LDS dyes were doped into zirconia-organically modified silicate (ORMOSIL) materials prepared by low temperature sol-gel technique. Embedded channel waveguides were fabricated using wet etching of glass substrates followed by deposition of the LDS 925-doped zirconia-ORMOSIL in the channel. Near infrared distributed feedback (DFB) laser action was induced in the LDS 925-doped sol-gel channel waveguide. Narrow line-width (<0.5 nm) tuning of the output wavelength was achieved by varying the period of the gain modulation generated by a nanosecond neodymium:YAG laser at 532 nm. Tuning range was from 787 nm to 933 nm. The dispersion behavior of the laser output was checked by comparing experiments with the predictions of Marcatili’s theory. Additionally, near infrared (NIR) wide-band tuning and high-order DFB lasing operation were realized in LDS dye-doped planar waveguides.
© 2005 Optical Society of America
The active device such as laser is a critical enabling technology to integrated optics. Thin film waveguide lasers are desired for their efficient coupling with planar lightwave circuit. The sol-gel method is particularly relevant for the fabrication of active devices since large number of functional components (e.g., rare-earth elements , semiconductors , organic dyes ) can be introduced into the glass matrix. Laser action from thin film structure can be induced using the distributed feedback (DFB) configuration . DFB waveguide lasers have been realized in dye-doped polymers [5–7] and in photopolymers [8, 9]. We demonstrated DFB laser action in sol-gel silica slabs , in titania-silica films  and in zirconia and zirconia-organically modified silicate (ORMOSIL) films [12, 13] on glass or fused quartz substrates. Multimode lasing and wide-band tuning were achieved in these sol-gel DFB waveguide lasers .
In most practical applications in integrated optics, the rectangular dielectric waveguide is the most commonly used structure on which many of the active or passive devices (i.e., waveguide filters, optical switches, multiplexers, etc.) are in fact based. The rectangular waveguides are usually rectangular dielectric strips embedded in other dielectrics of lower refractive index. Active centers or junction structure must be built in the dielectric strips to render the waveguides optically active (e.g., waveguide lasers). Casalboni et al. reported light amplification in dye-doped sol-gel channel waveguides . We recently fabricated dye-doped zirconia channel waveguides upon quartz substrates and achieved DFB lasing in rectangular channel waveguides with width at as narrow as 5µm and depth at 3µm .
LDS (Styryl) series dyes show positive gain from red to near infrared (NIR) spectral range and thus are useful for applications that require NIR laser sources. In 1980s, NIR dye lasers based on LDS dye-doped solution were extensively researched [16–21]. However, relatively little work has been reported on the properties of solid-state LDS dye lasers. M. Zevin and R. Reisfeld prepared LDS 730-doped zirconia thin films and observed the strong fluorescence emission . Y. Oki et al. demonstrated NIR DFB laser action in LDS dye-doped plastic thin films [23, 24]. T. Kobayashi et al. realized the NIR laser emission from LDS 821-doped plastic waveguides . In this work we report the fabrication of LDS dye-doped zirconia-ORMOSIL channel waveguides using sol-gel method and the demonstration of DFB laser action in the waveguides tunable in the NIR. The dispersion behavior of the laser output was checked by comparing experiments with the predictions of Marcatili’s theory. Additionally, narrow line-width DFB laser action was achieved for the first, second and third Bragg orders in LDS 925-doped zirconia-ORMOSIL planar waveguides. NIR wide-band continuous tuning was demonstrated by using four types of LDS dye-doped zirconia-ORMOSIL planar waveguides (viz., LDS 759, LDS 798, LDS 867 and LDS 925).
Dye-doped zirconia-ORMOSIL thin films of high optical quality were used in the DFB laser experiments. Undoped zirconia-ORMOSIL thin film shows excellent optical transmission from visible to NIR range. The sol-gel method for the preparation of dye-doped zirconia-ORMOSIL layer was reported previously [12, 22]. Briefly the starting solutions consisted of zirconium n-propoxide and acetic acid. After the solutions were magnetically stirred for an hour, a few drops of 2-propanol were added to adjust the viscosity that in combination with the speed of spin coating determined the thickness of the films. Then γ-glycidyloxypropyltimethoxysilane (GLYMO) was introduced to make the glass matrix more flexible and crack-free. The molar ratio of zirconium n-propoxide to GLYMO was kept 1:1. The water needed for hydrolysis was mixed with acetic acid (1:3 by volume) and introduced drop by drop to the solutions. The molar ratio of zirconium n-propoxide to acetic acid was about 1:4 in the final solutions. Finally, laser dyes were added until the desired concentration was reached while some propylene-carbonate (PC) was dropped in since that PC was a better agent for fabricating films of LDS dyes in high concentration. Four types of LDS dyes were adopted in this experiment, viz. LDS 759, 798, 867 and 925. Typical dye concentration was 5×10-3 M. Prepared at room temperature, the refractive index of the zirconia-ORMOSIL layer was around 1.53 determined by a commercial prism coupler (Metricon model 2010) at 633 nm.
