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We demonstrate 308 mW of single-mode laser emission at 464 nm from a frequency doubled picosecond fiber based Master Oscillator Power Amplifier (MOPA). The laser system consisted of a gain-switched and spectrally narrowed Fabry-Perot laser diode emitting at 928 nm, which was amplified in a two-stage amplifier based on W-type double-clad Nd-doped fibers. Output pulses with a duration of 90 ps at a repetition rate of 41 MHz were frequency-doubled in a periodically poled MgO-doped Congruent Lithium Niobate. A conversion efficiency of 14.8% was achieved in single-pass configuration.
© 2010 Optical Society of America
1. Introduction
High power CW or pulsed blue laser sources are of great interest for a variety of applications, including biomedicine, optical sensing or displays. Numerous applications require both diffraction-limited and single-frequency beam in a compact and efficient laser system. Most of practical realizations of such sources rely on intracavity second harmonic generation (SHG) of CW Nd-doped solid state lasers operated on the three-level transition near 946 nm [1,2]. However, these systems are limited to certain wavelengths and, most of all, it is difficult to maintain a good spectral purity and a diffraction-limited beam in a high power regime, mainly because of thermal effects. Up-conversion lasers based on Tm-doped ZBLAN are another approach but photodegradation at high pump power limits the output power to less than 250mW [3]. Laser diodes are probably one of the most promising coherent sources for CW blue laser emission at some specific wavelengths in the 405-470 nm spectral range. However, short lifetimes and low beam quality are still an issue and performances are much lower in pulsed regime whereas this regime is needed for many applications such as fluorescence lifetime imaging in biophotonics. As for example, gain-switched GaN laser diodes are commercially available and are able to generate pulses with duration as short as 50 ps at 40 MHz [4] but maximum peak power is still very low (< 1 W).
An attractive approach to overcome all these problems consists in using fiber laser sources, which have the great potential to combine high power, high beam quality and relatively compact size. For example, a record power of 60 W at 530 nm has been obtained by frequency doubling an Yb-doped fiber laser [5]. At the present time, blue light generation by frequency doubling a fiber laser was mainly attempted using an Yb-doped fiber laser emitting at 980nm [6]. The best results in terms of average power were obtained by A. Bouchier et al. with 83 mW at 489 nm by frequency doubling using a periodically poled lithium niobate (PPLN) waveguide [7]. Another interesting approach to reach shorter wavelengths in the blue spectral domain is the frequency doubling of an Nd-doped fiber laser operated on the three-level laser scheme 4F3/2 → 4I9/2 near 930 nm. A Q-switched Nd-doped depressed clad hollow optical fiber laser already permitted to generate 50 mW at λ = 464 nm [8] after a frequency doubling with a BiB3O6 non-linear crystal. For efficient SHG an alternative to CW or Q-switched lasers consists in using a fiber MOPA which may combine high peak power and high average power. Recently, gain switched laser diode have been utilized as seeding source in high-power fiber MOPA emitting at 1 μm and 1.5 μm [9,10]. These developments have led to multi-stages fiber MOPA systems able to produce kilo-watt level output power in CW regime and 300 W of average power in 20 ps pulses at 1 GHz repetition rate. Compared to Q-switched or mode-locked laser sources, MOPA systems offer a much better flexibility in terms of the pulse shape, repetition rate and spectral width while keeping a high efficiency and a high beam quality. In this paper, we report the performances of a 90 ps pulsed Nd-doped fiber MOPA emitting at 928 nm in a narrow optical spectrum and its frequency doubling using a periodically poled lithium Niobate crystal. 308 mW of average power was achieved at 464 nm in a diffraction-limited beam.
2. Picosecond fiber MOPA at 928 nm
Amplification usubg a Nd3+-doped fiber near 930 nm is based on a three-level laser scheme (4F3/2 → 4I9/2) and hence suffers from the competition with the 4F3/2 → 4I11/2 transition near 1050 nm which is much more efficient due to its four-level nature. For this reason, W-type core refractive index profile was necessary to filter out the amplified spontaneous emission (ASE) and spurious lasing at 1060 nm [11]. This design creates a LP01 mode cut-off and longer wavelengths suffer from high losses distributed along the fiber which sufficiently decrease the gain at 1060 nm to allow efficient lasing or amplification at 930 nm. Refractive index profile was designed to induce an effective cut-off wavelength λc near 1060 nm. During experiments, this wavelength could be shifted towards shorter wavelengths by a fine adjustment of the bending radius of the coiled fiber [12]. Consequently, very high core attenuation of more than 50 dB/m at 1050 nm was achieved in our fibers without incurring any losses at 930 nm.
