Tapered Amplifiers

A tapered amplifier (TA or TPA) is a specially designed semiconductor optical amplifier (SOA) that can reach an output power of several watts. The special feature of tapered amplifiers is the preservation of the good spectral characteristic of the seed laser such as small linewidth and a strong side mode suppression ratio (SMSR). While reaching the output power level of broad area lasers tapered amplifiers are able to maintain a high beam quality with an M² factor well below 2.

A tapered amplifier comprises two sections that are monolithically integrated on one semiconductor chip.

The first section is a ridge wave guide with a width of a few microns that allows only a single mode propagation. The input light is injected in this part of the TA. Our DFB lasers used as a seed laser match perfectly with our tapered amplifiers since the dimensions of the ridge waveguides are similar. Thus a simple focusing optic with 1 : 1 magnification enables a perfect optical coupling between seed laser and tapered amplifier.

The second section of the active area is trapeze-shaped with increasing lateral dimension towards the rear facet whereas the height remains equal to the first section. By this special design the beam quality stays high while the seeded light is amplified during the path through the steadily broadening active region. The amplification of the injected light is effected by a single path transmission through the two sections of the tapered amplifier. Front and rear facet of the chip have an anti-reflective (AR) coating. Tapered amplifiers can be used for more than one wavelength since the spectral profile of the gain is broad.

Tapered amplifiers are often used as part of a master oscillator power amplifier (MOPA) system. Single frequency lasers such as DFB and DBR lasers as well as ECDLs are appropriate seed lasers for all kind of MOPAs. The C-mount package of our TAs allows free access to both sides of the chip which is ideal for free beam setups whereas our miniTAs with 14 pin butterfly package enable a simple and robust fiber coupling with the seed laser.

Since it is difficult to suppress reflections from the optical elements placed between seed laser and tapered amplifier an optical isolator is required between the two components of the MOPA system. Often an additional isolator is needed at the output side because of the feedback sensitivity due to the high gain of tapered amplifiers.