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We demonstrate the use of a High Reflectivity Grating (HRG) as an intra-cavity element in a Semiconductor Disk Laser (or Vertical External Cavity Surface Emitting Laser) to stabilise its emission wavelength and polarization characteristics. Operation at 1058nm with up to 645mW of pump-limited output power and an M2~1.4 is achieved. We also show that this scheme permits tunable single-frequency operation.
©2007 Optical Society of America
1. Introduction
Semiconductor Disk Lasers (SDLs), also known as Vertical External-Cavity Surface-Emitting Lasers (VECSELs), are attractive diode-pumped semiconductor lasers which combine remarkable wavelength versatility with the operational flexibility afforded by an open-access cavity configuration [1]. Watt-power-level demonstrators, of fundamental emission ranging from 640nm to 2.4µm and frequency-doubled operation in the visible and ultra-violet, show the potential of these sources for applications such as laser projection displays, reprographics and printing, spectroscopy and telecommunications [1–6].
Emission with stable linear polarization and single spatial and longitudinal mode outputs is crucial to many of these applications and has thus motivated investigations on stabilization techniques. To-date, high-power (>100mW), narrow-linewidth operation of semiconductor disk lasers has mainly been obtained by using intra-cavity birefringent filters [2–5] or by exploiting the Fabry-Perot characteristics of a thin intracavity heatspreader [6]. An alternative but very attractive approach is to exploit intra-cavity grating-based techniques, as they readily offer large tunability with high spectral purity [7] and are better suited for use in compact geometries in particular when using micro-electro-mechanical integration [8]. However, because of the inherent low gain of SDLs, the implementation of such techniques has only been made possible by the recent emergence of two types of gratings with peak reflectivity greater than 99%: volume Bragg gratings [9] and high-reflectivity gratings (HRGs) [10–11]. The former kind of grating has been shown to allow high-power narrow-linewidth operation of SDLs [12] but lacked the linear polarization output.
In this letter, we report in full the characteristics of a 1060nm SDL which incorporates an HRG as an intra-cavity element. High-power, tunable, narrow-linewidth and polarization stable laser operation is achieved.
2. Semiconductor structure
The SDL epistructure, grown by Molecular Organic Vapour Phase Epitaxy, comprises a 30.5-pair AlAs/Al0.2Ga0.8As Distributed Bragg Reflector, an active region with ten strain-compensated InGaAs quantum wells (QWs) distributed over ten antinodes of the standing wave pattern, an AlGaAs confinement window and a 10nm GaAs cap to prevent oxidation [13]. A temperature-dependent backscattered photoluminescence (PL) study showed the maximum PL intensity is achieved for a temperature of 60°C suggesting an in-built offset of -4nm at 20°C between the quantum-well gain peak and cavity resonance. Thermal management of this semiconductor structure is obtained by liquid-capillarity bonding a 500-µm-thick, plane-plane, type-IIa natural diamond heatspreader to the epi-surface and subsequently mounting this composite in a brass-mount. The assembly temperature is kept constant using recirculating water at 10°C.
3. High Reflectivity Grating
The HRG used here is a resonant grating which exploits the proximity of a corrugation and a surface wave to achieve a high degree of polarization, incident angle and wavelength selectivity [10–11;14]. Its high reflectivity and very sharp wavelength selectivity result from the excitation of a TE-guided mode of the multilayer mirror as in [11]. However, this effect is designed here to occur under oblique incidence rather than surface normal illumination.
The HRG was designed to exhibit high-reflectivity at 1058nm for a ~7° angle of incidence. It was fabricated by first etching an 18nm-deep 518.6nm-period grating into a silica mirror substrate and subsequently depositing successively a SiO2/Ta2O5 guide and multilayer mirror by ion plating technology which replicates the surface morphology [11] as shown in the inset of Fig. 1(a). The grating was obtained by exposing a 300nm-thick positive resist to an interference pattern created by a 442 nm-emitting HeCd laser and subsequently transferring this pattern into the substrate by reactive ion etching (RIE). This fabrication sequence was chosen to minimize the loss by guided mode leakage [10].
