What Are Pump Lasers?

Pump lasers are laser sources dedicated to supplying the optical energy that excites gain media in other lasers or optical amplifiers rather than generating an output beam for direct use. The excited gain medium, whether a rare-earth-doped fiber, a solid-state crystal, or a semiconductor amplifier, absorbs the pump radiation and transitions to a higher energy state; stimulated emission then produces the amplified or lasing output at a longer wavelength. Pump lasers are therefore enabling components rather than end-use devices, and their performance, including output power, wavelength stability, spectral linewidth, and coupling efficiency into the gain medium, directly determines the gain, noise figure, and efficiency of the downstream optical system. The global telecommunications network, high-power fiber laser industry, and many scientific laser platforms depend on pump lasers as a critical subsystem.

The most important class of pump laser in deployed systems is the semiconductor diode laser, which converts electrical current directly to light with wall-plug efficiencies exceeding 60 percent at the pump wavelength. Rare-earth-doped fiber amplifiers, which form the backbone of long-haul optical communications, are almost universally pumped by single-mode or multimode diode lasers operating at 980 nm or 1480 nm. The RP Photonics encyclopedia entry on fiber amplifiers describes the role of pump lasers in erbium, ytterbium, thulium, and praseodymium amplifier configurations.

Optical Pumping Mechanisms

Optical pumping transfers energy from the pump photons to the gain medium by absorption at a transition between the ground state and a higher electronic level. In erbium-doped fiber amplifiers (EDFAs), pump photons at 980 nm excite erbium ions from the ground state to the short-lived 4I11/2 level, from which they rapidly relax to the metastable 4I13/2 level that provides gain at 1550 nm. Pumping at 1480 nm drives transitions directly into the upper manifold of the gain level, trading the cleaner population inversion of 980 nm pumping for a configuration that is more efficient for high-gain applications. In solid-state lasers such as Nd:YAG, pump radiation at 808 nm populates the upper laser level, and the crystal emits at 1064 nm. The fraction of pump photons converted to useful signal photons is called the Stokes efficiency, with the remainder appearing as heat in the gain medium. Managing this heat load is central to the engineering of high-power pump-laser systems.

Fiber and Solid-State Pump Laser Designs

The two dominant technologies for pump lasers are single-emitter semiconductor diode lasers and diode-pumped solid-state (DPSS) lasers. Single-emitter diode lasers at 976 nm are commonly packaged in a 14-pin butterfly module with a fiber pigtail for direct coupling to a doped fiber amplifier; typical output powers range from 100 mW to 2 W per emitter. High-power pump sources for large fiber lasers combine many diode emitters using wavelength-combining or spatially multiplexed schemes to deliver hundreds of watts into a double-clad fiber structure. DPSS lasers, in which a diode bar pumps a Nd:YVO4 or Yb:YAG crystal intracavity, offer excellent beam quality and frequency stability for pumping mode-locked oscillators and optical parametric amplifiers. The performance specifications for these devices are described in publications from Lumentum and other manufacturers in the optical communications segment, as well as in peer-reviewed work in IEEE Photonics Technology Letters.

Wavelength Selection and Coupling

The pump wavelength must be precisely matched to an absorption peak of the gain medium, and the wavelength must remain stable over temperature and aging to maintain consistent amplifier gain. Wavelength-stabilized diode lasers use fiber Bragg gratings or volume Bragg gratings to lock the emission wavelength to a narrow spectral window regardless of operating temperature. Coupling efficiency between the pump laser output and the gain medium depends on numerical aperture matching, fiber splice quality, and the geometry of the pump delivery scheme: co-propagating, counter-propagating, or bidirectional pumping. The IEEE Photonics Society is the professional community where advances in pump laser technology, including novel wavelengths for new dopant systems, are regularly reported.

Applications

Pump lasers have applications in a wide range of disciplines, including:

• Erbium-doped fiber amplifiers for long-haul optical fiber telecommunications
• High-power ytterbium fiber lasers for materials processing and welding
• Ultrafast laser amplifier chains for scientific research
• Raman amplifiers in optical communication systems
• Optical parametric oscillators and amplifiers for spectroscopy
• Medical laser systems requiring high-power solid-state gain media