Single and multiple frequency fiber lasers

Single frequency, low intensity noise, widely tunable lasers operating in the 1.5 {dollar}mu{dollar}m region have potential applications in future wavelength division multiplexed optical communications systems, fiber sensor arrays and high resolution spectroscopic measurements. A single frequency fiber laser having these characteristics will be described in detail. The laser cavity contains an erbium doped fiber gain module, fiber isolators to ensure unidirectional travelling wave operation and two fiber Fabry-Perot filters acting in tandem, which provide broadband tunability (1530 nm-1560 nm) combined with stable single frequency operation. Shot noise limited operation of this laser has been observed at frequencies greater than 300 MHz. At lower frequencies (1-300 MHz) the intensity noise has been characterized in terms of sidemode suppression ({dollar}>{dollar}60 dB of minimum sidemode suppression has been realized). Lower still (10 kHz-1 MHz) the intensity noise is dominated by the laser's relaxation resonance (30 kHz @ 1 mW output, {dollar}-{dollar}105 dBc/Hz). The linewidth of this laser has been measured to be less than 4 kHz using a loss compensated recirculating delayed self-heterodyne interferometer (RDSHI). The RDSHI is an improvement over the standard delayed self-heterodyne interferometer in that the effective delay line can be increased by a factor of 30 over the standard method, increasing the resolution by a corresponding amount. The RDSHI also allows measurement of the short term frequency jitter of a laser. In order to reduce laser frequency jitter, the Pound-Drever technique was employed to lock the laser frequency to an external fiber Fabry-Perot. The same technique also permitted the internal mode selection filter to track the laser frequency, completely eliminating residual mode hopping due to thermal length changes of the laser cavity. Finally, fiber laser configurations that allow multiple frequencies to be simultaneously produced in one laser cavity will be described.