1. Introduction

The development of photonic crystal fibers (PCFs) has created new opportunities for fundamental and applied laser science. Through enhanced control of the dispersion parameters, a small core PCF is an excellent instrument for the simplified generation of a spectral supercontinuum with femtosecond (fs) laser pulses [1]. The supercontinua generated by fs laser pulses in PCFs are being increasingly used in metrology, especially for frequency comb generation [2–4], and optical coherence tomography [5,6].

Most of the research on supercontinuum generated by fs laser pulses has been performed with Ti:Sapphire laser systems operating around 800 nm and delivering pulse energies of several nJ or even more. Recently Er-doped fiber lasers around 1560 nm have gained more importance as compact diode-pumped light sources for supercontinuum generation [7–10]. Er-doped fiber lasers enable not only the transfer of the fs-supercontinuum technology to the telecom wavelength around 1560 nm, but they represent also a more compact and more reliable light source than bulky Ti:sapphire laser systems. Until now, the generation of widely broadened supercontinua using these sources has been achieved in silica-based highly nonlinear fibers with a zero group velocity dispersion around 1500 nm. With such fibers, it is necessary to use nJ laser pulses at 1560 nm, or fiber lengths of several meters and more.

Though passively mode-locked Er-fiber oscillators allow the generation of ultra-short pulses with a duration down to 55 fs [11], it is necessary to amplify the pulse energy of several 100 pJ to higher levels for adequate supercontinuum generation. Er-doped fiber amplifiers (EDFAs) enable a compact all-fiber setup, but additional nonlinear effects and higher order dispersion have to be taken into account for the propagation of sub-100 fs laser pulses as they will increase the pulse duration without appropriate compensation.

In this paper we report an all-fiber diode-pumped stretched-pulse Er-fiber oscillator-amplifier system operating at 1560 nm. The oscillator was passively mode-locked by nonlinear polarization rotation [12] and the oscillator’s pulses were amplified and compressed in a compact all-fiber stretched-pulse Erbium-fiber amplifier setup. These amplified pulses were used for the generation of an octave-broad supercontinuum in an extruded PCF made out of SF6 [13], which shows a higher nonlinear refractive index and a different chromatic dispersion profile than PCF based on fused silica.

2. Erbium-fiber oscillator-amplifier system

The schematic setup of the all-fiber oscillator-amplifier system is shown in Fig. 1. The oscillator consisted of a normal dispersive Er-doped fiber (D=-22 ps/(nm•km)) and two anomalous dispersive fibers - SMF 1528 and Flexcor 1060 - with a total geometrical fiber length of 3.4 m resulting in an oscillator repetition rate of 59.1 MHz. The Er-fiber oscillator was pumped via a 980/1550nm-coupler at 980 nm in the propagation direction of the laser pulses. A Faraday isolator provided unidirectional operation and two polarization controllers were used to enable passive mode-locking.

The mode-locking was self-starting at a pump power of 130 mW and at 150 mW an average output power of 14 mW at 1560 nm was achieved. Due to the stretched-pulse setup the intracavity pulse was temporally broadened and shortened during one round trip, so that the extracted laser pulses normally had to be compressed outside the ring cavity for suitable characterization. As group velocity dispersion (GVD) is the dominant effect on the pulse chirp, this chirp can be easily influenced by the length of the output port fiber. The extracted pulses showed an up-chirp induced in the cavity by the normal dispersive Er-fiber. This chirp was reduced by approximately 75 cm of anomalous dispersive SMF 1528-fiber. The corresponding intensity autocorrelation function of the compressed pulses is shown in Fig. 2(a). The autocorrelation duration of the pulse was 90 fs; assuming a Gaussian pulse shape this corresponded to a pulse duration of 64 fs. The spectrum of these compressed laser pulses extended from 1500 nm up to 1640 nm (see Fig. 2(b)). As the limited gain bandwidth of about 30 nm to 50 nm of an Er-doped fiber does not allow the generation of pulses with a spectral bandwidth of more than 100 nm, spectral broadening effects like intracavity self-phase modulation (SPM) and stimulated Raman scattering (SRS) must have influenced the pulse generation in the oscillator and the amplifier.

