Multi-parameter tunable ultrafast fiber lasers have driven the development of many emerging areas of femtosecond biomedical photonics. Since it is difficult to independently tune the three parameters of center wavelength, repetition frequency and pulse width with guaranteed output pulse energy for solid-state ultrafast lasers, femtosecond biomedical photonics usually employs a pulse-selective fiber chirped pulse amplifier (pp-FCPA) coupled with an optical parametric amplifier (OPA) as the driving light source. However, the bulky spatial components of the OPA greatly affect the beam quality and environmental immunity of the system, and the tedious routine maintenance is beyond the knowledge of life scientists. Therefore, in order to replace the OPA technology and utilize the advantages of the pp-FCPA system, the authors developed a wavelength-tunable femtosecond light source based on supercontinuum generation.
Fig. 1 Three ways of supercontinuum generation
Figure 1 illustrates three common methods for generating a supercontinuum spectrum. Method 1 uses an all-fiber fusion architecture, which offers the most compact construction and excellent environmental stability, but the fiber carries mostly picosecond lasers, commonly found in commercial lasers. Method 2, on the other hand, uses a commercially available closure with fiber end caps and mode expansion as a nonlinear wavelength converter attached to a titanium sapphire oscillator to support wavelength conversion for femtosecond pulses. Method 3 is similar to Method 2, but combines a pp-FPCA front end with the advantages of fiber optics by coupling high-energy femtosecond pulses into a section of photonic crystal fiber to produce a coherent supercontinuum spectrum. It is this third approach that the authors employ in this paper.
However, it was experimentally found that the optical fiber used for supercontinuum generation is typically damaged after about 100 hours of cumulative operation. Irreversible optical damage greatly limits the lifetime of a supercontinuum source. Therefore, there is a great need to determine the principle of this optical damage in order to find means to circumvent it. If the optical damage is caused by airborne contaminants in a non-supercleanroom environment and/or spatial coupling of high peak power at the fiber endface, it can be addressed by commercially available photonic crystal fiber endcaps or by collapsing a specific aperture in the fiber endface.
Table 1 Three experimental scenarios for supercontinuum generation
Table 1 lists the three experimental schemes used by the authors to study fiber damage mechanisms. Scheme 1 coupled an input pulse with a central wavelength of 1030 nm, a repetition frequency of 10 MHz, and a pulse width of 280 fs into a 25 cm section of LMA-PM-15 fiber, and after repeated experiments, all of them found that the fiber was damaged after 100 ± 40 hours of cumulative operation. Scheme 2 used a different drive light source and photonic crystal fiber, but the peak power density coupled to the fiber end face remained the same as in Scheme 1. Scenario 2, however, shows optical damage within 10 ± 2 hours. The two schemes differ in where the optical damage occurs: the optical damage in scheme 1 is located <10 cm from the incident end of the fiber, whereas the optical damage in scheme 2 is located <1 cm from the incident end of the fiber. This difference suggests that the cause of the fiber damage is not airborne contaminants in the environment or the high peak power density during coupling, and that optical damage cannot be avoided by fiber end caps. Upon analysis, this fiber damage can be explained by the optical waveguide theory of long period fiber grating (LPFG). When a pulse is coupled into an optical fiber, part of the energy goes into the core while the other part of the energy passes into the cladding. When the light from the core mode and the cladding mode interferes with each other and produces a standing wave, an LPFG is written in the fiber.The shorter the period of the LPFG, the more cycles are contained in the same fiber length, and the more easily the fiber is damaged.
To verify this idea, the authors chose the LMA-PM-40-FUD fiber with a mode field diameter of up to 32 μm in Scheme 3. Its LPFG period is calculated to be about 9 cm, and the 9 cm fiber length is less than one cycle, so the theoretical fiber damage effect caused by LPFG will disappear. Experimentally, the optical system of scheme 3 does also remain stable after 2000 hours of accumulated operation.
Fig. 2 Schematic diagram of a multiparameter tunable femtosecond light source based on a fiber nonlinear converter
Figure 2 represents the multi-parameter tunable femtosecond light source built by the authors based on Scheme 3. The whole light source consists of a pp-FCPA system with a repetition frequency tunable from 1-10 MHz as the front-end, and a photonic crystal fiber that avoids the optical damage caused by the LPFG as the unit that generates the supercontinuum spectrum, i.e., the fiber nonlinear converter (FNWC). The spectrally broadened pulse is directed into a programmable pulse shaper. By selecting a specific filter window and amount of dispersion compensation, the center wavelength can be tuned in the range of 950-1110 nm and the pulse width can be tuned in the range of 40-400 fs. In addition, the final output pulse can be transmitted with a section of low-dispersion Kagome hollow-core fiber optic patchcord, allowing this light source to be easily switched between different application modules.
In summary, the authors have developed a reliable heavy-frequency, wavelength, and pulse width accessory for femtosecond fiber lasers that is substantially tunable, explains and suppresses optical damage in the coupled fiber system, and whose corresponding integrated laser system has a high degree of stability, which is expected to broaden the applications of tunable ultrafast lasers in the biological and medical fields.
