Distributed feedback waveguide lasers based on dye-doped sol-gel zirconia films with permanent grating structures were demonstrated. The permanent grating was realized by employing a novel epoxy-based azo-polymer that generates a surface relief grating by a photo-isomerization process induced by two interfering writing beams. When employing the rhodamine 6G dye, tuning of the output wavelength of the distributed feedback waveguide laser from around 575 nm to 610 nm can be achieved by adjusting the tilting angle between the orientation of the grating and the pump beam.
© 2007 Optical Society of America
Lasers with a distributed feedback (DFB) configuration require no external cavity or mirror . By employing a broadband gain medium such as an organic dye, the DFB configuration allows the fabrication of compact and tunable coherent light sources. DFB lasers that employ different materials such as Ce3+-doped LiSrAlF6 crystal , dye doped epoxy , dye-doped poly(methylmethacrylate) , dye-doped sol-gel silica  and conjugated polymer in polystyrene  were demonstrated. The sol-gel method is particularly attractive since a large amount of different kinds of functional components can be easily incorporated into the glass matrix. Zirconia (ZrO2) is a very useful optical material because of its wide optical transparency range, high mechanical strength and high resistance to chemical reaction. Zirconia films on glass or quartz substrates hold good promises for a wide range of applications in integrated optics [7–11]. Transient-grating DFB waveguide lasers based on 12, 13].
DFB lasing is made possible by the presence of periodic perturbations in the gain medium. The periodic perturbations are generated from the spatial modulation of the refractive index or the gain, or a combination of both, and can be permanent or transient. A transient periodic perturbation is typically achieved holographically by the interference of two crossing pump beams . The period of the perturbation can be adjusted by varying the intersection angle of the pump beams. The main advantage of a permanent grating over a transient grating is the larger modulation amplitudes, which allows reduction of the pumping threshold. Studies on the fabrication of permanent diffraction gratings have been performed using a variety of methods, including the use of a photo-mask, electron beam lithography, and holographic interference [14–19]. Photolithography using a photo-mask is not suitable for the fabrication of gratings with a sub-micrometer period due to the limitation by diffraction. Electron-beam lithography is a relatively expensive technology, and it is not suitable for fabrication of gratings over a large area. On the other hand, holography can produce fine patterns and it is easy to control the period, hence it should be the most suitable method for the fabrication of gratings with fine periods for DFB applications.
In the holographic method, the materials as well as the fabrication processes are crucial to the formation of the grating [20–25]. Lithographic techniques, which involve several deposition and etching steps, are rather complex and need very precise control to obtain a good quality surface structure. In recent years, films of azo-benzene-containing polymers (azo-polymers) have been extensively investigated [26–31] because of their ability to self-organized into regular structures on the micrometer and nanometer scales when assisted by light . One of the most interesting properties of the azo-polymers is that surface relief gratings (SRGs) can be induced on azo-polymer films by exposing to an interference of laser beams at modest intensities. Once formed, the SRG is stable when kept below the glass transition temperature (Tg) of the polymers. It can be removed by either heating the samples to a temperature above the polymer’s Tg or erased optically at below the Tg. It can be formed again by another exposure to an interference pattern. The SRG structure can be used to provide the periodic modulation that is needed for a DFB laser.
A few researchers have reported dye-doped polymeric DFB lasers based on azo polymers with photo-induced SRG [33–36]. In the present study, we demonstrate wavelength-tunable DFB waveguide lasers that are based on sol-gel derived zirconia utilizing the azo-polymer generated SRG. The device has a double-layered structure composed of a layer of azo-polymer film for generating the SRG, and an active layer of dye-doped zirconia film. A novel azo-polymer containing pseudo-stilbene type azo chromophores was used as the SRG material.
2. Experimental methods
The novel azo-polymer used in this experiments, BP-AZ-CA, was synthesized by a synthetic route reported earlier . The BP-AZ-CA polymer was dissolved in anhydrous N, N’-dimethylformamide (DMF) with a concentration of 7.9% by weight. It was filtered through a 0.45 μm membrane filter and spin-coated onto a glass slide. The film thickness could be controlled by adjusting the solution concentration and the spinning speed. The spin-coated film was dried at 70°C under a vacuum for 45 h. The waveguide properties of the film were characterized by a commercial prism coupler (Metricon Model 2010) at a wavelength of 633 nm. Both the TE0 and TM0 modes can be observed. The refractive index and the thickness of the BP-AZ-CA layer were simultaneously determined to be 1.76 and 0.35 μm, respectively.
