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The operation of a femtosecond Cr4+:YAG laser that incorporates a novel GaInNAsSb semiconductor saturable Bragg reflector is reported. In the mode-locked regime 230fs pulses centred at 1528nm were generated at an average output power of 280mW. The SESAM exhibited a low saturation fluence of 10µJ/cm2 and a short recovery time of 12ps.
©2008 Optical Society of America
1. Introduction
Ultrashort pulses from mode-locked solid-state lasers have found applications over the past couple of decades in a wide variety of research sectors including spectroscopy, biophotonics and telecommunications. A major enabling technology for applications is the exploitation of optimised semiconductor saturable absorber mirrors (SESAMs) as intra-cavity laser elements to passively initiate and maintain the pulse generation process with a greater degree of stability and fewer cavity design constraints than apply in Kerr-lens mode locking [1–3]. Favourable SESAM characteristics are the availability of saturable absorbers with low non-saturable losses, low saturation fluence, fast recovery time for absorption, a saturable loss modulation depth that is well matched to a given operational bandwidth and compatibility with integration on to broadband mirrors. The extension of the GaAs-based technology to the 1100–1700nm wavelength band with GaInNAs(Sb) quantum well [4–5] or In(Ga)As quantum dot absorbers [6] on monolithic high-contrast AlAs/GaAs or AlOx/GaAs distributed Bragg reflectors represents a significant advance towards high performance mode-locked near-infrared lasers. Here, we report the first use of a fast-recovery time GaInNAsSb/GaAs SESAM as an enabling technology for femtosecond operation at wavelengths beyond 1500nm [4,7–9]. The assessments described were performed with a Cr4+:YAG laser, selected for its broad luminescence extending from 1200nm to 1600nm and already demonstrated capabilities for the generation of 20fs-pulses [10–11].
2. GaInNAsSb SESAM
The GaInNAsSb anti-resonant SESAM epitaxial structure and its room temperature characteristics are shown in Fig. 1(a) and (b) respectively. The device consists of 3λ/4 GaAs cavity, that incorporates in its central antinode a ~1530nm saturable absorber, grown on top of a 29-layer-pair GaAs (113.4nm)/AlAs (132.1nm) distributed Bragg reflector that provides a ~171nm-stopband centered at 1577nm. The absorber structure comprises a 7.5nm-wide Ga0.62In0.38N0.02As0.935Sb0.045 quantum well (QW) having 2.5nm thick GaAs0.866N0.0134 planes located 2nm on either side. The relatively high concentration of antimonide in the QW is chosen not only to obtain a good epitaxial integrity and homogeneity resulting from the Sb surfactant effect [5] but also to minimise the broadening and weakening of the excitonic absorption by nitrogen clusters. Furthermore, as discussed in reference 12, the GaAsN side planes are introduced to shorten the absorber recovery time to a value measured to be as short as 12ps by pump-probe techniques.
For the fabrication, the sample was grown by solid source molecular-beam epitaxy (MBE) on semi-insulating GaAs (100) substrate. As and Sb were supplied in the form of As2 and Sb2 dimers from cracking effusion cells. The MBE reactor was equipped with a radio-frequency plasma source, used to generate active N species from high-purity N2 gas. The GaInNAsSb QWs and GaAsN layers were grown at a lower temperature of 410°C whist the remaining of the structure was grown at 600°C.
Fig. 1. (a). Layout of the SESAM device. The upper part shows the zoomed design around the absorber with the electric field distribution (red). (b). Reflectivity (red) and photoluminescence (blue) of the GaInNAsSb SESAM.
3. Laser setup and results
For the laser tests, we designed a highly asymmetric cavity, with a 20mm long, Brewster- cut Cr4+:YAG crystal as the gain material as shown in Fig. 2(a). The thermally stabilized Cr4+:YAG rod had a small-signal pump absorption coefficient of 1.2cm-1 and was placed between two focusing mirrors with -100mm and -75mm radii of curvature. The pump source was a 10W Yb:fibre laser (IPG Photonics) operating at 1064nm. For cw operation and subsequent measurements in the passively mode-locked regime the long arm of the cavity was set to 500mm. The length of the short arm was varied to achieve optimum mode locking and this was observed at an arm length of 250mm. This length remained fixed for the remainder of the assessments described. The beam waist radius (at the 1/e2 level) at the end of the short arm, where either a high reflectivity mirror or the SESAM was placed, was evaluated to be 255µm using an ABCD matrix formalism calculation. The cavity was designed to be at the centre of the stability region to eliminate KLM and during cw operation no pulsed behaviour was observed in the optical or radio frequency (RF) spectra. We interpret these observations as evidence that mode-locked operation was activated by the SESAM, via a soliton mode locking process [13].
At the outset of our investigations we monitored the laser performance by comparing the prism-slit tuning capabilities of the laser with an OC of 0.35%, at a constant absorbed pump power of 3.2W and with either the SESAM or an HR mirror terminating the short arm of the cavity. As shown in Fig. 2(b), the tuning range achievable with the SESAM reduces to the 1508–1560nm band, determined on the long wavelength side by the Cr:YAG gain profile and on the short wavelength side by the SESAM mirror stopband. The latter characteristic also explains the shift of the laser wavelength in the free-running regime (no intra-cavity prism) from 1480nm to 1520nm with the HR and SESAM respectively (indicated as crosses in Fig. 2(b), furthermore it inherently limits the accessible gain spectrum to produce shortest pulses.
