Fiber Laser Part 1

Outline Fiber Laser Ring cavity Linear cavity Single-wavelength laser Multi-wavelength laser  Hybrid linear and ring cavity Fiber laser at S, C and L band wavelength region.

What is fiber laser? Fiber lasers are usually meant to be a laser with doped fiber as gain medium, or just lasers where most of the laser resonator is made of fibers

Advantages Fiber laser good reliability and lifetime, high power high stability low amplitude noise high output stability immune to tough environment changes low threshold power narrow linewidth

Fiber laser applications optical sensing optical filtering high-resolution spectroscopy interferometric sensing optical frequency metrology narrow-bandwidth amplification optical fiber communications etc

Types Brillouin Fiber laser (BFL) Brillouin Erbium Fiber laser (BEFL) Brillouin Raman fiber laser (BRFL) Erbium-doped fiber laser (EDFL) Raman fiber laser (RFL) Erbium doped fiber laser (EDFL) FWM fiber laser etc

Nonlinear effect The nonlinear effect in optical fiber occurs due to the interactions between propagating light and the fiber. The nonlinear effects are weak at low powers, but can become much stronger when light reaches certain threshold values, which can occur when the power is increased The nonlinear effects also depend on transmission length of optical fiber. The longer the fiber length, more light interaction occurs and the nonlinear effects became stronger.

Nonlinear effect categories The first category covers the inelastic scattering phenomenon that can persuade stimulated effects such as SBS and Stimulated Raman-Scattering (SRS). Both of these phenomena are related to the interaction between photons and electrons.

Nonlinear effect categories The second category covers the nonlinear refractive phenomenon that arises from the power dependence of the refractive index in the optical fiber. This condition produces effects such as Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM) and Four -Wave Mixing (FWM). These optical nonlinearities can lead to interference, distortion, noise and excess attenuation of the optical signals, which limit system performances

Stimulated Brillouin Scattering (SBS) SBS is a nonlinear process in optical fiber that occurs from the interaction between intense pump light and acoustic waves and generate a backward propagation frequency shifted light The signal frequency was found to be downshifted by 0.08 nm (10 GHz) from the injected signal wavelength, through the Doppler effect

Stimulated Brillouin Scattering where: = refractive index = pump wavelength = acoustic velocity in the gain medium

Stimulated Brillouin Scattering

Stimulated Brillouin Scattering

Raman Scattering Raman Scattering process is caused by interaction of light with vibrational modes of molecules of lattice vibrations of crystals Raman Scattering is an inelastic process that occurs when a pump photon, excites a molecule up to a virtual level (intermediate state)

Raman Scattering Two types Spontaneous - Spontaneous Raman scattering occurs if the intensity of the incident field is below a threshold level. In this condition photon are scattered into random direction Stimulated - Stimulated Raman scattering (SRS) only occur when the pump power exceeds a certain threshold level

Ring and linear cavity configurations Ring cavity

Ring cavity configurations Single wavelength Multi wavelength

Optsystem example EDFL

Optsystem example EDFL

Ring cavity configurations Single wavelength BFL

Ring cavity configurations Single wavelength BFL To generate a single-wavelength BFL, BP signal is pumped into the ring-cavity from the BP source through the optical circulator. The BP signal will be guided into a Brillouin gain media. When the BP power exceeds the threshold power, SBS effect will be initiated and its power grows linearly with the BP power. The BS signal will be created which propagates in the opposite direction from the direction of the BP signal.

Ring cavity configurations Single wavelength BFL This BS signal will be guided in the ring-cavity through the optical circulator and the BS signal is taken from the optical coupler. The BP signal circulates in the anti-clockwise direction is blocked by the optical circulator. The optical spectrum of the single-wavelength BFL can be observed using an optical spectrum analyzer (OSA).

Ring cavity configurations Single wavelength BFL Results:

Ring cavity configurations Single wavelength BEFL

Ring cavity configurations Single wavelength BEFL The BEFL system consists of a 11 km dispersion compensating fiber (DCF), an optical circulator (Cir), an optical coupler, and an erbium-doped fiber amplifier (EDFA). The Brillouin gain is provided by the DCF while the Cir is utilized to guide the propagation of both the Brillouin pump (BP) and the BS signal into and out of the Brillouin gain media, respectively. The DCF has 20 µm2 effective area, 7.31 (Wkm)-1 nonlinear coefficient, -1328 ps/nm dispersion and 7.28 dB total loss. An optical coupler with different coupling ratios provides a medium through which the output of the laser cavity is connected to an optical spectrum analyzer (OSA) that is used for monitoring and measurement in the experiment.

Ring cavity configurations Single wavelength BEFL The EDFA gain block is composed of an 8.0 m erbium-doped fiber (EDF), a 1480/1550 nm wavelength division multiplexing (WDM) coupler, and a 1480 nm laser diode. The 1480 nm pump laser with a maximum power of 135 mW is used as the primary pump source for the EDF. The WDM coupler is used to multiplex the 1480 nm pump and the BP signal.

Ring cavity configurations Single wavelength BEFL The BP signal with 200 kHz narrow linewdith provided by an external cavity tunable laser source (TLS) was injected into the resonator. The injected BP signal from the TLS is guided into the EDFA by the optical circulator. First it is amplified by the EDFA and then guided into the Brillouin gain media (i.e., the DCF). When the BP signal exceeds the threshold power of the Brillouin gain media, the SBS effect is initiated and thus the first-order Brillouin Stokes (BS) signal that propagate in the opposite direction to the direction of the BP signal is generated. This BS signal with 0.08 nm downshifted from the BP signal is amplified by the EDFA and then transported to the optical coupler via the circulator from its port 2 through port 3. For each of the selected coupling ratios of the optical coupler, part of the BS signal is guided into the resonator and the other part is measured by using an OSA through the output port of the optical coupler.

Ring cavity configurations Single wavelength BEFL Results:

Ring cavity configurations Single wavelength BEFL Results: