1
History of Optically Pumped Semiconductor Lasers - VECSELs

Mark E. Kuznetsov

Axsun/Excelitas Technologies, Billerica, MA, USA

1.1 Introduction

Optically pumped semiconductor lasers (OPSLs) are an old concept that originated in the early days of semiconductor lasers in the 1960s, and that remained a scientific curiosity until the mid-1990s, when its potential capabilities for high power with excellent beam quality were first fully demonstrated, spurring subsequent rapid development of the science and technology of these versatile lasers. Distinguishing features of OPSLs today are light emission normal to the plane of the semiconductor chip, laser cavity external to the chip to stabilize fundamental transverse laser mode and to enable insertion of intracavity functional optical elements, and the use of optical pumping for efficient and high output power operation. A wide range of applications is enabled by additional remarkable properties of this laser family, such as wavelength operation from ultraviolet (UV) to mid-infrared (IR) and even terahertz range, and passively modelocked operation with output pulses shorter than 100?fs. These lasers are also widely known by two other names, descriptive of their geometry: vertical external-cavity surface-emitting lasers (VECSELs) and semiconductor disk lasers (SDLs). Alternatively, VECSELs can also be electrically pumped, but achievable laser output powers are then typically much lower than for the optically pumped version.

OPSL development in the 1990s was spearheaded by Aram Mooradian in Micracor, a small start-up company that spun out technology from Aram's group in the MIT Lincoln Laboratory. I worked with Aram in Micracor to carry out this development. In 2011, the first annual VECSEL conference was held at SPIE Photonics West, with the VECSELs-XI conference scheduled for 2022. The first book about these lasers, Semiconductor Disk Lasers: Physics and Technology [1], was published in 2010; it was edited by Oleg Okhotnikov from the Tampere University of Technology in Finland and described the then state of the art in chapters contributed by researchers from around the world. Since the publication of this book, science, technology, and applications of VECSELs have made a significant step forward, and hence the present book to bring VECSELs overview up to date. This chapter describes the history of OPSLs, the people that took part in their development, and it's also a personal story of the OPSL development by our team at Micracor. Sadly, both Aram Mooradian and Oleg Okhotnikov, who have contributed so much to the early development of these lasers, have passed away since the publication of the first book. This historical chapter is dedicated to their memory.

1.2 OPS-VECSELs: Concept and History

The first laser invented in 1960 was a flashlamp-pumped solid-state ruby laser. Other laser gain media and pumping schemes soon followed, and in 1962 a semiconductor diode laser pumped by current injection in a semiconductor p-n junction was demonstrated [2-6]. Semiconductors offered the possibility of operating at different wavelengths, depending on the material composition - already in 1962, together with the near-IR operation of binary GaAs lasers, ternary alloy GaAsP semiconductor diode laser in the visible was also demonstrated [5]. Electron-hole pairs for laser excitation in semiconductors can be created by various means. Current injection pumping of diode lasers, although requiring more complex device fabrication, is appealing for its simplicity of use and the possibility of direct laser output modulation by current modulation. Yet other schemes for semiconductor laser pumping were also investigated, including optical and electron beam pumping. Semiconductor diode lasers have seen tremendous development from the 1960s to the 1990s, driven primarily by applications in optical fiber communication, CD and DVD optical disk readout, and pumping of solid-state and fiber lasers and amplifiers. One challenge had remained, however. While diode lasers could produce very large, from watts to hundreds of watts, powers, this power was produced with poor beam quality: in highly transverse multimode, high aspect ratio output beams, or from large arrays of lasers. Single transverse mode output, and especially circular Gaussian output beam, could be produced at only much smaller power levels of at most a few hundred milliwatts. Optical pumping had remained an essentially experimental tool to demonstrate lasing capability of the semiconductor gain medium or laser structure, on the way to the ultimately useful diode-current-injection electrical pumping.

