Architectures for linewidth reduction in cascaded Raman fiber lasers and applications

Abstract

Fiber lasers have been increasingly used as a high-power source due to several advantages, such as superior beam quality, better thermal management, high efficiency, and compact and reliable design. In fiber lasers, rare-earth dopants are used to provide gain, and feedback is provided by fiber Bragg gratings (FBGs). Rare-earth dopants such as Ytterbium (Yb) provide gain in 1050 - 1120 nm, Erbium (Er) or Erbium-Ytterbium (Er-Yb) in 1530 - 1590 nm, and Holmium/Thulium (Ho/Tm) in 1900 - 2100 nm. There are substantial white spaces in output wavelengths of rare-earth doped fiber lasers in the 1000 - 2000 nm range. Cascaded Raman fiber lasers (CRFLs) can fill these gaps by using stimulated Raman scattering (SRS) to achieve gain in intermediate wavelengths. CRFLs transfer power from a pump to a Stokes at a longer wavelength. This process can be cascaded, where Stokes can act as a pump and transfer power to higher Stokes at longer wavelengths. The gain provided by SRS is dependent only on the underlying fiber medium. Using a tunable pump in CRFLs allows the SRS gain wavelength to be tuned. While FBGs provide feedback at a fixed wavelength, the backscattered part of Rayleigh scattering (RS), which occurs due to random inhomogeneities in the fiber, provides wavelength-independent random distributed feedback (RDFB) over a long fiber length. Broad feedback at one end reduces the generation threshold of Raman Stokes. A wavelength-tunable pump and broad feedback are reported to provide power at any wavelength in the 1000 to 2000 nm range.

However, broad feedback leads to the broad output line widths of CRFLs, affecting the spectral density, which is the amount of power within a fixed wavelength span. Spectral density can be increased by increasing the spectral purity, the ratio of power in desired Stokes by total power, and having a narrower linewidth output. Using a low-intensity noise pump, the spectral purity can be very high (>99\%). It is seen that the distribution of power inside an RDBF CRFL with feedback at one end is not uniform, and the backward lasing power is significantly lower than the forward lasing power. This allows low-power components like filters to be used in the backend of CRFLs even when the output is at significantly high power. Different filters, such as Fabry-Parot filters, air-etalons, thin film filters, etc., have been reported to reduce the linewidth but are limited to a single Stokes shift or tunability. A reflective Fourier spectral shaper was used as feedback to provide arbitrary spectral filtering to the feedback. It leads to a significant reduction in output linewidth with multi-watt output power. However, linewidth reduction could be achieved till the 3rd Stokes shift, limited by the bandwidth of spectral shaper. Having a spectral shaper with higher bandwidth providing feedback to all the cascaded Stokes with narrower linewidth is difficult.

It was observed in the spectral shaper experiments that the reduction of the linewidth of desired Stokes is independent of the linewidths of previous Stokes. Hence, it is proposed to use a simple dual feedback mechanism consisting of broad feedback due to flat cleave and a narrow wavelength of tunable feedback due to grating to narrow the linewidth. Significant linewidth reduction of about 5 times with ~10 W power in the desired wavelength is achieved for multiple cascaded Stokes from 1100 nm to 1500 nm. The linewidth reduction can be easily extended to higher wavelengths, limited only by the loss of the fiber. Thus, the dual feedback is a simple mechanism to reduce linewidth at any output wavelength of CRFLs without compromising the wavelength tunability.

Frequency doubling of CRFLs with tunable Yb-pump can generate power at any visible wavelength from green to red and beyond. As the near-infra-red (NIR) pump’s linewidth becomes more than the phase-matching bandwidth of the nonlinear crystal, less NIR power is utilized for doubling, reducing efficiency. Because of the narrowed output of linewidth, higher efficiency of visible generation was achieved. By frequency doubling the output of CRFL with dual feedback, 100 mW power was achieved in wavelengths from green to yellow. Longer visible wavelengths can be generated using appropriate nonlinear crystals. Significantly higher visible power can be generated by making the CRFL system all polarization-maintaining.

In summary, mechanisms for linewidth control of cascaded Raman fiber laser output at any wavelengths were explored. Due to the linewidth narrowing, high-efficiency frequency doubling is achieved to get wavelength-tunable visible power from green to red and beyond.