By adopting the wet etching approach, channels in glass substrates were created. The refractive index of the glass substrates was 1.51. Using the standard photolithographic technique to make a photo-mask and the subsequent isotropic etching of the glass substrates in HF: NH4F: H2O solution, we obtained channels with half-round cross-section. The dimensions of the channels in glass substrates were examined by alpha-step profiler and optical microscopy. The width of the top ranged from 50 to 13 µm. Unlike the wet etching of silicon wafers, the isotropic wet etching of the glass substrates will create the channels with depth less than half of the width. A microscopy image of the cross-section of a channel with the top width of 30 µm and depth of 12 µm is shown in Fig. 1(a).
The dye-doped zirconia-ORMOSIL layer was then deposited on top of the glass substrate by spin coating. The zirconia-ORMOSIL layer outside of the channel was removed leaving that inside to serve as the laser medium. The spin speed and the viscosity of the sol-gel solution determined the depth of the dye-doped zirconia-ORMOSIL layer. It typically was 2–4 µm. A microscope image is shown in Fig. 1(b) for a wet-etched channel with the top width of 30 µm. Visually the dye-doped zirconia-ORMOSIL layer stood out in red tone against a transparent background. At room temperature, the zirconia-ORMOSIL channel waveguides can be kept crack-free over ten days.
A home-made scanning ellipsometer fitted with synchronously rotating polarizer and analyzer [26, 27] was used to measure the refractive index and the extinction coefficient (n and k) of the LDS 925-doped zirconia-ORMOSIL films from 400 nm to 1200 nm. Three types of detectors, viz. photomultiplier tube (Hamamatsu R1104), photomultiplier tube (Hamamatsu R316) and InGaAs detector (Oriel 70348), were used in different spectral regions. The scanning ellipsometer approach yields spectroscopic information for n and k, both critical parameters that define the propagation and loss of an optical wave in the waveguide . The prism coupler (Metricon 2010) was also used to measure the waveguiding behavior . Silicon wafers were used as substrates. The measurements followed the standard procedures of ellipsometry. From the ratio of the intensities of the reflected polarized beams, the values of elliptical azimuth Ψ and phase angle Δ were extracted. n and k were then determined at 10 nm interval by a numerical routine . The ellipsometry results for a 0.9-µm-thick zirconia-ORMOSIL film with an LDS 925 concentration of 5×10-3 M are illustrated in Fig. 2. n and k were 1.523 and 7×10-4 around 900 nm, respectively. Hence the LDS 925-doped zirconia-ORMOSIL layer surrounded by the glass substrate of lower refractive index behaved as an embedded channel waveguide with low propagation loss.
The optical arrangement for DFB channel waveguide lasers was similar to that used in our previous experiments in obtaining DFB laser action in sol-gel silica block  and in sol-gel glass planar waveguides [11–13, 15]. The frequency-doubled output from the nanosecond Nd:YAG laser that served as the pump source was s-polarized according to the manufacturer. To be certain of the pump beam polarization, a polarizer was placed in the exit path of the Nd:YAG laser to filter out the unwanted p-polarization component if it does exist. More detail on the geometry of the polarized pump beams can be found in Ref. 10. Incident on a holographic grating, the pump beam was diffracted into ±1 orders of approximately equal intensities. The diffracted beams were redirected to combine on the films at an intersection angle of 2θ to create a periodical gain modulation. The output wavelength of the DFB sol-gel glass waveguide laser followed the Bragg condition of λL =ηλp /MSinθ, where η was the refractive index of the gain medium at λ L , λ P was the pump laser wavelength, and M (=1, 2, 3…) was the Bragg reflection order. For DFB laser action in rectangular waveguides, η takes on the values of the effective indices for modes or modes, where i, j are indices specifying the mode . Tuning of λ L can be achieved by varying the intersection angle and thus the gain period. A 0.3 m spectrograph/ICCD detector system was employed for spectral measurement.