Why would semiconductor lasers with high multiwatt output power and beam quality be useful and important? The alternative high-power laser technologies, e.g. solid state, gas, and atomic vapor lasers, rely on discrete atomic transitions and thus are restricted to discrete unique operating wavelengths. Semiconductor lasers, via material composition and bandgap-engineered quantum-confined structures, can produce a very wide range of operating wavelengths, from UV to mid-IR, both directly and using nonlinear optical, including intracavity, conversion. This allows, by design, an essentially continuous coverage of this spectrum and even dual wavelength laser operation. High-power good beam quality semiconductor lasers can offer unique operating parameters not accessible by other types of lasers. If you add to this femtosecond pulse capability with high peak powers, potentially compact size and low fabrication cost, this makes such semiconductor lasers useful, and sometimes possibly unique, for a wide range of applications.

Early semiconductor lasers were edge emitting and emitted light in the plane of the wafer, so that enough gain could be accumulated over the length of the device. Such edge-emitting geometry, both gain and index guided, limits transverse profile beam quality for high powers. Vertical cavity surface-emitting lasers (VCSELs) were first described by Kenichi Iga at the Tokyo Institute of Technology in 1979 [7, 8] and further developed to efficient low threshold operation by Jack Jewell at Bell Laboratories, Holmdel, in 1989 [9-11]. VCSELs surface-emitting geometry, with light emitted normal to the plane of the wafer, because of low gain of the short gain region, requires very high reflectivity semiconductor or dielectric multilayer mirrors. Such vertical cavity geometry allows single transverse mode circular Gaussian beam output, but typically only for milliwatt level powers, limited by the difficulty of heat dissipation from the small mode area of a few microns in diameter.

Optical pumping of semiconductor lasers has a long history; it has been used for various purposes, such as characterization of novel semiconductor laser materials and structures, generation of higher output powers, or short pulse generation. As early as 1965, an OPSL has been demonstrated by Robert Phelan and Robert Rediker at MIT Lincoln Laboratory [12], where an edge-emitting InSb laser was pumped by an edge-emitting GaAs diode laser. Remarkably, both concepts, optical pumping of semiconductors and the use of diode lasers as pumps, are already in use this early in the history of lasers. In 1966, Nikolay Basov's group at the Lebedev Physical Institute in Moscow introduced the concept of a "radiating mirror" [13], Figure 1.1a: a thin semiconductor gain layer placed on top of a mirror and a heatsink, with an external output coupling mirror completing the laser cavity. Both optical and electron beam pumping were envisaged and demonstrated as the possible excitation sources. Large lateral extent of the gain medium, greater than its thickness, would ensure effective heat removal and thus the possibility of high output power. This is essentially the concept of a "disk laser" geometry, which would prove so effective many years later in both solid-state [14, 15] and semiconductor [1] laser configurations. Basov's 1966 "radiating mirror" concept, Figure 1.1a, is remarkably similar to the 1996 Micracor OPSL configuration, Figure 1.1b. In his paper, Basov reported operation of a "radiating mirror" laser with optical pumping of CdSe using two-photon absorption of a Q-switched Nd-doped glass pump laser. So as a concept, SDL had been already introduced and demonstrated in 1966; however, its full potential was yet to be explored and developed.

In the late 1960s, Nick Holonyak's group at the University of Illinois, Urbana, reported several studies of optically pumped CdS, GaAs, and GaAsP semiconductor thin platelet lasers [16], some using GaAsP diode laser pumping, and considered both edge- and surface-emitting laser geometry. Transparent sapphire heatsinking windows had been used to remove heat and help improve power performance of the devices, foreshadowing the future use of such transparent heatspreaders. Optical pumping here is mainly used to explore lasing in different semiconductor materials. In a 1973 publication from Aram Mooradian's MIT Lincoln Lab group, Stephen Chinn demonstrated pulsed operation of optically pumped edge-emitting bulk GaAs semiconductor lasers [17], with the goal of efficient high power generation. Later in 1981, Julian Stone, Jay Wiesenfeld, Andrew Dentai, and coworkers from the Bell...