The demands of global bandwidth and distribution are rising rapidly as Internet usage grows. This fundamentally means that more photons are flowing within optical cables. While transmitting sources launches some optical power, the majority of the optical power that is present within modem telecommunication systems originates from optical amplifiers. In addition, modem optical amplifiers offer flat optical gain over broad wavelength bands, thus making possible dense wavelength de-multiplexing (DWDM) systems. Optical amplifier performance, and by extension the performance of the laser pumps that drive them, is central to the future growth of both optical transmission and distribution systems.
We describe advances in the development of high-power diffraction-limited lasers for single- mode fiber-coupled sources. The development of the tapered amplifier has led to the realization of a monolithic MOPA diode laser, which provides up to 3 W cw of single-spatial- mode output power. We further describe the implementation of the MOPA in fiber-coupled architectures that provide up to 1.2 W cw coupled into a single-mode optical fiber and some of the optical considerations unique to devices based on tapered amplifiers.
A high-power monolithically integrated master oscillator flared power amplifier is demonstrated which operates at approximately 860 nm to an output power greater than 1.3 cw with a far field pattern consisting of a single, diffraction-limited lobe.
Single-mode strained-layer lasers have been fabricated which use buried second-order gratings for distributed Bragg reflectors. The lasers contain a strained GaInAs quantum well in the active region and operate in an edge emitting fashion with CW powers in excess of 110 mW. Single longitudinal and transverse mode operation is maintained at about 971.9 nm up to 42 mW. Total power conversion efficiencies as high as 28 percent have been observed. The longitudinal and transverse mode behavior is stable under 90 percent amplitude modulation with 50 percent duty cycle pulses at 10 kHz and 10 MHz. Preliminary life-test data at 40 C also indicate room temperature lifetimes in excess of 45,000 hours.
The coherent operation of one-dimensional linear arrays of grating-coupled surface-emitting lasers is experimentally investigated, for different laser designs (gain lengths, grating parameters. For laser arrays with shallow grating teeth and strong interelement coupling, a diffraction-limited far field of 0.012 deg FWHM was obtained from up to six coupled lasers extending over a length of 3.5 mm.
The steady-state properties of a two-gain section/three-grating surface-emitting laser are studied theoretically as a function of length-induced phase variations. In particular, the optical length of one gain section is varied with respect to the other by up to one grating period. It is found that several mode hops occur. Although the number of these can be reduced by optimizing the design, stable single-mode laser operation requires control of the optical length of each gain and grating section to better than one wavelength.