 

Fig. 1. Schematic experimental setup of the all-fiber Erbium oscillator-amplifier setup and the photonic crystal fiber (PCF) for supercontinuum generation: Green fiber=Er-doped fiber, black fiber=SMF 1528, blue fiber=Flexcor 1060, red fiber=SF6 PCF, PBS=polarization beam splitter, PC=polarization controller, ISO=Faraday isolator, WDM=980/1550nm-wavelength-division-multiplexer, DC=diagnostic coupler, M1=dichroic mirror HR980nm-HT1550nm

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A fiber-optic polarization dependent isolator (isolation 40 dB) was used at the laser output to protect the laser against spurious back reflections from the amplifier. Furthermore a 50/50-coupler was integrated to analyze the seed signal for the amplifier, so that the amplifier was seeded with a power up to 7 mW.

The amplifier consisted of a Er-fiber with normal GVD and two fibers with anomalous GVD. The fiber lengths were arranged similarly to the oscillator, so that the laser pulses were also stretched and compressed during propagation through the fiber. The amplifier fiber propagation length was optimized for minimal total second order dispersion resulting in laser pulses with minimal linear chirp. Third order dispersion or nonlinear effects were not compensated in this arrangement, so that the total length of the amplifier was kept short to avoid unwanted pulse broadening by these effects.

The Er-doped fiber length was chosen to be about 1.1 m to achieve an acceptable pump light absorption (above 95%) and a moderate amount of higher order dispersion. The Er-fiber was pumped in the propagation direction with a pump power of up to 450 mW at 980 nm. At the end of the amplifier stage, a polarization controller was used to define the polarization of the extracted laser pulses. Then, the amplified pulses were collimated and the residual 980 nm pump light was separated by a dichroic mirror. A polarization beam splitter (PBS) was used to produce linearly polarized pulses in combination with the polarization controller at the end of the fiber. After this PBS the average output power was 59 mW at a pump power of 450 mW, resulting in a pulse energy of 1 nJ. The corresponding intensity autocorrelation is shown in Fig. 2(c). The width of this autocorrelation function was about 85 fs, resulting in a pulse duration of 60 fs assuming a Gaussian pulse shape. The spectral bandwidth (see Fig. 2(d)) extended from 1480 nm up to 1650 nm. Furthermore, the shape of the amplified spectrum showed a structure, which indicated the influence of higher order dispersion and nonlinear effects on the pulse propagation despite the short fiber length of the amplifier. Detailed investigations of pulse amplitude and phase are in progress and will be published in the near future.

 

Fig. 2. (a) Intensity autocorrelation function of the fiber laser (autocorrelation function width of 90 fs resulting in 64 fs pulse duration assuming a Gaussian pulse profile); (b) Output spectrum of the fiber laser; (c) Intensity autocorrelation of the amplifier (autocorrelation function width of 85 fs resulting in 60 fs pulse duration assuming a Gaussian shape profile); (d) Output spectrum of the fiber amplifier

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3. Supercontinuum generation

A half-waveplate and a PBS were used before the focal lens for variable adjustment of the pulse energy launched into the PCF. The PCF was a 30 cm long extruded SF6-fiber with a core diameter of 2.6 µm and zero group velocity dispersion around 1.3 µm [13]. With a coupling efficiency of about 20% a launched average power up to 12.5 mW was measured after the PCF, corresponding to a pulse energy of about 210 pJ at 1560 nm.

