The output power of single-frequency fiber amplifiers is usually limited by nonlinear effects such as stimulated Brillouin scattering (SBS). To obtain higher power thresholds for the onset of unwanted nonlinear effects, the mode area of the fibers in use needs to be increased. Specialty fibers can provide larger mode areas and thus push the current power limits of single-frequency fiber amplifiers while maintaining single-mode beam quality as required by next generation gravitational wave detectors. Fibers with a large core diameter, depressed cladding around the core and a confined doping (DCCD-fiber) inside the core are by now commercially available and address the need for large mode area fibers while maintaining single-mode operation. The depressed cladding leads to a smaller effective refractive index difference for Higher Order Modes (HOM) in comparison to the fundamental mode which results in a significant increase of bending losses for the HOM. The confined doping results in a selective gain increase for the fundamental mode. Here, we present a forward pumped single-frequency amplifier based on an Yb3+-doped DCCD fiber. With this fiber, an output power of 400W was achieved with a slope efficiency of 75\%, and a PER of 15 dB. The amplifier showed no signs of SBS or parasitic lasing of the amplified spontaneous emission. This work will evaluate the potential of the used DCCD fiber in the context of next generation gravitational wave detector lasers.