The SRG was then inscribed in the BP-AZ-CA azo-polymer layer. The mechanism of the grating formation involves successive trans-cis photo-isomerization and thermal cis-trans relaxation, leading to the alignment of the azo-group in the direction perpendicular to the polarization direction of the incident light . Both the intensity and polarization gradients of the writing beams are necessary for forming the grating, so that a much higher writing efficiency was observed when the film was exposed to p-polarized than s-polarized writing beams. For p-polarized writing beams, the simultaneous presence of an intensity gradient and a non-zero component of the electric field along a direction perpendicular to the grating grooves leads to a higher modulation depth. Fig.1 shows schematically the set-up for the grating formation. A vertically polarized continuous-wave laser beam at 488 nm from an argon-ion laser was used as the writing light source. The laser beam was expanded and collimated by two lenses f 1 and f 2. A Soleil-Babinet compensator was used to convert the polarization direction of the beam from vertical to horizontal. The beam was then divided into two beams of equal intensities by a beam splitter. The two beams were redirected by two mirrors to combine on the azo-polymer films at an intersection angle of 2θ, forming the intensity and polarization interference pattern. By changing the intersection angle 2θ of the writing beams, we can easily modify the grating period, which is an important parameter for controlling the DFB laser emission. The grating period can be calculated according to
where λw = 488 nm is the wavelength of the writing beams, θ is the incident angle of one of the writing beams on the film, and Λ is the period of the surface relief grating obtained. The wavelength of the argon-ion laser falls into the absorption band of the BP-AZ-CA azo-polymer, so that it is effective for inducing the trans-cis photoisomerization. During the writing process, an unpolarized, low power He-Ne laser probe beam at 633 nm was used to monitor the grating formation by measuring the intensity of the first order diffracted beam from the grating.
The SRG forming behavior of the BP-AZ-CA polymer was characterized by the inscription rate and the saturation levels of the grating. When the film was irradiated by two p-polarized beams with a power density of about 150 mW cm-2, sinusoidal surface-relief structures with a regular spacing were formed. The surface modulations approach a maximum after 20 min of irradiation at room temperature. A plot of the first order diffraction efficiency for the He-Ne probe beam with respect to the exposure time is shown in Fig. 2. A diffraction efficiency of about 20% was obtained. We have also directly measured the surface relief profile obtained using an atomic force microscope in the contact mode. A typical SRG profile is shown in Fig. 3. The surface modulation depth (Δh) is about 150 nm, and can be adjusted by the irradiation energy. The period of the modulation is about 400 nm. From Eq. (1), with a wavelength of 488 nm for the writing beams and an intersection angle of approximately 75° for the two interfering beams, the grating period Λ calculated is consistent to the value of 400 nm from a direct measurement. In addition, the He-Ne probe laser could also be used to determine the grating period Λ from the grating equation
where θ and ϕ are the incident angle and the diffracted angle, respectively, M is the diffraction order, and λp = 633 nm. The grating period obtained by this method was also consistent with the above measurements. By varying the intersection angles of the writing beams, several surface relief gratings of different periods were inscribed at different areas on a single azo-polymer film.
After the formation of the photo-induced grating, an active layer was fabricated on the surface of the modulated BP-AZ-CA film. In this work, a rhodamine 6G (R6G) dye-doped sol-gel zirconia thin film was used as the active layer. The sol-gel method for the preparation of the dye-doped zirconia layer was reported previously . Briefly, the starting solution consisted of zirconium n-propoxide and acetic acid. After the solution was stirred for 30 min, a few drops of 1-propanol were added to adjust the viscosity that, in combination with the speed of spin coating, determined the thickness of the films. The water that was needed for hydrolysis was mixed with the acetic acid, and was introduced drop-wise to the solution. Finally, the laser dye R6G was added to the solution to give a dye concentration of 4 × 10-3 M. A R6G-doped zirconia layer was obtained by spin-coating the solution on top of the BP-AZ-CA layer. We have also prepared a single-layer R6G-doped zirconia film by spin-coating directly on a plain glass substrate. The refractive index of this film was measured to be 1.56 at 633 nm, with a thickness of around 0.9 μm using the prism coupler. Since the refractive indices of both the R6G-doped zirconia layer (1.56 at 633nm) and the BP-AZ-CA azo-polymer layer (1.76 at 633 nm) were higher than those of the glass substrate (1.51) and air, the double-layer structure behaves as an asymmetric waveguide. The thickness of the double layer supports both the lowest transverse electric (TE0) and transverse magnetic (TM0) modes in a planar waveguide.
The samples were pumped by the frequency-doubled output of a nanosecond Nd:YAG laser, which delivers pulses of about 6 ns in duration with a wavelength of 532 nm. The pump beam was incident normally on the surface of the sample. A combination of half-wave and quarter-wave plates was used to produce a linear or circular polarization state of the pump beam as desired. A cylindrical lens with a focal length of 10 cm was used to focus the pump beam, so that at the sample the pump beam has the shape of a narrow strip with an approximate length of 2 mm and a width of 300 μm. By adjusting the relative orientation of the sample and the cylindrical lens, the long side of the effective pumping area can be controlled to make an angle Ψ with respect to the k-vector of the surface relief grating (the direction of the k-vector is normal to the grooves of the SRG). Thus the pumping was transverse, and by varying Ψ the effective period of the DFB modulation can be tuned. We initially set Ψ = 0. A 0.3 m spectrograph/array detector system with an optical fiber coupler was employed for the spectral measurement of the DFB laser output.
