Spectral and Spatial Filtering Using Waveguide Grating Mirror Ivan Avrutsky, Rabi Rabady, Kirill Zinoviev Department of Electrical and Computer Engineering Wayne State University, Detroit, MI 48202
[email protected]
Abstract: Resonant reflection of light by a waveguide grating provides simultaneous spectral and spatial filtering of the reflected beam. Narrowband transmission filter based on a plasmon resonance shows high out-of-band suppression in a wide spectral range. Dielectric waveguide grating mirror improves spatial coherence of a semiconductor laser. OCIS codes: (050.1950) Diffraction gratings; (130.2790) Guided waves; (140.3410) Laser resonators; (240.6680) Surface plasmons; (120.2440) Filters.
Introduction Excitation and re-emission of guided modes in a planar waveguide grating structure is known to result in sharp features in wavelength and angular reflectance spectra. The resonant reflection was predicted in 1965 [1] and experimentally demonstrated in 1985 [2, 3]. By physical nature, the resonant reflection from a waveguide grating is similar to the Wood anomalies of light diffraction at metallic gratings associated with excitation and re-emission of surface plasmons. Due to much lower absorption in dielectric structures, the quality factor of the resonant waveguide mode excitation can be very high. To the best of our knowledge the highest reported finesse is 1.5⋅104 [4]. Potential applications include spectral filtering [5-8], optical switches [4], optical sensors [9], polarization control in lasers [10], and spatial filtering of lateral modes in large volume lasers [11]. Special cases of normal incidence [12] and two-dimensional gratings [13] have been considered. In this talk we summarize recent achievements in study of the resonant reflection of light by waveguide gratings and report our latest results on lateral mode control in lasers using the waveguide grating mirror. Narrowband filter using plasmon resonance Theory predicts 100% maximal reflection for a lossless waveguide grating and infinite size incident beam. Out-of-resonance reflection can be to a certain degree suppressed by applying antireflectance coatings. Thus, narrowband filters with low insertion losses and high out-of-band suppression can be fabricated using the resonant reflection. The resonant wavelength λ in such a filter depends on the incident angle θ through the phase matching condition of waveguide excitation:
λ Λ
± sin(θ ) = n* ,
(1)
where Λ is the grating period and n* is the modal index. In practice, however, it is hard to provide low out-of-band reflectance in a wide spectral range (e.g., communication systems require about 20-30dB rejection over S- through U-bands covering the spectral range from 1440nm to 1675nm). We have found that using of a transmission resonance associated with long-range plasmons supported by a periodically corrugated thin silver film with symmetric claddings provides narrowband filtering suitable for application in optical communication systems employing coarse WDM [14]. OSA TOPS Vol. 75, Diffractive Optics and Micro-Optics Robert Magnusson, ed. ©2002 Optical Society of America
285
Diffractive Optics and Micro-Optics
In general, a thin film with symmetric claddings supports two plasmon modes: symmetric and asymmetric. The asymmetric mode is weakly localized so it experiences lower losses. Consequently, in the reflection and transmission spectra one can see two resonances (Fig. 1).
Reflection & Transmission
1.0 0.8
Reflection t = 40 nm σ1 = σ2 = 5nm Λ = 623nm
0.6 0.4 0.2
Transmission 0.0 1500
1520
1540
1560
1580
1600
Wavelength, nm Fig. 1. Double resonance associated with symmetric and asymmetric plasmon modes supported by a thin silver film.
The film and the grating, however can be engineered in such a way that the two resonances overlap forming a single second order resonance (Fig. 2). 1
θ Λ
Transmission
θ = 40
o
θ = 45
o
θ = 50
o
0.1
0.01
1E-3 1450
1500
1550
1600
1650
Wavelength, nm
Fig. 2. Simulated light transmission spectra through thin periodically corrugated silver film.
