A flexible polymeric Bragg reflector is fabricated for the purpose of demonstrating widely tunable lasers with a compact simple structure. The external feedback of the Bragg reflected light into a superluminescent laser diode produces the lasing of a certain resonance wavelength. The highly elastic polymer device enables the direct tuning of the Bragg wavelength by controlling the imposed strain and provides a much wider tuning range than silica fiber Bragg gratings or thermo-optic tuned polymer devices. Both compressive and tensile strains are applied within the range from −36000 με to 35000 με, so as to accomplish the continuous tuning of the Bragg reflection wavelength over a range of up to 100 nm. The external feedback laser with the tunable Bragg reflector exhibits a repetitive wavelength tuning range of 80 nm with a side mode suppression ratio of 35 dB.
©2010 Optical Society of America
Wavelength tunable lasers have been investigated in an attempt to extend their tuning range to cover the entire bandwidth for wavelength division optical communications . Compared to single wavelength semiconductor lasers, tunable lasers reduce the burden of inventorizing a series of fixed wavelength lasers with specific wavelengths corresponding to the WDM grids. Moreover, the use of tunable lasers enables the direct routing of a certain wavelength through the optical cross connecting network by changing the carrier wavelength. Cost-effective tunable lasers have a wide range of application areas, such as the spectral response measurement for biomolecule detection , optical sensors based on wavelength interrogation , and optical coherence tomography .
Sampled grating devices based on the Vernier effect exhibit a wide tuning range, though there is some difficulty involved in controlling the two gratings along with the phase section [5,6]. The use of an acousto-optic tunable filter in LiNbO3  and a coupled ring resonator in a silica waveguide  were also reported to achieve a wide tuning range. Compared to the other principles, the direct modulation of the grating period in a Bragg reflector provides the most convenient way to tune the lasing wavelength of an external cavity laser.
By virtue of the significant progress made in developing novel organic materials, optical polymer materials for fabricating waveguide devices have been advanced to demonstrate magnificent properties. Fluorination of the polymer substituting the lossy O-H bond leads to the reduction of the optical loss in the communication wavelength range, as well as the improvement of the chemical stability during high temperature operation and the enhancement of the optical damage threshold . Since low-loss polymer materials became available, various integrated optical devices indispensible for optical communications have been realized, including optical switches and optical variable attenuators [10,11].
Polymeric optical devices have several peculiarities compared to inorganic ones. They have a large negative thermo-optic effect along with low thermal conductivity, which are beneficial for fabricating array type waveguide devices requiring a low driving power consumption. In terms of the device fabrication process, the injection molding and nano-imprinting processes can be used for the fabrication of polymer devices [12,13]. Moreover, the excellent elastic properties of polymers enable the optical devices to be fabricated on a flexible substrate to cover any curved surface .
The thermo-optic tuning of a polymeric Bragg reflector was recently employed to demonstrate an external cavity laser with a tuning range up to 26 nm . However, the reliability of the heating electrode fabricated on the polymer material limits the tuning range, because the heater temperature required achieving the maximum tuning range approaches 100 °C. Additionally, the local heating in the middle of the waveguide introduces an abrupt discontinuity of the refractive index along the propagation direction, so as to cause additional radiation loss.
Compared to other inorganic optical devices, the polymer waveguide has many unique properties including its good elastic properties. Polymers can be elongated by over 10% before they reach their elastic limit, whereas less than 1% of elongation is available in silica fiber . Hence, the significant enhancement of the tuning capability could be achieved if one could produce a Bragg reflecting waveguide on a flexible polymer substrate, as demonstrated in our preliminary experiment . In this work, the flexible Bragg reflector is incorporated into a superluminescent laser diode (SLD) in order to demonstrate a widely tunable compact laser. The reflection spectrum of the polymer Bragg reflector is optimized to obtain single mode lasing. To extend the tuning range, both tensile and compressive strains are imposed on the device, in order to achieve a total tuning range of 100 nm. The external cavity laser exhibits excellent side mode suppression and reproducible wavelength tuning behavior proportional to the imposed strain.
2. Design of the polymer waveguide Bragg reflector
Polymeric Bragg reflector consisting of a rib waveguide structure and a grating located at the interface of the core and cladding polymers is shown in Fig. 1 . To impose a strain along the light propagation direction and change the grating period, an appropriate fixture to hold the flexible sample is prepared. The SLD has a mirror at one side and the Bragg reflector is connected to the other side, in order to provide the feedback of the wavelength to be lased. As the grating period changes due to the imposed strain, the reflection wavelength is shifted to shorter or longer wavelengths. Polymer materials have a negligible strain-optic effect  and, consequently, the tuning wavelength is predominantly determined by the mechanical elongation.
