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We demonstrate that a combination of ultrafast wafer bonded semiconductor disk laser and a bismuth-doped fiber amplifier provides an attractive design for high power 1.33 µm tandem hybrid systems. Over 0.5 W of average output power was achieved at a repetition rate of 827 MHz that corresponds to a pulse energy of 0.62 nJ.
© 2014 Optical Society of America
1. Introduction
Semiconductor disk lasers (SDLs) utilize the advantages of the disk laser concept, e.g. low thermal lensing and good heat dissipation [1–3]. Semiconductor gain media offers spectral versatility that stems from a wide portfolio of available compounds [3–8]. In particular, recently demonstrated technologies of wafer fusion [9, 10] and low-temperature wafer bonding [11] allow high power operation at the spectral range of 1.2-1.7 µm that is difficult to cover utilizing the monolithic epitaxial growth [12, 13]. Such sources have already proved their significance in medicine [14, 15], astrophysics [16], broadband data transmission [17], optical clocking [18], and sampling [19].
The extensive efforts of the last decade have resulted in substantial progress in SDLs passively mode-locked by semiconductor saturable absorber mirrors (SESAMs) [20–26]. Since SDLs are highly sensitive to the cavity losses owing to a low single-pass gain, the mode-locked operation typically exhibits reduced output power, as compared with the continuous-wave regime, due to the inevitable loss induced by the saturable absorber. A practical solution for boosting the power of mode-locked SDLs could be an external amplification. Fiber amplifiers offer a highly practical solution due to their robust scheme with low-maintenance and considerable power scaling capability. To date, external amplification of mode-locked SDLs has been demonstrated using ytterbium-doped fiber at 1.05 µm [27, 28]. Such a scheme has also been employed for supercontinuum generation at 1.05 µm [29] and at 1.57 µm [30]. The concept can be further extended to the spectral range of 1.3 µm using Raman and bismuth-doped fiber amplifiers [31, 32].
In this Letter, we demonstrate a picosecond mode-locked SDL emitting at 1.33 µm, which is amplified in a bismuth-doped fiber amplifier. The 1.33 µm SDL was fabricated using low-temperature bonding technology that allows the integration of GaAs/AlAs distributed Bragg reflectors (DBRs) with InP based gain sections [11]. The bismuth-doped fiber demonstrated recently [33] reveals record characteristics at 1.33 µm and could naturally be utilized as a power amplifier with a mode-locked seed laser. This arrangement provides an opportunity for building a master oscillator - power amplifier (MOPA) system operating at the spectral range of 1.3 µm. However, the bismuth-doped fibers developed to date exhibit a relatively low local gain and, consequently, the length of amplifier typically exceeds 10 m. This characteristic makes fiber-based pulsed oscillator impractical since the long length of the gain fiber leads to large amounts of dispersion. Furthermore, the appropriate dispersion compensating fibers would result in pulse distortion through high nonlinearity and high-order terms of dispersion [34]. Therefore, the MOPA system should preferably implement master oscillators other than a bismuth fiber laser.
The key objective in this work is to combine the recent advances in SDLs with the substantial progress in bismuth-doped fibers. The 1.3 µm MOPA system uses an SDL seed oscillator made by wafer bonding technology that allows the integration of non-lattice-matched semiconductors and a newly developed bismuth fiber amplifier. Since the fiber amplifier is core-pumped by a 1.18 µm SDL, the whole MOPA concept uses an SDL as a short-pulse master oscillator and a pumping source. The described system, therefore, takes full advantage of superb beam quality, elevated power and wavelength versatility inherent to SDLs.
2. Experimental details
2.1 The 1.33 µm seed master oscillator
The performance of SDLs with InGaAs quantum wells (QWs) is limited by increased lattice mismatch to GaAs when moving to wavelengths beyond 1.2 μm [35]. The operation wavelength can be tailored to longer wavelengths by adding nitrogen into the InGaAs quantum wells (QWs), but the GaInNAs/GaAs material system can suffer from enhanced nonradiative processes and requires critical optimization of growth parameters [36]. On the other hand, InP-based materials are ideal for long wavelength emitters, but they are impaired in lasers with vertical cavities since the materials lattice-matched to InP exhibit low refractive index contrast. Consequently, high reflective DBRs in this material system require a large number of layer pairs, which increases the thermal resistance of these devices and makes monolithic InP technology of low practical value [37]. This problem can be overcome by wafer fusion that allows to combine the advantages of InP-based active materials with high quality GaAs/Al(Ga)As DBRs [38, 39]. In this work, we utilize an alternative technique where the GaAs-based DBR and the InP-based active region are bonded via an intermediate bonding layer [11].