Fig. 2. (a). Cr4+:YAG laser cavity layout: Pump light from a Yb:fibre laser is coupled via telescope optics and a focusing lens (FL) into the cavity. M1 (HR, RC=-75mm) and M2 (HR, RC=-100mm) produce a cavity mode radius of 40µm in the Brewster cut Cr4+:YAG crystal. An output coupler (OC) and an HR mirror/SESAM terminated the cavity. (b). Measured tuning range of the Cr4+:YAG laser with a HR mirror (blue – solid dots) and the SESAM (red – circles). The crosses indicate the free running wavelength, without wavelength selection by the intracavity prism-slit combination. The reflectivity characteristic of the SESAM is overlaid for comparison.
To avoid wavelength-related inaccuracies, the subsequent laser performance analysis was made at the fixed central wavelength of 1530nm. Cw laser performance for output couplers of 0.1%, 0.35% and 1.4% are shown in the inset of Fig. 3. With the 1.4% output coupler a maximum output power of 400mW at 4.43W of absorbed pump power was achieved. From the slope efficiencies the total cavity round-trip losses were estimated to be 0.5%.
Power transfer characteristics of the laser were measured for cavity parameters identical to those of the cw operation to enable an assessment of the linear and nonlinear properties of the SESAM. The slope efficiencies in the mode-locked regime were deduced to be 3.0%, 8.5% and 10.2% for the 0.1%, 0.35% and 1.4% output couplers respectively (see Fig. 3).
Fig. 3. Threshold and slope efficiency characteristics for the HR mirror (inset) and the SESAM for three different output couplers (black – triangles=0.1% OC, red – crosses=0.35% OC and blue – dots=1.4% OC). The switching points between cw and mode locking are indicated with arrows.
With the 1.4% output coupler in place a maximum output power of 275mW was achieved. It should be noted that higher output powers were achievable for both cavity configurations at the expense of lasing instabilities due to thermal-lens effects.
From the Findlay-Clay analysis [14] the difference in internal losses of the cavity was estimated by plotting the absorbed pump power at lasing threshold for the HR mirror and for the SESAM against the logarithm of the reflectivity (-ln(R)) of different output couplers. The difference in y-intersect (Δδ) can yield an approximation for the losses introduced by the SESAM as shown in Fig. 4.
Fig. 4. Absorbed Pump Power at lasing threshold versus the logarithm of the reflectivity of the output coupler for the HR mirror (blue) and the SESAM (red). The difference in offset of the fitted lines (marked as Δδ) indicates the losses of the SESAM to be of the order of 0.3% (low signal absorption).
These losses introduced by the SESAM were estimated to be 0.3% and can only give a qualitative indication of both, the modulation depth and the non-saturable losses, due to the approximate nature of this measurement method. Owing to an increased nitrogen content of this device and indicated by the change in slope efficiencies, one would expect the non-saturable losses to be the main contributing factor to this value.
With the calculated spot size on the SESAM and the pulse repetition frequency of 164MHz, the mode-locking threshold was reached at an intracavity fluence on the SESAM of 12µJ/cm2. These results confirm the expected the low saturation fluence (<10µJ/cm2) of this GaInNAsSb SESAM in the 1500nm spectral region.
For femtosecond operation, two fused silica prisms with a tip-to-tip separation of 180mm were incorporated into the long arm of the laser resonator to compensate for positive group velocity dispersion. With the SESAM in place, the mode locking was self-starting at pump powers as low as 2.7W (laser threshold=2.4W) and Fig. 5 shows a typical spectrum and intensity autocorrelation for the output pulses from this laser when operating at a centre wavelength of 1528nm. By assuming a sech2 intensity profile, the pulse duration was determined to be 230fs and thus with the spectral width of 11nm, the deduced time-bandwidth product was 0.327, indicating that the pulses were near-transform limited. For a 1% output coupler the average powers were typically 280mW thus confirming the low-loss nature of the SESAM. Importantly, no intra-cavity slit was needed to enhance the stability of this laser implying that the modulation depth of the GaInNAsSb device was sufficient to readily maintain mode locking over extended operational periods.
Fig. 5. (a). Measured optical spectrum. (b). Corresponding intensity autocorrelation of the output from the mode-locked Cr4+:YAG laser.
4. Conclusion
We demonstrated efficient and stable mode-locked operation of a Cr4+:YAG laser that incorporated a novel GaInNAsSb anti-resonant SESAM. The modest saturation fluence of 10µJ/cm2 of this SESAM enabled a low-threshold and stable self-starting generation of neartransform- limited pulses with durations of 230fs at an average output power of 280mW. We appreciate that sub-40fs pulses have been obtained from a Cr:YAG laser with a GaInAs structure [15]. However it could be expected that with an optimized modulation depth shorter pulse durations can be achieved using a GaInNAsSb SESAM. Nevertheless, these data thus illustrate clearly a robust femtosecond-pulse generation at high average output power levels from a solid-state laser system in the near-infrared region using a GaInNAsSb-based SESAM.
With its fast recovery time of 12ps prevailing GaInNAs structures, this SESAM should also facilitate mode locking at higher pulse repetition rates in the optical communications relevant 1200–1600nm band.
Acknowledgments
We wish to acknowledge overall funding support from the UK Engineering and Physical Sciences Research Council.
References and links
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