The experimental setup is shown in Fig. 1 . A Fabry-Perot (FP) laser diode emitting near 928 nm was gain-switched by a home-made nanosecond electric pulse generator with a DC bias current, based on the scheme given in Ref. [13]. The output beam was sent through a etalon filter which acts as a wavelength filter to reduce the large spectral width of the gain-switched laser diode. The beam was then coupled in a fiber loop in order to ensure a self-seeding effect and consequently wavelength locking [14]. In our configuration, the fiber delay imposed a round-trip delay time of 0.24 μs, which allowed us to operate the source at a repetition rate of a multiple of 4.1 MHz. By carefully adjusting the repetition rate, single longitudinal mode operation was achieved with 30 dB of side-mode suppression ratio at a repetition rate of 41 MHz. The linewidth of the mode was measured to be less than 50 pm, limited by the resolution of the optical spectrum analyzer. It is thus particularly adapted to second harmonic generation in a periodically poled crystal owing to their low spectral acceptance. In order to operate the gain-switched laser diode in picosecond regime, its average output power was limited to ~0.5 mW at a repetition rate of 41 MHz. Because of fiber coupling loss and transmission loss in the isolator which was originally designed for 980 nm, only 70 μW of average signal power was injected in the Nd-doped fiber amplifiers (NDFA). Therefore, a pre-amplifier stage was necessary to reach a power level able to saturate the gain of a second power amplifier and avoid strong ASE. This signal was injected into the first NDFA through a pump/signal combiner which also coupled the pump power from a pigtailed multimode laser diode at 808 nm. The fiber amplifier consists of 8 m length of Nd-doped double-clad fiber with a cladding diameter of 125 μm, a core diameter of 5 μm, NA 0.11 and a pump absorption coefficient of 0.44 dB/m at λ = 808 nm. This fiber geometry ensures a compatibility with standard single-mode fiber components and hence allows an all-fiberized setup. The radius of the depressed region around the core was 6.6 μm with an average refractive index of 1.452 whereas the average refractive index of the core and the clad were respectively 1.462 and 1.457. Using this index profile, a bending radius of 6 cm was still necessary to totally suppress the ASE near 1050 nm. In our setup, the two-stage amplifier is all built from fiber or pigtailed components which are single-mode at 930 nm and spliced together, except from the bulk second isolator because of the lack of commercial pigtailed isolators having a sufficient isolation factor at λ = 930 nm (i.e. > 30 dB). With 1.43 W of launched power at 808 nm, the first amplifier provided an effective gain of + 11 dB, which is sufficient to saturate the second amplifier and low enough to avoid the rise of ASE power. Figure 2 shows the signal spectrum at the output of the first amplifier. ASE is contained in a spectral range from 918 to 934 nm but represents only 10% of the total output power. Peak power is sufficiently low to avoid nonlinear effects and no spectral broadening was observed.
Fig. 2 Optical spectra of the signal at 928 nm from the self-seeded laser diode (light gray curve), the output of the first amplifier (dark gray curve) and the output of the power amplifier at an average power of 2.3 W (black curve). Inset: zoom-in of the signal peak.
The power amplifier was almost identical to the first amplifier except that four pigtailed multimode laser diodes at 808 nm are launched through a pump/signal combiner to achieve a total pump power of 13 W. Nd3+ concentration was also lowered to reduce neodymium ions clustering, which are responsible for a strong decrease in the efficiency of the laser conversion [15]. As a consequence, a fiber length of 25 m was used, resulting in a pump absorption of 80%. With 1 mW of seed power and for a pump power of 13 W, the power amplifier produces 2.3 W of output power, corresponding to a slope efficiency of 20% with respect to the launched pump power, as shown in Fig. 3 .
From these results, we believe that the fraction of clustered ions is still relatively high and a better core composition with other codopants would be needed to reduce this detrimental effect. At the maximum output power the amplified signal was 18 dB above the ASE (Fig. 2). As expected, the beam quality was measured to be diffraction limited with a M2 better than 1.03. Figure 4 shows an autocorrelation traces that revealed at full power a 90 ps FWHM pulse duration assuming a Lorentzian pulse shape. Therefore, a peak power of ~500 W and the long interaction length cause spectral broadening owing to self-phase modulation (SPM) as shown in the inset of Fig. 2. It can be observed a large pedestal and the laser linewidth was broadened up to 0.34 nm at the maximum average output power of 2.3 W. This spectral broadening is consistent with theoretical expectations.