The flexibility of polymer waveguides is achieved by fabricating them on a flexible substrate. Low loss polymer materials based on fluorinated acrylates with low absorption loss in the communication wavelength range are used to form the waveguide. The refractive indices of the polymer materials chosen for the device design are 1.455 and 1.430 for the core and cladding layers, respectively. The large index contrast of the waveguide is favorable for increasing the reflectivity in a compact device with a limited grating length. Even in the case of a large contrast, a single mode waveguide with a mode size comparable to that of a single mode fiber is obtainable by adopting an oversized rib structure . The calculation of the effective index results in a single mode waveguide for a waveguide core of 4 x 6 μm2 and a rib height of 1.6 μm.
To design the Bragg reflector, the change in the effective index of the guided mode caused by the presence of the grating element is calculated and then, the transmission matrix method is utilized to find the reflectivity of the grating. When the thickness of the grating is 200 nm, it introduces an effective index modulation of 4 × 10−4 and reflectivity of 35.8% in a 2-mm long grating. The grating period is designed to be 530 nm for an initial Bragg reflection wavelength of 1535 nm.
3. Fabrication of flexible polymeric waveguide device
The direct fabrication of polymer waveguide devices on top of a flexible polymer substrate is complicated, due to the unstable ground which is easily bent by heating, poor heat transfer through the plastic, and difficulty of spin coating. Hence, we adopted the post lift-off process based on the selective adhesion property of SU-8 material . The post-lift off of the flexible layer, after it is fabricated on a hard silicon substrate, increases the reproducibility of the flexible device fabrication process.
The device was fabricated on a silicon wafer coated with a pattern of Au/Cr layer for the selective lift-off. The flexible substrate consisted of SU-8/NOA61/SU-8 layers with a total thickness of about 100 μm. To provide good flexibility, NOA61 exhibiting a good flexibility is used to form the thickest layer instead of using the stiff SU-8 single layer. Over the flexible substrate, the waveguide layer was formed by the spin coating and UV-curing of two ZPU polymers (product of ChemOptics Inc.) for the core and claddings. A 2-mm long Bragg grating was fabricated on top of the lower cladding layer by laser interferometry with a 442-nm He-Cd laser and subsequent plasma etching. Without incorporating an additional high index material, by over coating the core material, the periodic modulation of the core thickness was achieved to form the Bragg grating. To improve the grating uniformity, a black matrix polymer was used to prevent any undesired interference patterns .
The waveguide rib was defined by dry etching the core layer in an oxygen plasma with a photoresist as the etch mask. The device was made to be flexible through the lift-off of a selected area covering the Au pattern. Figure 2 shows the 5-mm long flexible Bragg grating waveguide device with glass blocks attached for the fiber pigtail. Without additional polishing, the single mode fibers inserted in a quartz ferrule were pigtailed.
Before the sample was held by the fixture used to apply pressure, PDMS blocks were attached by sandwiching the flexible part, so as to prevent the bending of the thin film when the sample was compressed. The thickness of the PDMS block compared to the length of flexible substrate determines the limit of the compressive strain. The length of the flexible part became 2.8 mm after the glass block was attached, and the thickness of the PDMS sandwiching structure was 2.2 mm.
4. Characterization of the external cavity laser with tunable Bragg reflector
The spectral response of the fabricated tunable Bragg reflector device was characterized using the setup shown in Fig. 3 . The superluminescent laser diode has antireflection coatings on both ends of the waveguide for the purpose of preventing the emission stimulated by the external feedback, and it has a 3-dB bandwidth of 60 nm and a center wavelength of 1550 nm. An optical circulator and a polarization controller were used to launch the TE polarization on the device throughout the measurement, and the reflected light was measured using an optical spectrum analyzer. The reflection and transmission spectra measured from the flexible Bragg reflector device are shown in Fig. 4 , along with a comparison with the calculated results for the 2-mm long Bragg reflector. A small transmission dip of 0.8 dB was observed and the reflection spectrum exhibited a 3-dB bandwidth of 0.7 nm. The reflectivity of the experimental result was lower than the simulation result, which may be caused by the insufficient etching depth of the grating due to the photoresist residue. However, a small reflectivity with a narrow bandwidth is preferable for single mode lasing in an external feedback laser.
