We report a PM all-normal, all-in-fiber passively mode-locked laser operating at 1030 nm. The main pulse shaping mechanism is provided by a tilted chirped-FBG. The laser delivers nanojoule range highly chirped pulses at a repetition rate of about 40 MHz. The FWHM of the optical spectrum is up to 7.8 nm leading to sub-500 fs compressed optical pulses. The influence of the filtering bandwidth and the output coupling ratio has been investigated.
© 2012 OSA
Femtosecond fiber lasers are potential sources for many applications such as multi-photon imaging, micromachining, THz generation etc. In particular, ytterbium-doped fiber lasers are of great interest and have therefore been extensively investigated during the last years. Regarding ultra-short pulses ytterbium fiber lasers, the dispersion properties of silica fibers are not ideal for implementing soliton-like sources. Indeed, silica fibers exhibit high level of normal dispersion around 1 µm which has to be balanced in soliton-like sources, leading in some cases to bulky solutions employing free space elements. One alternative is to operate in All-Normal Dispersion (ANDi) regime. In this case a particular care has to be taken on the laser design to ensure a stable mode-locking operation. This is the solution we will present here with a SESAM and an original filtering element for ensuring a stable and self-starting mode-locking operation in an all-in-fiber Polarization Maintaining (PM) cavity.
As already mentioned, the use of bulk diffraction gratings in the laser cavity has been initially proposed in the literature to compensate for the high normal chromatic dispersion [1,2] of silica fibers around 1 µm. Such laser configuration proves to be very interesting in terms of the achieved performances but in addition to being bulky, it is also very sensitive to the optical alignment and to temperature changes. An alternative way to manage the chromatic dispersion in all-in-fiber configuration is to use a Photonic Crystal Fiber (PCF) exhibiting anomalous dispersion at 1 µm. However such fibers generally limit the maximum achievable pulse energy because of their low mode field diameter and are relatively expensive, difficult to handle and to splice. Recently the generation of 131 fs pulses has been demonstrated from a dispersion-managed soliton cavity involving this kind of fiber with limited pulse energy (55 pJ) . The soliton regime is very interesting for producing ultra-short pulses and can be realized in all-in-fiber configuration by using a PCF for the dispersion management. Unfortunately this regime is limited in terms of the achievable pulse energy compared with all-normal dispersion cavities.
In 2006, it was demonstrated that ANDi fiber lasers can deliver high-energy and ultra-short pulses without the need for dispersion management in the cavity . Here the pulse shaping relies on the use of a spectral filtering element. Several works dealing with pulse shaping by spectral filtering have been reported in the last few years, most of them based on the Non-Linear Polarization Rotation (NLPR) technique. Pulse energy higher than 1 nJ has been demonstrated by several groups [5–7]. To our knowledge, the shortest pulse duration achieved is 76 fs  after compression, in a laser cavity comprising a small core fiber for enlarging the optical bandwidth with nonlinear effects. Despite of their very good performances, most of the ANDi lasers making use of the spectral filtering for pulse shaping are still built with free-space optic elements [4,5,7].
In comparison, only a few works based on the use of saturable absorbers has been reported so far. Saturable absorbers generally permit a more reliable mode-locked operation compared to the NLPR. Also SA based lasers h ave a self-starting operation and, unlike NLPR lasers, are less sensitive to the temperature and to the birefringence of the fibers. Another advantage of SA-based mode-locked lasers is the possible linearly polarized operation, which is required for a number of applications such as frequency doubling. Despite all the above advantages, all-in-fiber ultrashort lasers based on the use of saturable absorbers in all-normal dispersion regime are scarcely found in literature. A PM SESAM-based ANDi fiber laser has been proposed by Ortaç and al . This laser delivers stable pulses with energy close to 1 nJ, nevertheless the FBG used in reflection induces high chromatic dispersion in the cavity leading to a dechirped pulse duration larger than 1,5 ps. A fiber laser in all-fiber configuration making use of a carbon nanotube saturable absorber was proposed by Kieu and al . Nevertheless the laser requires a polarization controller for an optimized operation. According to the authors, Non-linear Polarization Evolution (NPE) is also involved in the pulse shaping mechanism since the filter used is polarization dependant and can therefore act as a polarizer. A low repetition rate laser has been reported in reference  which is of particular interest because the laser has an all-in-fiber configuration and the mode-locking is achieved with a SESAM. The authors achieved here a few nanojoules energy. However the pulse duration is in the order of few hundreds of picoseconds with limited pulse recompression capability (spectral width less than 1 nm) which is certainly due to the large amount of chromatic dispersion in the cavity.
In this paper, we present, for the first time to our knowledge, an ultra-fast all-normal dispersion and linearly polarized all-in-fiber laser for which mode-locking is initiated by a SEmiconductor Saturable Absorber Mirror (SESAM) and pulse shaping is mainly ensured by a Tilted-Fiber Bragg Grating (T-FBG) used in transmission.