Both the DBR and the active region were grown by solid source molecular beam epitaxy (MBE). The DBR comprised 25.5 pairs of λ/4 thick GaAs-AlAs layers and was grown on GaAs substrate. The active region was grown on InP substrate and comprised 10 compressively strained AlGaInAs QWs. The QWs were arranged in four groups located at the antinodes of the optical field. More details about the resonant structure can be found in [11].
The intermediate bonding layer comprised a self-assembling monolayer (SAM) of (3-Mercaptopropyl) trimethoxysilane (MPTMS) that was deposited on the SiO2-covered DBR surface. The DBR surface was prepared for the MPTMS deposition by dipping it into NH4OH that rendered the surface hydrophilic with NH2 and OH groups [40]. The DBR was then placed in a low vacuum chamber with an open container of MPTMS [41]. Covalent bonding occurs when the OH and NH2 groups on the DBR surface react with the hydrophilic silanol groups Si(OCH3)3 of the MPTMS vapor. Consequently, the DBR surface became terminated by hydrophobic SH groups that can bond to a hydrophobic InP surface [42]. The top surface of the InP-based active region was then rendered hydrophobic by a dip in 0.5% HF. The hydrophobic wafer surfaces were then placed face to face and pressure was applied at the center of the “sandwich” assembly to initiate room temperature bonding via van der Waals forces [40]. For covalent bond formation, the assembly was placed under uniform pressure of 0.5 MPA and annealed at 200 °C overnight. The InP substrate was removed by wet etching using HCl. In order to ensure efficient heat removal from the gain structure, a 3 × 3 mm2 type IIIa chemical vapour deposition (CVD) intracavity diamond heat spreader was capillary bonded to the top surface of the gain sample. The diamond had a wedge angle of 2° and an average thickness of 300 µm. In order to reduce the diamond-induced scattering losses, the top surface of the diamond was antireflection coated with TiO2/SiO2 layers.
The gain mirror was pumped with a fiber-coupled diode laser emitting at 980 nm. The pump was focused onto a spot with a diameter of 200 μm at the gain element. The temperature of the copper block was kept at 10°C throughout the measurements. The gain element was tested in continuous-wave (CW) operation in a V-cavity and a W-cavity shown in Fig. 1.The output characteristics for both cases are shown in Fig. 2.The output power reaches 3.9 W for the SDL operating with the V-cavity comprising the gain element as an end mirror. The output power dropped to 2.7 W for W-cavity when the gain element acts as a folding mirror. This power reduction is expected, because the number of parasitic reflections from the front facet of the CVD diamond is doubled in the W-cavity where the gain mirror is used as a folding mirror contrary to the V-cavity where the gain chip acts as an end mirror [43]. The W-shaped cavity shown in Fig. 1 was then implemented to obtain mode-locked operation. The fundamental repetition rate of the cavity was 827 MHz. The mode-locked disk laser used as a master oscillator for the MOPA system employed a SESAM described earlier in [24]. The SESAM operating at this wavelength range is still under extensive development and detailed characterization is currently under way. The mode diameter on the SESAM was about 60 µm.
Fig. 1 A schematic of the W-cavity used for mode locking the seed SDL. The distances between the components SESAM - HR1, HR1 - gain mirror, gain mirror - HR2 and HR2 - POC were approximately 20 mm, 60 mm, 65 mm and 35 mm, respectively. The folding angles of HR1, gain mirror and HR2 were 15°, 12° and 10°, respectively. The pump beam was positioned at an angle of 40° with respect to the gain mirror. HR1 and HR2 have radius of curvatures (RoC) of 30 mm and 150 mm, respectively. The transmittance of the plane output coupler (POC) was 0.5%. HR: high reflective.
In pulsed operation, the average output power measured after propagation through an optical isolator was ~120 mW. The reduction of the output power in mode-locked regime is primarily associated with the loss introduced by the SESAM. The cavity geometry, e.g. V- or W-configuration, and positioning of the elements within the cavity also have an effect on the laser performance [43].