3. Results and discussions
The output wavelength of a DFB laser follows the Bragg condition :
where n is the refractive index of the gain medium at the output wavelength λL, ϕ is the grating period, and M is the Bragg diffraction order. For a DFB waveguide laser, n takes the value of the effective index for the TE or TM modes. Therefore, the emission wavelengths of the DFB laser are determined by the grating period. The tuning of the DFB laser wavelength is limited by the spectral bandwidth of the emission of the dye used. This bandwidth is more than 20 nm in our case.
With adequate pumping on a SRG structured double-layered film, we obtained a narrow laser emission corresponding to the second order of diffraction (M=2), with a full width at half maximum equal to 0.5 nm, which was in fact the resolution limit of our detection system. On the other hand, only very weak and broad amplified spontaneous emission (ASE) was observed from the double-layered structures without an inscribed SRG. Figure 4 shows the laser emission intensity of a SRG structured double-layered film as a function of the pump pulse energy. The grating period was about 390 nm, and the modulation depth was about 150 nm. The threshold pump energy was about 2.5 μJ per pulse, corresponding to an energy density of 0.4 mJ cm-2. The output laser energy was found to be linearly dependent on the pump energy above threshold. Since the azo-polymer layer has a higher refractive index than the sol-gel active layer, the energy density of the guiding mode is higher in the azo-polymer layer than in the active layer. It is expected that the threshold pump energy can be lowered if the refractive index of the sol-gel layer can be increased. By varying the crossing angle of the argon-ion laser writing beams, SRGs of different periods, ranging from about 380 nm to 400 nm, can be written in different regions on a single substrate. The output spectra obtained from the regions with different SRG periods are shown in Fig. 5, which demonstrates that the wavelength of the DFB laser emission can be varied from 587 nm to 610 nm using different gratings on the same film. However, since the SRG periodicity is very sensitive to the crossing angle, it is difficult to finely tune the output wavelength through adjusting the crossing angle.
The laser output intensity was investigated as a function of the polarization of the pump beam. Three states of polarization for the pump beam, viz. s-polarized, p-polarized and circularly polarized light, were used. Here, p-polarized and s-polarized pumping refers to light polarized parallel and perpendicular to the k-vector of the grating, respectively. The maximum DFB laser output was found to occur when the waveguide was pumped by an s-polarized pump beam. At a constant pumping intensity, the laser output efficiency drops to near 50% for a circularly polarized pumping, and to nearly 30% for a p-polarized pumping, compared to that for an s-polarized pumping. For s-polarized pumping, the DFB laser emission is highly polarized with a degree of polarization, which is defined by (ITE - ITM)/ (ITE + ITM), where ITE and ITM are the laser intensities for the TE and TM output of the waveguide laser, of 90%.
Figure 6 shows the pair of TE0 and TM0 laser output spectrum for a circularly polarized pumping. The thickness of the active layer was about 1.2 μm, and the grating period corresponds to 392 nm. The separation of the two modes is found to be about 0.9 nm. According to Eq. (3), and using the measured values of 1.535 and 1.533 respectively for the effective indices for the TE0 and TM0 modes, we obtained a theoretical separation value of 0.8 nm. This is in good agreement with the measured value, considering that the spectral resolution limit of our detection system was only 0.5 nm.
By varying the relative orientation of the focused pump beam with respect to the surface relief grating direction (i.e. the angle Ψ), the effective periodicity of the distributed feedback can be tuned. The effective period, ϕeff, is given by
where Λ is the grating period. Together with Eq. (3), the output wavelength of the DFB laser is given by
Thus, if we ignore the minor variation of the effective index of the waveguide mode on the output wavelength, then the output wavelength should be linearly dependent on 1/cosΨ. This dependence of the output wavelength of the DFB laser on the angle Ψ is experimentally demonstrated and the results are shown in Fig. 7. The result also demonstrated that the output of our sol-gel zirconia waveguide DFB lasers based on SRG can be continuously tuned from around 575 nm to 610 nm, which covers the range of the gain bandwidth of the R6G dye.
In summary, we have demonstrated a bilayer structured distributed feedback waveguide laser that is composed of a novel azo-polymer layer for surface relief grating formation and a dye-doped zirconia sol-gel active layer for lasing. Tuning of the output wavelength of the distributed feedback waveguide laser from 575 nm to 610 nm can be achieved by adjusting the tilting angle between the orientation of the grating and the pump beam.
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