Basically, for an initially unchirped pulse, spectral broadening can be expressed as
where Ppeak is the pulse peak power, Leff is the effective fiber length, τ is the pulse duration, and Aeff the effective mode area. According to this relation, picosecond pulses propagating in a single-mode core may be detrimental and SPM is clearly the limiting nonlinear effect for applications such as frequency conversion owing to the limited spectral acceptance of nonlinear crystals. For this reason, it might not be judicious to amplify pulses shorter than 100 ps, except if higher repetition rate is of interest as peak power will decrease for the same average power. It can be noted that large mode area (LMA) fiber design is well adapted to reduce the threshold of nonlinear effects because shorter fiber length may be used while mode area is increased. However, further calculations using the method published in Ref. [16]. revealed that a fiber core diameter higher than 7-8 μm would be unsuitable for efficient wavelength filtering using W-type refractive index profile.3. SHG at 464 nm in PP MgO:LN
For generation of the second harmonic, the diffraction-limited beam at 928 nm was coupled into a periodically poled MgO-doped Congruent Lithium Niobate (CLN). The crystal was manufactured by HC Photonics, has a grating period 4.426 μm and was 5 mm long with a 0.5 × 2 mm aperture. It was heated using a thermoelectric Peltier module at an optimal operation temperature of 43°C. The output beam from the MOPA was collimated with a 4.5 mm focal length aspheric lens and a simple 30 mm focal length plano-convex lens was used to focus the beam with a spot radius of 20 μm. The polarization state was adjusted using a polarization controlling loop, providing a polarization extinction ratio better than 90%.
In Fig. 5 the output power of the second harmonic at 464 nm is depicted as a function of the incident fundamental power. For an average fundamental power of 2.08 W, we measured 308 mW of blue light, corresponding to conversion efficiency of 14.8%. A thermal acceptance of 3.8°C has been measured and agrees well with the theoretical value calculated using the Sellmeier equation [17]. With the rise of the fundamental power, the temperature of the PPLN crystal had to be adjusted within 1 to 2°C to compensate for the thermal effects. However, even with this compensation, the SHG power deviates from the theoretical tanh2 curve and a fast saturation effect followed by a roll-off can be observed for the highest incident powers. This effect could not be compensated by adjusting the PPLN temperature or the incident polarization so that it might be related to photorefractive effects or/and a temperature gradient along the crystal caused by a residual pump or signal absorption.
Fig. 5 SHG output power as a function of the fundamental power at a crystal temperature of 43°C. Inset: measured M2 along x- and y-axis.
In addition, we believe that the SHG efficiency is also strongly limited by the spectral broadening of the fundamental power owing to SPM. The wavelength acceptance bandwidth of the PPLN crystal can be evaluated using the following equation [18]
where L is the length of the nonlinear crystal and n the refractive index calculated at the fundamental and second harmonic wavelengths using the Sellmeier equation [16].This equation gives an acceptance bandwidth of 0.45, 0.23 and 0.11 nm for the respective standard crystal lengths of 5 mm, 10 and 20 mm. For this reason, a crystal length of 5 mm was chosen for the frequency doubling of the MOPA source, which provided a maximum laser linewidth of 0.34 nm (FWHM) at the maximum output power. However, the spectrum of the MOPA presents a large pedestal component (Δλ = 1.25 nm at 1/e2) and only 50% of the power is contained in the 0.45 nm bandwidth of the PPLN crystal. The spectrum of the blue laser emission at its maximum output power is presented in Fig. 6 . At an average fundamental power of 2.1 W, the laser line presents a nearly gaussian shape with FWHM of only 0.25 nm which also confirms that a large part of the fundamental power was not converted during SHG. In addition to the lower conversion efficiency caused by the pulsed regime, this observation mainly explains why the measured conversion efficiency deviates from the ideal case of CW or nanosecond pump source with potential narrow linewidth, for which 40% of conversion efficiency has already been demonstrated [19]. No beam degradation has been observed after SHG and the generated blue light is still diffraction-limited with a measured beam propagation factor of M2 < 1.03 in both directions as shown in the inset of Fig. 5.
4. Conclusion
In conclusion, a picosecond blue laser source at 464 nm has been demonstrated with the maximum output power of 308 mW by using a seeded neodymium doped amplifier and a bulk periodically poled MgO:LN for SHG. To the best of our knowledge, this is the highest blue power reported so far from a frequency-doubled fiber-based laser source. Conversion efficiency in second harmonic was 14.8%, mainly limited by the spectral broadening owing to self-phase modulation. In addition, the blue source presents a diffraction-limited beam and picosecond pulse regime, which is particularly adapted for biological applications such as fluorescence lifetime imaging in confocal microscopy.
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