3. Results and discussions
Figure 3 shows the traces of absorption, fluorescence and amplified spontaneous emission (ASE) of the LDS 925-doped zirconia-OMORSIL film. Fluorescence and ASE were measured along the optical axis of the waveguides. The thickness of the film was 1 µm and the refractive index was 1.53 at 633 nm. LDS 925 concentration was 5×10-3 M. The absorption peak was at 490 nm which was identical with the measurement result of the ellipsometer shown in Fig. 2(b). Wide absorption band allowed the relative efficient pumping at 532 nm by frequency-doubled Nd:YAG laser. Broad fluorescence emission centered at 746 nm with a line width of about 153 nm was observed. At higher pump energy (≥150 µJ), narrowing of the spectral width (down to 35 nm) indicative of ASE was achieved. The peak of ASE was at 841 nm. Compared with the peak of the fluorescence, ASE redward shifted 95 nm, which can effectively extend the output wavelength of DFB lasers.
DFB laser action was observed in LDS 925-doped channel waveguides in glass substrates when the pump laser energy exceeded 300 µJ. Accounting for diffraction and transmission loss, the actual energy deposited in the film was about 20 µJ. Figure 4 shows the DFB laser emission spectrum and the angle tuning results for a zirconia-ORMOSIL channel waveguide with a width of 30 µm and a depth of 4 µm embedded in a glass substrate. The line width of the DFB laser was less than 0.5 nm, which was the resolution limit of the spectrograph/ICCD system. The tuning data followed the solid line, which is the prediction by the Bragg resonance condition (M=2) for a zirconia-ORMOSIL layer with a refractive index of 1.53. η takes on the value of the effective index for mode, which is deduced by the well-known Marcatili theory by approximating the half-round channel waveguide to a rectangular channel waveguide .
The DFB laser output wavelength for channels of various widths and a fixed depth of 1.8 µm at an intersection angle of 28.7° was plotted in Fig. 5. The solid line is the prediction based on the Marcatili theory of rectangular waveguides . The prediction was for mode for a rectangular waveguide of a depth of 1.8 µm. Refractive index for zirconia-ORMOSIL was 1.53 and that for glass was 1.51. Experimental data correspond to channel waveguides with widths at 15.6 µm, 25 µm and 30 µm, respectively. Reasonable agreement between theory and experiments is observed, attesting to the high optical quality of the sol-gel zirconia-ORMOSIL channel waveguides.
DFB lasers operated at high Bragg orders has been reported in the early work of DFB lasers . We studied DFB lasing of LDS 925-doped zirconia-ORMOSIL thin films at first, second and third orders of the Bragg condition. The films were obtained by spin-coating glass substrates with the dye-doped zirconia-ORMOSIL sol-gel solution. Cladded on one side by the low index substrate and the other by air, the film on substrate structure behaved as an asymmetric waveguide. DFB lasing at first, second and third orders was achieved by crossing the pump beams at the intersection angles required by the Bragg condition. The results of the tuning of DFB lasing at different Bragg orders are summarized in Fig. 6(a). For first-order Bragg operation, tuning from 825 nm to 943 nm was realized. For the second Bragg order, the tuning range was from 809 nm to 932 nm. The larger intersection angle (>76°) limited the short-wavelength tuning of DFB lasing at first Bragg order. For the third Bragg order, the tuning range was from 809 nm to 881 nm. The tuning narrowed considerably as the Bragg order increased, corresponding to a decrease of the intersection angle. High-order DFB lasing operation can be employed to realize the surface-emitting lasers [33, 34].
We also demonstrated the NIR wide-band tuning of DFB waveguide lasers. By adopting four types of LDS dye-doped zirconia-ORMOSIL thin films (viz., LDS 759, LDS 798, LDS 867 and LDS 925), continuous tuning from 696 nm to 932 nm was obtained for the second Bragg order. Figure 6(b) shows the experimental data of the angle tuning versus the theoretical fit of Bragg condition (solid line), the refractive indices were determined independently by the prism coupler. Good agreement was seen.
In this work, we fabricated LDS dye-doped zirconia-ORMOSIL channel waveguides using the sol-gel methods. Tunable NIR DFB laser action was demonstrated in the channel waveguides. Dispersion characteristics were studied. NIR wide-band tuning was achieved and DFB lasing at both low and high Bragg orders (up to the third) was observed.
This work is supported in part by RGC Earmarked Research Grant of the Hong Kong SAR Government 4233/03E.
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