2.2 The 1180 nm disk laser for pumping the bismuth-doped fiber amplifier
The gain chip structure for the 1180 nm SDL was grown on 2” n-GaAs substrate by solid source MBE. The active section was grown on top of a 25.5-pair AlAs/GaAs DBR. It comprises 10 highly-strained GaInAs QWs with thicknesses of 7 nm and indium molar fraction of about 35%. The QWs were spaced half wavelengths apart with 32-nm GaAsP strain compensation layers between each QW. The arsenic molar fraction of the strain compensation layers is about 89%. The structure was designed to be anti-resonant and completed with a 20 nm Al0.33Ga0.67As window layer and a 10-nm GaAs cap layer. The standard growth conditions were applied for all layers except for the QWs, whose high strain required them to be grown at a low temperature. For the growth of the QW layers, the manipulator temperature was set to 460 °C. After the growth, the wafer was diced into 2.5 × 2.5 mm2 chips. A chip was then capillary bonded onto a 2° wedged diamond heat spreader and the whole assembly was pressed against a water-cooled copper heat sink. The mean average thickness of the diamond is 300 µm. The top of the diamond was covered by an antireflection (AR) coating in order to reduce pump light reflection and to alleviate parasitic reflection originating from the heat spreader wedged geometry.
The wavelength of the pump disk laser was set to 1180 nm. This is the optimal wavelength for obtaining the peak gain in the bismuth fiber amplifier at 1.33 µm and it matches the high absorption band of the bismuth-doped fiber. The diameters of the cavity mode and the pump spot at the gain element were both 300 µm. The gain medium was pumped with a fiber-coupled diode laser emitting at 788 nm. A highly reflective mirror with a radius of curvature of 200 mm was used as a steering mirror with a folding angle of 10°. A plane output coupler with 2.5% transmission and the gain mirror were placed as the end mirrors. Gain element temperature was kept at 15 °C. The free-space and fiber-coupled power characteristics are shown in Fig. 3(a) with maximum powers of 12.23 W and 8.38 W, respectively. The fiber-coupled power measured before and after the pump combiner reveals that the loss induced by the pump combiner limits the pump power to 4.13 W. Figure 3(b) shows the optical spectra of the pump disk laser at various pump powers. The beam quality parameter M2 was measured to be below 1.1 in both orthogonal directions at whole range of pump power. Instabilities were not observed up to the maximum output pump power.
Fig. 3 (a) The power characteristics of the SDL used for pumping the fiber amplifier. (b) Optical spectra of the pump laser for various excitation powers.
2.3 The bismuth-doped silica fiber for optical amplification at the 1.3 µm spectral range
The fiber MOPA (FMOPA) is recognized as a highly practical approach for boosting the optical power in numerous applications. The power scaling of the 1.33 µm master oscillator source was performed here in a fiber amplifier built using bismuth silica fiber. The fiber preform was fabricated by modified chemical vapor deposition (MCVD). The core/cladding diameters of the fiber were 4.5/125 µm. The bismuth concentration in the fiber core was below the accuracy of measurements of 0.02%. The phosphor oxide concentration was about 5.7 mol%. The refractive index profile of optical fiber perform presented in Fig. 4 shows a cladding/core index difference of about Δn = 5 × 10−3. The fiber cut-off wavelength of ~0.9 µm ensures that both the seed and the pump waves propagate as single-mode beams providing a large and a stable overlap with reduced twinkle effect.
It should be noted that bismuth-doped silica fibers introduce notable losses in the spectral range 0.9-1.4 µm. The remarkable attribute of these losses is their bleachable character. The bleaching of absorption was measured in the 1230 nm spectral band, as shown in Fig. 5(a). The absorption at 1230 nm decreases from 0.6 dB/m to 0.088 dB/m when the pump power exceeds 0.5 W. The optical loss and the gain spectra obtained with the 1230 nm excitation are presented in Fig. 5(b). The net gain in this fiber is observed in the spectral range 1260-1500 nm with a maximum value of 0.22 dB/m at λ = 1330 nm. Figure 5(b) also shows the variation in the fiber transmission spectrum that occurs during the switching on/off the pumping power (on/off gain).
Fig. 5 (a) Pump dependent bismuth fiber absorption at 1230 nm. (b) Spectra of optical loss and gain in bismuth-doped phosphosilicate fiber.
3. Results and discussion
A schematic of the amplifier setup is shown in Fig. 6. A 100 m long bismuth-doped fiber used in an optical amplifier was pumped with the 1180 nm SDL. Residual/unabsorbed pump power up to 75 mW was measured at the output of the bismuth-doped fiber. In order to dispose the unabsorbed pump radiation, two fiber filters were placed at the output of the amplifier.
