1. Introduction

Ultrafast fiber lasers and their supercontinuum generation have attracted much attention in many applications, such as the optical coherence tomography, spectroscopy, and frequency metrology [1–3]. Particularly, for an optical frequency comb, spectrum over one octave of bandwidth is necessary for the f–to–2f interferometer technique [4, 5].

The high repetition rate Yb:fiber lasers are desirable to achieve sufficiently large comb line spacing for high-precision calibration of astronomical spectrographs. The nonlinear polarization evolution (NPE) mode-locked fiber ring lasers with large modulation depth and essentially instantaneous response support a broad spectrum for sub-50 fs pulse generations [6, 7], which becomes a popular technique for the seed pulse source of laser frequency combs. However, limited by the ring cavity length, high fundamental repetition rate is still challenging for Yb:fiber ring lasers. The repetition rate operation up to 570 MHz in Yb:fiber laser with free-space coupled high pump power (1.4W) was demonstrated by Wilken et al [8]. Recently, a newly-developed technique of wavelength-division-multiplexing (WDM) collimator [9–11] succeeds to remove the pigtailed fiber and has been proved to be an efficient way to obtain a higher repetition rate in fiber ring laser. On the other hand, all-normal dispersion (ANDi) fiber laser without the intracavity grating pair can contribute to higher repetition rate [10, 13]. However, the large net normal dispersion of ANDi fiber laser increases the noise level and timing jitter, which degraded the signal noise ratio of the detected carrier envelop offset frequency (fceo) [14, 15]. Therefore, a near zero dispersion cavity is preferred for a frequency comb, and the grating pair is usually used to adjust the cavity dispersion.

Higher repetition rate can directly lead to lower delivered pulse energy, which makes the succeeded spectrum broadening difficult. To date, external amplification has been applied to obtain enough pulse energy for the octave spanning spectrum in a nonlinear fiber with the laser repetition rate above 386 MHz [12, 16]. However, the amplification process may introduce amplified spontaneous emission (ASE) noise, and make the system more complex and instable. To avoid all of these, high-nonlinearity fibers have been employed such as the soft glass fibers, tellurite glass fibers and chalcogenide fibers [17–21] to adopt lower pulse energies. Supercontinuum generation with further lower pulse energy is demanding as the pulse repetition rate is increased.

In this letter, we report a 528 MHz repetition rate ring cavity femtosecond fiber laser and its spectrum broadening in a tellurite microstructured fiber to generate octave-spanning supercontinuum. The spectra obtained are from 750 nm to 1700 nm with the pulse energy above 10 pJ. To our knowledge, this is the lowest pulse energy to obtain the octave-spanning spectrum at the repetition rate above 500 MHz.

2. Set up of the 528 MHz Yb:fiber ring laser

The schematic of Yb:fiber laser is shown in Fig. 1 . The fiber section comprises 12 cm length of Yb doped fiber with the absorption of 1600 dB/m at 976 nm. Both of the WDM collimator and fiber collimator had a length of 4.5 cm, giving a total fiber length 21 cm, the free-space section was ~22 cm long containing a bulk Faraday rotator, wavelength plates, two polarization beam splitters (PBS) and a pair of 1000 lines/mm fused silica transmission gratings providing anomalous dispersion. The output pulses are rejected from the PBS and dechirped with a pair of gratings outside the cavity.

 

Fig. 1 Schematic of 528 MHz Yb fiber ring laser (PBS: polarization beam splitter, FR: faraday rotator, λ/2: half-wave plate, λ /4: quarter-wave plate, YDF: Yb doped fiber).

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Compared with our previous experiment [12], the major difference is that the anomalous dispersion fiber is replaced by a transmission grating pair. This makes the laser more efficient. Another benefit is the adjustability of the grating separation that allows the fine tuning of the dispersion for achieving the broadest spectrum.

The cavity dispersion was carefully designed. The calculated group delay dispersion (GDD) of the total dispersion of for a 12 cm long Yb:fiber is + 2760 fs². The 9 cm long single mode fiber is + 2160 fs². The dispersion of the 1000 lines/mm grating pair is calculated to be −6300 fs²/mm for double pass and is slightly adjustable.

Two 650 mW 976 nm laser diodes were combined into a single mode fiber as the pump laser which delivers a maximum effective pump power of 1.07 W. The pump power is coupled into the Yb doped fiber with a WDM collimator, as described in [11]. The laser threshold is 850 mW. At the maximum pump power, the output power of the mode-locked laser was 410 mW which exhibits an optical-to-optical efficiency of ~40%. The output power after the grating pair compressor is 307 mW. The laser was self-starting and last for a week without covering with a box. This super stability is possibly due to the very short fiber length and the compact of the laser structure.

The output pulse spectra and the corresponding autocorrelation traces after the grating pair compressor are presented in Fig. 2 . The measured pulse spectra widths were 19 nm, 33 nm and 50 nm for the intracavity grating separation of 1.4 mm,1.1 mm and 0.8 mm respectively. The corresponding minimum pulse durations were from 96 fs, 62 fs and 46 fs for the Gaussian profile assumed. The broadband spectrum and the sub-50-fs pulse agree well with our prediction that a shorter intracavity fiber will result in short pulse output. Sub-50fs pulses are favorable for the subsequent spectrum expansion in the high nonlinear fiber.

