We demonstrate wavelength locking of a diode laser at 760 nm with feedback from an elastic transmission grating in the Littrow configuration. The laser was in a single longitudinal mode with a side-mode suppression of 20 dB. By stretching the grating the laser could be tuned over a few nm. The grating was fabricated in a silicone elastomer (polydimethylsiloxane) by a moulding technique, and coated by a thin layer of Ti and Au to achieve an increased diffraction efficiency needed for efficient locking.
© 2006 Optical Society of America
Tunable lasers are used in a wide field of research and applications and can be built in many different ways. Grating stabilized external cavity diode laser (ECDL) are frequently used in evaluation of telecommunication components and for spectroscopy applications, where usually only a few mW of power is needed, but a high spectral purity is required. These lasers primarily rely on reflection gratings in Littrow configuration which requires a sophisticated mechanical design to avoid beam steering when the laser is tuned by tilting of the grating [1–5]. A simpler construction using a transmission grating was demonstrated by Laurila et al. . Their design gives a more compact tunable single-mode laser, still with a high output power and without beam steering.
The idea behind this work is to build a simple external cavity laser at 760 nm using an elastic transmission grating in Littrow configuration. The laser should be tunable in wavelength by stretching the grating and hence changing the grating period. Silicone (polydimethylsiloxane) was chosen as the grating material as it is a good optical material with high transparency and at the same time very elastic. Furthermore it can be moulded to reproduce a surface with very high accuracy. Silicone rubber as an optical material is not very frequently used, though it has some interesting features. Compared to many other polymers it can be fabricated with low absorption in the visible and near IR, with a typical loss of 0.1 dB/cm in the 800 nm band and with a refractive index of 1.43. Waveguides have been fabricated in UV-curing silicone for optical interconnect applications  and relief silicone gratings have been used for various sensing applications, where the elastic properties are utilized [8–11]. However, no-one has previously reported the usage we demonstrate, locking and tuning of a laser.
2. Experiments and discussion
To fabricate flexible gratings a commercial heat curing silicone (polydimethylsiloxane) elastomer (Sylgard 184, Dow Corning Corporation) was used. It was carefully mixed with 10% of curing agent and air bubbles were removed by putting the liquid mixture under low pressure in an ultrasonic bath for approximately 10 minutes. The mixture was poured onto a glass substrate and the master mould, which was a standard holographic reflection grating (1800 l/mm), was laid face down on top of the silicone solution. To control the thickness of the silicone, the edges of the master mould were laid on two microscope slides. This assembly was then put in an oven at 85°C for 100 minutes. After it had cooled down, the moulded grating was removed from the master.
The diffraction efficiency of the as fabricated grating was evaluated with a diode laser at 760 nm, approximately in the Littrow angle. A relatively low diffraction efficiency of a few per cent was observed for the first order reflection. However, a larger value is needed to lock the diode laser stably. To increase the diffraction efficiency we coated the gratings with thin semitransparent metal films. Here one should consider several things; of course first that the film remains partly transmitting, then that the grating profile is preserved so that the first order reflection becomes high, and finally that the grating can be stretched without having the metal film falling off. Several different metal and thickness combinations were evaluated and the best results were obtained by first depositing a 2 nm thick Ti-film followed by a 30 nm thick Au-film using an electron-beam evaporator. For TM polarized light at 760 nm in the Littrow configuration the first order reflection was 57% (TE 25%) and the transmission 16% (TE 4%). Measurements of the grating profile were carried out with an atomic force microscope (AFM) in contact mode, whose conical tip has a full angle of 30 degrees and a height of 20 µm. The results for a Ti-Au coated grating can be seen in Fig. 1. The profile is sinusoidal and closely reproduces the master grating with a groove modulation of 0.25 (peak to peak). The grating period was approximately d=0.55 µm, which corresponds to the groove frequency of 1800 lines/mm.