Polarization-resolved transmission spectral characteristics of the HRG were measured using as a source the amplified spontaneous emission of an Yb-doped singlemode fibre collimated into a 4mm-diameter beam and on the detection side an optical spectrum analyzer with 0.06nm resolution. Fig. 1(a) shows the results when the component is illuminated at 7.64° of incidence. As expected, the characteristics show a broadband reflection of about 91% with ripples due to the uncoated, polished back surface of the 6mm-thick silica substrate. The TE-polarized reflection presents the desired additional narrow peak (<1nm halfwidth) of reflectivity>99%. The latter feature is expected to ensure both wavelength and polarization selectivity once the grating is inserted in the laser cavity. Temperature assessments indicate that the wavelength corresponding to the peak reflectivity varies at a rate of 0.06nm/K. Angular characterization (see Fig. 1b) shows that the HRG peak shifts at ~5nm/degree for angles greater than 6.5° and disappears for low angles of incidence (absent at 0°).
Fig. 1. (a) HRG reflectivity curves for 7.64° incidence angle. Inset: Atomic Force Microscope image of part of the HRG top surface (b) Angular dependence of the HRG TE- reflectivity.
4. Laser Operation
4.1 Cavity Setup
A four-mirror laser cavity was setup (see Fig. 2) with the HRG positioned at one of the folds of the cavity with the etched grooves oriented vertically (i.e. out of the plane of the figure). The curved mirror of radius of curvature R=-100mm was positioned ~57mm away from the semiconductor chip and ~304mm away from the HRG. The planar HR mirror was placed 37mm away from the HRG. Cold-cavity calculations using Winlase™ show that, for a 7° angle of incidence, the vertical and horizontal laser mode sizes at the HRG are respectively 177µm and 209µm and are close to the mode matching condition at the semiconductor wafer for the ~80µm-diameter pump spot size obtained by relaying up to 9.8W of the output of a 100µm-core fibre-coupled 808nm-diode array using a 14mm/8mm collimator/focuser.
4.2 Operation using conventional output coupling mirrors
The laser performance was initially assessed using the cavity of Fig. 2 but with conventional plane output coupling mirrors instead of the HRG. The output power measured at the output 4 of the laser under maximum pumping conditions (Pp=9.8W) is optimum for a mirror of 97% reflectivity as shown in Fig. 3(a). The recorded power transfer (Fig. 3a) indicates a threshold of 0.865W, a maximum output power of 824mW and a slope efficiency of 9.2%. It should be noted that the output 3 of the laser shows power performance similar to output 4 and that the maximum power achieved is mainly limited by the evolution of the slope efficiency with output coupling. As illustrated in Fig. 3(b), this gain-controlled operation is characterized by a multi-peaked spectrum with a peak-to-peak separation of 0.4nm corresponding to the diamond etalon and shows a thermally-induced redshift of 0.42nm/W. The laser emission was found to be circularly polarized, a state attributed to undesirable birefringence of the heatspreader [15].
Fig. 3. (a) Power transfer (ROC=97%) and (b) spectral characteristics of the SDL using conventional output couplers. The pink star corresponds to the best laser results obtained when using the HRG intra-cavity (see next section).
4.3 Operation with the high reflectivity grating
To evaluate the impact of the HRG on the laser performance, we recorded the laser spectral and power characteristics for a variety of angles of incidence (5.98 to 9°) on the HRG, rotating the flat HR end-mirror appropriately for feedback. Fig. 4 summarizes the main tuning characteristics of the laser: threshold, maximum output power and the minimum pump power required to have the source to operate at the grating peak wavelength.
Fig. 4. Tunability characteristics of the SDL with intra-cavity HRG. The laser emission wavelength corresponding to particular incidence angles are given below.