The spectrum after the PCF was measured with an optical spectrum analyzer (OSA). Due to the limits of the OSA we were only able to investigate the spectrum between 350 nm and 1750 nm. Figure 3 shows measured spectra on a logarithmic scale after the PCF for different launched pulse energies. In each spectrum a narrow peak around 980 nm appeared, which corresponded to a small amount of residual pump power from the amplifier. For low launched pulse energies up to 85 pJ spectral broadening was only observed close to the seed spectrum around 1560 nm. In comparison, for a launched pulse energy of 50 pJ a significant Raman peak around 1650 nm appeared in the spectrum, resulting in a spectral supercontinuum extending from 1300 nm beyond 1750 nm. This Raman peak, strongly reminiscent of the self-frequency shifting solitons observed at much shorter wavelengths using 800 nm excitation in silica fibers, shifted to longer wavelengths for higher pulse energies, indicating the generation of spectral components beyond 1750 nm. For launched pulse energies up to 150 pJ the spectrum around 1560 nm became broader and new spectral components appeared, e.g. at 600 and 800 nm. For pulse energies above 170 pJ the supercontinuum around the laser center wavelength of 1560 nm broadened significantly and merged with the increasing peak around 800 nm to a continuous spectrum. For the maximal launched pulse energy of 210 pJ the supercontinuum spectrum spanned over an octave from 750 nm to 1750 nm with two major peaks around 800 nm and 1550 nm. Furthermore, separated spectral peaks existed around 400 nm and 600 nm, so that the generated supercontinuum extended unevenly from 400 nm to beyond 1750 nm. The broadening effects in the SF6 fiber look broadly similar to those of silica fibers: a detailed analysis of the supercontinuum evolution and especially of the origin of the visible components is in progress and will be discussed in a later publication.

 

Fig. 3. Optical spectrum of the all-fiber oscillator-amplifier system (OAS) before the PCF and optical spectra of different launched pulse energies after 30 cm of extruded SF6-fiber

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For the investigation of the visible part of the supercontinuum spectrum a SF10 prism was used for spatial separation of the spectral components. The separated spectrum was displayed on a white-bleached sheet of paper and photographed with a digital camera. Figure 4 shows the visible spectrum of the supercontinuum generated at a launched pulse energy of 210 pJ. The spectral section consisted of three peaks at 550 nm, 600 nm and 800 nm, which corresponded perfectly to the spectrum measured with the OSA. Due to the infrared sensitivity of the digital camera the peak at 800 nm appeared only as a blue/white spot on the photograph. The spectral intensity around 400 nm was too weak for suitable monitoring with the digital camera, but its fluorescence was visible to the eye on the paper.

 

Fig. 4. Spectrum measured with an OSA (above) and digital picture of the visible spectrum of the generated supercontinuum separated by a SF10 prism (below). The letters indicate the relation between the observed wavelengths at 550 nm, 600 nm and 800 nm.

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4. Conclusion

We reported a diode-pumped Er-doped all-fiber oscillator-amplifier system, which delivered compressed 1 nJ, 60 fs laser pulses at a center wavelength of 1560 nm. These laser pulses were used for supercontinuum generation in a 30 cm long extruded SF6 fiber. We achieved a more than octave-broad spectrum spanning from 400 nm to beyond 1750 nm with launched pulse energies of about 200 pJ. We also observed the evolution of the supercontinuum spectrum as a function of launched pulse energy. The evolution indicated that there is also a significant supercontinuum generation at wavelengths beyond 1750 nm by Raman shifting of spectral components, in the wavelength regime where the fiber dispersion is anomalous.

In comparison to previous experiments on fs supercontinuum generation with passively mode-locked Er-doped fiber laser systems, the use of the extruded SF6 fiber instead of a silica based fiber enabled the generation of a broader supercontinuum containing visible components, with a low pulse energy of less than one nJ and a fiber length less than one meter.

In conclusion, this more than an octave-broad supercontinuum light source based on a passively mode-locked Er-fiber oscillator-amplifier system and an extruded SF6 photonic crystal fiber is a compact and reliable system for applications in metrology and spectroscopy (e.g., frequency comb around 1560 nm or optical coherence tomography).