 

Fig. 2 (a) (b) (c): spectra with the separation of grating pair at 1.4 mm (a), 1.1 mm (b) and 0.8 mm (c); (d) (e) (f): the corresponding measured autocorrelation traces of the compressed pulses. The dechirped pulse width was calculated to be 96 fs (d), 62 fs (e) and 46 fs (f) with Gauss profile assumed.

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The measured repetition rate is 527.7 MHz with signal to noise ratio of 60dB (Fig. 3(a) ). The phase noise spectrum was measured till 1 MHz and is shown in Fig. 3(b). The root mean square of timing jitter was calculated to be 1.7 fs by the integration of the phase noise spectrum from 1 kHz through 1 MHz. The relative intensity noise was measured to be < 130 dBc/Hz@1MHz.

 

Fig. 3 (a): Radio frequency spectrum from 0 GHz to 1.1 GHz at the resolution bandwidth of 1MHz. Inset: Radio frequency spectrum at the resolution bandwidth of 10 kHz. (b): phase noise spectrum. PSD: power spectral density.

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3. Description of the tellurite glass based microstructured fiber

The electron microscope image of the cross section of the tellurite fiber used in this experiment is shown in Fig. 4 . The fiber structure is similar as the fiber in [21]: three very fine filaments 5 μm long and ~100 nm wide supports a 1.2 μm diameter core. The nonlinear coefficient of this fiber is estimated to be 1348 km⁻¹W⁻¹(with n₂ = 2.5 × 10⁻¹⁹m²W⁻¹).

 

Fig. 4 Dispersion curve of the tellurite fiber with the zero dispersion wavelength around 1 μm. Inset: scanning electron micrograph of the core region in the tellurite fiber.

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The zero-dispersion-wavelength (ZDW) of the bulk tellurite glass is beyond 2 μm, and the suspending-core structure can shift the ZDW to shorter wavelength. The modeled dispersion of this fiber is shown in Fig. 4. It is seen that its ZDW was about 1 μm, a little below the supercontinuum pump wavelength at 1.03 μm. Thus, the pump pulses propagate in the region of anomalous dispersion, suitable for supercontinuum generation. The experimental data about the dispersion at 780 nm and 920 nm were −500 ps/nm/km and −70 ps/nm/km, in accordance with the modeled results.

4. Octave-spanning spectrum generation

A 13 cm long tellurite fiber was used for supercontinuum generation. The dechirped pulses were directly coupled into the tellurite fiber with a 1.5 mm focal length aspheric lens. The launched pulse energy was increased from 1.3 pJ to 17.5 pJ. The resulting spectra, shown in Fig. 5 , are plotted on a logarithmic scale for different launched pulse energies. An octave-spanning spectrum from 750 nm to 1700 nm was found at the pulse energy below 20 pJ.

 

Fig. 5 Spectrum evolution for the coupled pulse energy from 1.3 pJ to 17.5 pJ in the tellurite fiber.

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It can be seen that the spectral broadening appeared at the pulse energy as low as 1.3 pJ. After the pulse energy was increased to 3.7 pJ, the Raman peak shifted to 1150 nm in the long-wavelength regime, and a peak appeared at 850 nm in the normal dispersion regime of the fiber. Further broadening to 800nm on the short-wavelength side of the pump and the Raman peak around 1650 nm were observed, when the launched energy reached 9.8 pJ. As the pulse energy is further increased from 9.8 to 17.5 pJ, more Raman soliton peaks filled in gaps between 1250 nm to 1600 nm, together with the spectral intensity increased. There are two peaks located at 770 nm and 1540 nm in the spectrum. To the best of our knowledge, this is the lowest pulse energy reported for the generation of the octave-spanning spectrum directly from Yb-based fiber laser oscillator.

Such a low-pulse-energy broadened spectrum not only offer the possibility for the direct generation of frequency comb from a Yb:fiber laser, but is of particular interest in the development of astro-combs, which requires high repetition rate up to tens of GHz.

The octave-spanning spectra with longer tellurite fiber lengths were also tested. It is easy to obtain octave-spanning spectrum across two peaks at 700-800 nm and 1400-1600 nm, respectively. These peaks offer the source for the f -to-2f interference signal for Yb fiber laser combs.

Mid-infrared signals beyond 1700 nm were also recorded by an infrared spectroscopy (ocean optics NIR quest) in this experiment. With 17.5 pJ launched pulse energy, Raman peaks at 2100 nm and 2300 nm were observed, with an intensity of 27 dB lower than the maximum signal at 1030 nm. Because the tellurite glass has a high transmission through the mid-infrared to 5 μm [22], this technique is potentially used for mid-infrared spectroscopy and mid-infrared frequency comb. Further study in this wavelength region will be reported in the near future.

5. Conclusions

We have developed a compact 528 MHz repetition-rate and 46 fs Yb:fiber ring laser, and obtained octave-spanning spectrum pumped by this laser in a microstructured tellurite fiber at the coupled pulse energy of tens of pJ. The transmission grating pair and compact cavity make the laser more efficient and deliver shorter pulses. Owing to the high output power and short pulses, the spectrum expands a wavelength range from 750 nm to 1700 nm in a 13 cm tellurite fiber. This is the first time to show the octave spanning spectrum at >500 MHz repetition rate and at the lowest pulse energy. Such a broad spectrum can be used to develop a simple and compact frequency comb with the f-to-2f interference technique. Furthermore, the experiment also shows the capability of the fiber to expand the spectrum to mid-infrared with low pulse energies.