The observed behavior can be divided into three distinct regions (α≤6.36°, 6.36°<α<7.64° and α>7.64°) and can be simply explained using the convolution of the spectral responses of the gain region and of the HRG. For 6.36°<α<7.64°, the gain peak and HRG peak reflectivity are closely matched, and grating-controlled operation is achieved from threshold to the maximum available pump power. In this regime, the grating sets the wavelength (see Fig. 5(a)) and polarization of the laser. Emission with up to 645mW of output power (see Fig. 6) at output 4 (see Fig. 2), an M2~1.4 (measured for 500mW of output) and a linear polarization (13:1 polarization ratio) aligned with the grooves of the grating (±2°) is demonstrated in this configuration. For α≤6.36°, grating-controlled operation is not possible, an observation consistent with the disappearance of the HRG narrowband reflection feature for low angle of incidence, further enhanced by the relatively small beam size at the grating plane used here [16]. For α>7.64°, the grating peak wavelength redshifts away from the wavelength corresponding to the maximum gain of the active region (~1054nm under maximum pump power), the maximum output power decreases and the minimum pump power required to reach the grating-controlled regime (see represented by triangles in Fig. 3) increases sharply. From the above, the combined wavelength tuning span achieved in grating-controlled tunable operation (α>6.36°) is 15nm with an upper limit of 1064nm (see Fig. 4). Though only discrete tuning at the wavelengths corresponding to the diamond etalon modes was achieved here, continuous tuning should be possible provided an anti-reflection coated wedged diamond is used instead of the plane-plane heatspreader [17].
However, there are subtleties in the laser behavior (see Fig. 4, 5(b) and 6) which originates from the fact that HRG base reflectivity is not quite low enough to prevent gain-controlled operation in all conditions. In particular, the latter regime of operation is achieved for α≤6.36° i.e. when the HRG narrow reflection peak is absent. Similarly, for α>7.64°, and as shown in Fig. 5b, there is a transition from the gain-controlled to the grating-controlled regime which reflects the change in the maximum net gain as the pump is increased i.e. as the semiconductor-chip gain broadens and shifts towards longer wavelengths.
Fig. 5. Laser spectral characteristics for an incidence angle of (a) 7.64°, (b) 8.26° and 10°C water cooling. The spectra are deliberately vertically offset by 30dB for improved visibility
The power transfer characteristics corresponding to the three regimes described above are provided in Fig. 6. It should be noted that the slope efficiency measured between 1 to 4 Watts of pump power is greater for grating-controlled operation (α=7.64°) than for gain-controlled operation (α=6.36 and 8.26°). This is consistent with the over output coupling occurring when the laser operates on the HRG base reflectivity (see Section 4.1). To evaluate the effective reflectivity of the HRG at its peak wavelength, we measured the output powers of the 5 numbered output beams (see Fig. 2) whilst the laser was operated in grating-controlled mode (α=7.64°). The obtained values (P1=1.13mW, P2=0.89mW, P3=116mW, P4=102mW and P5=1.22mW, respectively) confirmed the reflectivity of the HR mirrors to be of 99.9% and revealed that the peak reflectivity of the HRG was of 94.8%. The discrepancy between the latter figure and the maximal reflectivity measured with the transmission experiment (see Fig. 1) is believed to be due to the influence of beam aperturing effects occurring for small (<200µm-diameter) beams [16]. With this evaluation, it can be observed from Fig. 3(a) that spectral narrowing does not significantly affect the laser performance.
Fig. 6. Laser power transfers recorded for 3 angles of incidence representative of the three respective regimes of operation.
To investigate further the quality of the spectral output in the grating-controlled regime, we performed a detailed spectral analysis of the laser output where the incident angle on the grating is of 7.89° i.e. the laser emits at 1060.24nm. For 200mW of output power, the scan obtained with a commercial scanning Fabry-Perot interferometer with a 1GHz free spectral range shows (see Fig. 7(b)) a measured linewidth of ~90MHz with no feature appearing at the 375 MHz inter longitudinal mode spacing of our 398mm-long cavity, establishing that the laser emission is single longitudinal mode.
Fig. 7. Spectral analysis of grating-controlled regime with α=7.89° and 200mW output power using (a) an optical spectrum analyzer and (b) a commercial Fabry-Perot interferometer.
5. Conclusion
We have explored the use of a high reflectivity grating (HRG) as an intra-cavity element in a semiconductor disk laser to obtain high-power narrow linewidth operation with stable linear polarization. In particular, it has been established that when the grating peak reflectivity is close to the gain maximum, grating-controlled operation with output power up to 645mW and M2~1.4 was achievable using this technique. Finally, we also showed that the scheme was suitable to realize single-frequency tunable operation by demonstrating singlemode emission at 1060nm with 200mW output power and tunability over 15nm by simultaneously rotating the HRG and an end mirror.
Acknowledgements
The authors would like to thank the European Union for financial support of the Framework 6 programme NATAL, Dr T. Kim and colleagues of the Samsung Advanced Institute of Technology for providing the semiconductor epistructure used in this work.
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