and larger arrays. Rather than mounting single devices on copper submounts, and thus limiting the modest amount of steady state waste heat which can be dissipated, the laser designer is now faced with the challenging task of fabricating increasing numbers of devices in increasingly smaller volumes and operating these units at increased duty cycle or average power. We are faced with the same evolution of technology which IC chip manufacturers have faced for years; however, the semiconductor lasers are far more temperature sensitive than silicon-based electronic circuits and the task is ever more challenging.

Depending on the application, the utility of any laser is governed by its ability to deliver photons to a remote location, which in turn depends on the beam quality.

Introduction

Phase-locked arrays of diode lasers have been studied extensively over the last 15 years. Such devices have been pursued in the quest to achieve high coherent powers (> 100 mW diffraction limited) for applications such as space communications, blue-light generation via frequency doubling, optical interconnects, parallel optical-signal processing, high-speed, high-resolution laser printing and end-pumping solid-state lasers. Conventional, narrow-stripe (3–4 μm wide), single-mode lasers provide, at most, 100 mW reliably, as limited by the optical power density at the laser facet. For reliable operation at watt-range power levels, large-aperture (≥100 µm) sources are necessary. Thus, the challenge has been to obtain single-spatial-mode operation from large-aperture devices, and maintain stable, diffraction-limited-beam behavior to high power levels (0.5–1.0 W).

By comparison with other types of high-power coherent sources (master oscillator power amplifier (MOPA), unstable resonators), phase-locked arrays have some unique advantages: graceful degradation; no need for internal or external isolators; no need for external optics to compensate for phasefront aberrations due to thermal- and/or carrier-induced variations in the dielectric constant; and, foremost, beam stability with drive level due to a strong, built-in, real-index profile. The consequence is that, in the long run, phase-locked arrays are bound to be more reliable than either MOPAs or unstable resonators.

Introduction

Diode lasers were first demonstrated in 1962. These structures were broad area homojunction devices which could only be operated at 77 K with short (< 100 ns) pulse widths. From those early lasers, much progress has been made in increasing the output power, reliability, operating temperature and efficiency of diode lasers and diode laser arrays. For example, at present, monolithic laser diode arrays have been operated at temperatures in excess of 100°C at output power levels in excess of 100 W cw and at power conversion efficiencies of greater than 50% with multiwatt output powers. This chapter will describe the history of the improvements made in high-power diode laser research and the failure mechanisms responsible for the limitations in output power of high-power diode lasers. We also present the results of reliability testing at high output power for a variety of semiconductor laser structures and review environmental testing for laser array products. The design considerations for long life operation at high power are also presented.

7.1.1 History of high-power diode laser arrays

Following the early introduction of homojunction diode lasers, single heterostructure and double heterostructure (DH) diode lasers were demonstrated in the late 1960s. The double heterostructure laser was the first laser to exhibit cw room temperature operation This DH laser offered two major improvements over the original homojunction device.

Introduction

Diode laser arrays were conceived and fabricated in the late 1970s as an attempt to overcome the inherent power limitations of single-aperture diode lasers. Increasing the output power by simply increasing the lateral aperture size from the usual ≈ 5 μm to values of 50–100 μm had already been tried with disappointing results. These so-called ‘broad-area’ devices exhibited such poor modal characteristics that the power focusable into a diffraction-limited spot barely increased at all. Arrays were thus seen as a means of increasing output power by phase-locking several diode lasers together so that they operated as a single (hopefully) diffraction-limited source. The last decade has seen a tremendous research effort directed towards the design and fabrication of high-power arrays exhibiting good mode control, with (as usual) mixed results. Today's multi-watt arrays certainly provide more focusable laser power than could be obtained by simply stacking together individual diode lasers. On the other hand, achieving the desired mode control and coherence from arrays has proved considerably more difficult than was originally envisioned. As a result, virtually all the high-power arrays commercially available at present emit their radiation into two broad far-field lobes instead of the desired single diffraction-limited lobe.

This situation has arisen because the first simple types of diode arrays that were easily understandable and relatively easy to fabricate have been shown to exhibit poor mode discrimination, so that even devices that lased in the desired in-phase mode near threshold became multi-mode at higher currents.

Introduction

The oldest, best developed, and still most sophisticated heterostructure materials system for semiconductor diode laser arrays is the AlGaAs—GaAs system. This system has a unique combination of practical physical, electrical, optical and chemical features that has withstood the test of time. These features include a tractable chemical system that lends itself equally well to basic epitaxial growth methods, such as liquid phase epitaxy (LPE), and to more sophisticated thin layer epitaxial growth methods, such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Controlled thin layer epitaxy has, in turn, allowed the development of AlGaAs—GaAs quantum well heterostructure lasers. Perhaps the single most important attractive feature of the AlGaAs—GaAs heterostructure system is the fortunate coincidence that AlAs and GaAs have, for all practical purposes, the same lattice constant (Δa/a < 0.12%). This allows the design of any structure, with any combination of layer compositions, without regard to lattice mismatch or the associated dislocation formation.

The range of wavelengths available from lattice matched AlGaAs—GaAs conventional double-heterostructure lasers and quantum well heterostructure lasers is from λ ≈ 0.88–0.65 μm. The long wavelength limit is defined by the band edge for GaAs, and AlGaAs active layers or the quantum size effect, or both, are used to shift the emission to shorter wavelengths.