The amplifier gain parameters versus the seed signal power are shown in Fig. 7. The measurements were performed at pump powers of 1.96 W, 3.03 W and 4.13 W. They show the saturation characteristics of the amplifier. Gain up to 14 dB was obtained for 11 mW of seed signal at a pump power of 4.13 W. The highest powers observed at the output of the amplifier for pump powers of 1.96 W, 3.03 W and 4.13 W were 280 mW, 414 mW and 516 mW, respectively. The noise characteristics of the amplifiers were measured using a standard procedure [44]. The noise figure of the amplifier was found to be 4.5 dB.
Fig. 7 The amplifier gain with respect to the seed signal power. The 1180 nm pump powers are 1.96, 3.03 and 4.13 W.
Figure 8(a) shows the optical spectra of the mode-locked seed laser at a pump power of 22.5 W before and after amplification. The shape of the optical spectrum indicates the strong pulse up-chirping, as expected for passively mode-locked SDLs [45]. Up-chirped pulses with the long-wavelength components in the leading edge of the pulse experience a blue shift in the optical spectrum, because the SESAM attenuates preferably the leading edge of the pulse. Consequently, the pulse spectrum is weighted at the shorter wavelengths and is largely determined by the amount of chirp [45]. Therefore, estimating the transform-limited pulse width based on spectral analysis is not esteemed. After amplification, the spectral bandwidth of the pulse exhibited minor broadening from 4 nm to 5 nm. The pulse spectrum shifts towards the gain peak of the bismuth-doped fiber amplifier [33]. This shift depends on the relative detuning between the pulse and fiber gain spectra. Using pump blocking filters at the output of fiber amplifier, the level of the residual pump power was 50 dB below the signal level. The oscilloscope trace of the laser output in the time domain is shown in Fig. 8(b). A 3.5 GHz New Focus 1592 photoreceiver with a rise time of 115 ps (3-dB bandwidth “DC to 3.5 GHz”) and an Agilent Infiniium DCA-J 86100C digital communications analyzer with a 12 GHz bandwidth were used in the measurement.
Fig. 8 (a) The optical spectra of the mode-locked seed signal at a pump power of 22.5 W before and after amplification in the bismuth fiber. (b) Oscilloscope trace of the mode-locked signal.
The radiofrequency (RF) spectrum was measured with a 12 GHz New Focus 1577 photoreceiver with a 34 ps rise time (3-dB bandwidth 12 GHz) and examined with a Hewlett Packard E4407B ESA-E Series spectrum analyzer (9 kHz - 26 GHz). Figure 9(a) shows the RF spectrum at the fundamental repetition frequency of 827 MHz; the inset shows the wide scan over a 12 GHz range. The 3dB peak width at 828.57 MHz is 1.496 kHz and the resolution bandwidth is 1 kHz.
Fig. 9 (a) The RF spectrum of the amplified signal at the fundamental repetition frequency. Inset shows the RF spectrum over a 12 GHz frequency range. (b) The autocorrelation traces of the 1.33 µm mode-locked semiconductor disk laser at the input and the output of the bismuth fiber amplifier.
The pulse shape measurements were performed using a noncolinear autocorrelator Femtochrome FR-103 XL. The pulse measured over 120 ps scan range reveals no pedestal. The autocorrelation was normalized to its maximum amplitude. Figure 9(b) shows the autocorrelation traces of the input and the amplified mode-locked signals with pulse durations of 5.77 and 5.73 ps, respectively. The pulse durations derived from sech2 fitting indicate negligible pulse distortion in the bismuth fiber. The maximum pulse energy achieved for 516 mW of average output power was 0.62 nJ.
4. Conclusions
We demonstrate a bismuth-doped fiber amplifier that was optically pumped and seeded with a semiconductor disk laser. The study shows the attractive potential of hybrid technology that exploits semiconductor disk lasers and bismuth fiber amplifiers to build architecture for ultrafast high-power sources operating in the 1.3 µm spectral range. A 100 m-long bismuth-doped fiber was used to amplify the signal pulse from a mode-locked semiconductor disk laser up to an average output power of 516 mW. The MOPA system delivered 5.7 ps pulses with a pulse energy of 0.62 nJ at a 827 MHz repetition rate.
Acknowledgment
This work was supported by EU project PIRSES-GA-2010-269271.
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