For the wavelength tuning experiment, the sample was carefully attached to the motorized micro-translation stage, as shown in the photograph of Fig. 3. In this setup, one can impose both tensile and compressive strain along the longitudinal direction, in order to change the actual period of the Bragg grating. The initial Bragg reflection peak was located at 1536.9 nm. By controlling the motorized stage in steps of 2 μm, the sample was stretched or compressed to impose the strain, and reflection spectra were measured for each step, as shown in Fig. 5 . When the sample was compressed 102 μm by imposing a compressive strain of 36429 με (3.64%), the reflection peak was shifted by 51 nm to 1486 nm. When the sample was stretched 98 μm by imposing a tensile strain of 35000 με (3.50%), the reflection peak was shifted by 49 nm to 1586 nm. The tensile strain range was limited by the spectrum distortion, while the buckling of the flexible film limited the range of compressive strain. To ensure the reproducibility of the measurement, the total tuning range was limited to 100 nm in this experiment. The maximum strain is still much lower than the elastic deformation limit of a polymer material. Hence, the tuning range could be further increased by modifying the packaging configuration.
By connecting the tunable Bragg reflector to an SLD with a mirror at one end, a tunable laser was constructed, where the output light was directed toward the other end of the Bragg reflector. The SLD has a center wavelength of 1535 nm, a 3-dB bandwidth of 50 nm, and a slope efficiency of 0.105 A/W at 25 °C. The output spectrum of the laser exhibited an initial lasing wavelength of 1536.9 nm with a side mode suppression ratio of 35 dB, as shown in Fig. 6 . The spectrum measured with a resolution of 0.05 nm is shown in the inset, exhibiting a 20-dB bandwidth of 0.12 nm and 3-dB bandwidth of 0.05 nm, which was hard to measure due to the resolution limit of the optical spectrum analyzer. The output power of the laser was −3.5 dBm, but could be improved by polishing the end facet to reduce the pigtail loss.
The tuning characteristics of the laser were measured by the same procedure as that used for the reflection spectrum tuning. For a tuning range of 10 nm, the spectra were measured with a maximum resolution of 0.05 nm as shown in Fig. 7(a) . The output optical power was −10 dBm in this sample, due to the rough end facet as diced. There was no significant change of the lasing spectrum during the tuning process. The tuning range was extended to 80 nm for a total applied strain of 57143 με (5.7%), as shown in Fig. 7(b). Throughout the entire tuning range, the output power fluctuation was less than 0.5 dB. No abrupt change of the lasing wavelength was observed when the strain crossed over the tension-compression border.
To investigate the repeatability of the wavelength tuning, the sample was compressed and stretched repeatedly for 2 cycles. The measured lasing wavelength was plotted in Fig. 8 by overlapping multiple data. Each wavelength was repeated 4 times and the results were well overlapped with a deviation of less than 0.2 nm and there was no hysteresis. The lasing wavelength was linearly proportional to the imposed strain with a ratio of 1.40 pm/με. For the total tuning range of 80 nm, the lasing wavelength was dependent on the displacement of the motorized stage with no other complicated effects being observed.
Tunable wavelength lasers with a wide tuning range based on the strain tuning of a flexible Bragg reflecting polymer waveguide were demonstrated. The highly elastic property of the flexible polymer device which was fabricated by the post lift-off technique enabled the direct tuning of the Bragg reflection wavelength by controlling the imposed strain. An external cavity laser was constructed comprising a Bragg reflector and an SLD light source. Both compressive and tensile strains were imposed on the flexible polymer device to tune the Bragg reflection wavelength over a range of 100 nm. To ensure the reproducible operation of the tunable laser, it was tuned over a range of 80 nm, from 1495 nm to 1575 nm, corresponding to a total strain of 57000 με. The laser exhibits excellent wavelength repeatability and linear characteristics proportional to the imposed strain with a tuning efficiency of 1.40 pm/με. Compact tuning mechanics incorporating commercially available piezo electric motors will be developed for its practical application.
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant (2009-0079553), the Brain Korea 21 program, and the World Class University program through the National Research Foundation of Korea (R31-2008-000-20004-0). All of the resources originated from the Ministry of Education, Science and Technology, Korea.
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