2. Experimental setup
The experimental setup is shown in Fig. 1 . The amplifying medium is a 55 cm-long double-clad ytterbium-doped fiber which exhibits a very high absorption at 976 nm. The 976 nm pump radiation is launched through a 976/1030 PM fibered multiplexer. A PM coupler in which the fast axis is blocked is used both as the output of the laser and the polarizer. Two different output ratios have been tested: 30/70 and 50/50. A PM circulator is used to sustain unidirectional ring operation and to insert the ultrafast (τrelax = 500 fs) and high contrast (ΔR = 40%) SESAM that permits a reliable self-starting mode-locked operation. A cleaving angle of 10° has been used at the output of the laser for avoiding any back-reflection during the measurements. No rotation of the polarization can occur in the cavity because all the components and fibers used are polarization sensitive and birefringent respectively.
It is well-known that an additional pulse shaping mechanism is required for a stable ultra-short pulsed operation in ANDi cavities . We chose to insert a 3° T-FBG in the cavity as the filtering element. The T-FBGs are typically fabricated by the exposure of the optical fiber to the beam of a UV laser passing through a quartz phase mask. In order to have a tilted photo-inscription of the FBG, the phase mask is tilted with an angle of 3° according to its plane. Therefore, the FBG is written with a small angle compared to the fiber normal axis instead of being written perpendicularly to the optical fiber (as it would be in a standard case) in such way that the reflected light will be extracted from the core of the fiber. The made T-FBGs induce high losses (see Fig. 2 ): Losses higher than 10 dB for wavelengths higher than 1034 nm for the first T-FBG (filter 1) and losses higher than 10 dB for wavelengths higher than 1037 nm for the second T-FBG (filter 2). The two filters exhibit background losses close to 2 dB. During laser operation, the long wavelength operation is filtered by one of the T-FBG based filter whereas the short wavelengths (<1020 nm) are naturally filtered since they cannot experience enough gain per cavity round trip (i.e. the gain provided by the ytterbium-doped fiber is lower than the cavity losses).
3. Experimental results
By using filter 1 and 30% output ratio (configuration named CFG30-F1), mode-locking operation is self-starting for pump power close to 200 mW. For this range of power, single pulse and stable mode-locking operation is sustained as it can be seen in Fig. 3 . Beyond 230 mW, we observed multi-pulse unstable behavior. We assume that the SESAM operates at pulse energies much higher than its bleaching threshold and inverse saturable absorption or detrimental non-linear effects others than saturable absorption can occur.
For 220 mW pump power, the average output power is 20.1 mW and the repetition rate is around 35.5 MHz. Therefore the pulse energy reaches 0.57 nJ. This is about 5 times higher than the highest possible energy with soliton mode-locked lasers [3,13] which are generally limited to 0.1 nJ due to the soliton energy limitations. This is lower than the 20 nJ reported in the reference . This reference is the highest energy obtained with a standard single mode fiber laser cavity but in a non-fully fibred configuration. The profile of the optical spectrum exhibits steep edges as usually observed for ANDi lasers [4–12]. The FWHM of the optical spectrum is 7.8 nm corresponding to Fourier transform-limited pulse durations shorter than 400 fs. The deconvolution factor of the autocorrelation trace has been evaluated to 1.43 by calculating the Fourier transform limited pulse from the experimental optical spectrum. The output pulse is highly chirped and its duration is close to 7 ps as it can be seen from Fig. 4 .
In order to optimize the delivered pulse energy, we replaced the 30/70 output coupler with a 50/50 coupler. In this configuration (named CFG50-F1), stable mode-locking is obtained for 240 mW pump power. The average output power significantly increased up to 52 mW, corresponding to a pulse energy of 1.36 nJ. By removing the output coupler, the cavity length is slightly changed; therefore the repetition rate becomes equal to 38.1 MHz. The optical spectrum bandwidth is in this case 7.2 nm at FWHM.
A third configuration (named CFG50-F2) was also tested. In this configuration the 50/50 output coupler is combined with the second tilted FBG (filter 2) which has a broader transmission than filter 1. Similar results in terms of pulse energy were obtained. Nevertheless the delivered optical spectrum is narrower (FWHM = 4.6 nm) in this case, as shown in Fig. 5 .
A pulse compression experiment has been performed using the CFG50-F1 configuration. The optical compressor is based on a pair of transmission gratings with density equal to 1600 lines/mm. The transmission gratings have been used at Littrow angle (55.5° @ 1030 nm) and the overall transmission of the setup is 69%. The distance between the gratings is 1.3 cm. A fiber isolator collimator has been spliced at the output of the oscillator for collimating the beam with a 2 mm diameter. The setup is described in Fig. 6 .
In general, we observed that lower output coupling ratios and narrower filtering lead to better results in terms of achievable short pulse duration since the optical spectrum is broader whereas high output ratios lead to higher pulse energy. Some similar results about the impact of the filtering have been already published [12,15]. All the conducted experiments are summed up in Table 1:
In conclusion, we present a new and simple approach to build an ultra-short pulse PM ytterbium fiber laser. For the first time to our knowledge, the pulse shaping mechanism is mainly ensured by a tilted fiber Bragg grating.
Some guidelines have been proposed to optimize the laser operation both in terms of pulse energy and duration. The best configuration exhibits nanojoule picosecond pulses that can be compressed below 500 fs. The repetition rate is about 40 MHz and the average output power is up to 50 mW. We believe that this configuration is very promising and can open the way to reliable all-in-fiber oscillators operating with high pulse energy.
The author acknowledges the support of the EU funded FP7 ALPINE Project, n. 229231.
References and links
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