For the optical experiments we used a 760 nm diode laser (QLD-760-10S, Qphotonics Inc.) with a coating to reduce the reflectivity to 2% on the output face, as specified by the supplier. The temperature controlled laser was first characterized without feed-back from the grating. It operated transversally and longitudinally single-mode at room temperature with an output power of 10 mW for 70 mA of drive current. The temperature tuning was 0.07 nm/°C and the current tuning was 0.025 nm/mA. The linewidth was measured by means of a scanning Fabry-Perot interferometer (FPI) to ΔλFWHM=450 MHz at 55mA and 20°C (6 mW). For operation in the extended cavity, the laser was collimated and launched at the metal coated silicone grating placed in Littrow configuration, with a total cavity length of 25 cm, see Fig. 2. The grating was clamped between two pairs of small metal plates, and stretching was achieved coarsely, by means of a micrometer screw, or finely, with a piezoelectric element. The mount could be rotated around the two axes perpendicular to the beam direction to allow the reflected light to come back into the semiconductor chip. Thanks to the high diffraction efficiency of the grating, the locking of the diode laser was rather easily achieved. When the laser locked to the grating feed-back, the linewidth was reduced to below ΔλFWHM=310 MHz, limited by the resolution of the FPI. At the same time, the side-mode suppression increased by almost 10 dB to approximately 20 dB. The modal spectrum as measured with an optical spectrum analyzer (OSA) is shown in Fig. 3.
In a classical Littrow configuration, the laser wavelength, λ, is tuned by tilting the grating according to mλ=2dsinα, where α is the tilt angle of the grating. However, in this experiment the tilt angle was fixed and the groove spacing d was changed by stretching or compressing the grating. The experiment was started by locking the diode at the gain peak maximum (758.3 nm) where side-mode suppression was the highest. In order to be able to both increase and decrease the groove spacing, the grating was slightly stretched from the beginning. Stretch tuning is then achieved by fine tuning the grating pitch with the piezoelectric element.
An example of the wavelength tuning is given in Fig. 4 as measured with the OSA, plotted versus applied strain. The diode was here tuned over 2 nm with a grating strain increase of 0.15%. The side-mode suppression was always better than 16 dB. It can be seen that the wavelength dependence on the applied strain is approximately linear, which is reasonable in a region where the grating length expansion is linear with the applied strain. The relative wavelength shift is then proportional to the length change, as Δλ/λ=Δd/d. However, the experimental point do not follow the theoretically predicted line closely. This we believe is partly due to the fact that the laser would only lock at wavelengths corresponding to the Fabry Perot modes of the diode laser itself. In general, the wavelength chosen by the laser depends on the spectral properties of the gain peak of the semiconductor, the Fabry-Perot modes of the diode-laser chip itself, the external cavity modes as well as the grating feedback. In our case, the residual 2% reflection of its output face cause the diode-laser Fabry-Perot modes to be dominating. Ideally, for smooth tuning, the semiconductor chip should have a good AR on the output facet to get a wide mode-hop-free tuning range.
Stretching of the grating up to 10% of the initial length has been tried while maintaining a good quality of the reflected beam. However, the diode did not follow over such a wide wavelength region without adjustment of the grating angle, α, primarily because of a limited gain bandwidth. Under extreme stretching it also happened that the metal film flaked off, but in the case of tuning of a few nm, the grating could be repeatedly tuned without any problem.
In conclusion, a high frequency grating has been moulded in a transparent heat-curing silicone elastomer. By coating the grating with a thin semitransparent metal film it was possible to frequency lock a diode laser at 760 nm in a transmission Littrow configuration. When stretching the grating, the laser wavelength could be tuned over a few nm with maintained longitudinal-single-mode operation.
Future plans involve work with better AR coated diode lasers and electronic feed-back controlled locking to atomic transition lines as well as fabrication of other soft optical silicone components like deformable mirrors and lenses.
References and links
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