In such a filter, the out-off-band rejection is strong due to absorption in a metal film, and the resonant transmission is high due to low optical losses for an asymmetric plasmon mode. In addition, the second order line shape provides better side-band suppression compared to the Lorentzian spectrum typical for a single resonance device. For example, in the spectra shown above the full width at –20dB level is only 4 times greater than the –3dB width. In the case of a Lorentzian shape this ratio is equal to 99 ≈ 10 .
286
Spatial filtering by dielectric waveguie gratings
Although the wavelength and angular domains appear rather symmetrically in the mode excitation condition (1), and analysis of the spectral filtering is widely covered in the literature, the spatial filtering by a waveguide grating mirror has not been studied comprehensively yet. Some evident spatial phenomena have been noticed long time ago: the guided mode brings light beyond the illuminated spot, which results in the reflected beam shifted and widened in the propagation direction of the waveguide mode [15]. Normal incidence, naturally, provides no shift because of simultaneous excitation of two oppositely directed guided modes. The widening of the reflected beam can also be treated as a spatial filtering: strong reflection takes place only for a narrow range of incident angles, which results in wider size of the reflected beam. In a laser resonator, multiple reflection from a waveguide grating mirror accumulate this effect resulting in a fundamental mode operation of a large volume laser. We report an experimental study of a wide area semiconductor laser with external waveguide grating mirror. The waveguide grating mirror improves both the output spectrum and the far-field pattern of a wide-area semiconductor laser (Fig. 3). We used commercially available lasers and fabricated specially designed waveguide grating mirrors using rf-magnetron deposition of silica/titania films and deep-UV holographic grating technology. 1 .4 1 .2 P o w e r, a rb . u n
Output power, arb. un.
3.0
2.0
1.0
0.0 793
1 .0 0 .8 0 .6 0 .4 0 .2 0 .0
794
795
796
-3
797
Wavelength, nm
-2
-1
0
1
2
3
A n g le , d e g .
Fig. 3. Emission spectrum (left) and far-field pattern (right) of a semiconductor laser with waveguide grating mirror.
At close-to-normal incidence the excitation condition (1) is slightly modified to include possible second order Bragg interaction between the modes propagating in opposite direction: ⎛κ ⎞ ⎛λ *⎞ 2 ⎜ − n ⎟ − Sin θ = ⎜ B ⎟ ⎝ 2k ⎠ ⎠ ⎝Λ 2
2
(2)
where κ B is the Bragg coupling strength and k = 2π / λ is the vacuum propagation constant. In fact, it is not evident how the spectral and spatial filtering can work simultaneously. Considering a wavelengthangle map of locations for the reflectance resonance, one finds (2) lines λ (θ ) corresponding to total reflection rather than some resonant points (λ, θ). It means that the waveguide grating mirror will likely provide fundamental lateral mode operation at one wavelength (resonance at θ = 0 ) and high-index lateral mode operation at another wavelength (resonance at θ ≠ 0 ). We have analyzed role of optical losses and found that they improve to a certain degree the filtering properties of the waveguide rating mirror (Fig. 4). It is very unusual: typically the losses deteriorate filter performance. In the case of the 287
waveguide grating mirror, the normal incidence resonance corresponding to excitation of the two modes, is less affected by losses compared to the oblique incidence resonance. The lossy waveguide, thus, provides simultaneous filtering in both spectral and spatial domains. 810
810
805
805
λ, nm 800
800
795
795
790
790 -1
-0.5
0
0.5
1
-1
0
-0.5
θ, deg
θ, deg
0.5
1
Fig. 4 (Color). Two-dimensional plot of resonant reflection versus wavelength and incident angle for a losless waveguide (left) and a waveguide with losses comparable to coupling strength of the grating.
To verify this conclusion we have measured transmission spectra T(λ, θ) of a titania/silica waveguide structure at incident angles θ in the range from –1o to +1o and presented 1-T(λ, θ) as a 2D plot similar to Fig. 4. Because of light absorption/scattering in a waveguide, it is not exactly equal to the reflectance. Nevertheless, the experiment shows perfect qualitative agreement with the theory and confirms simultaneous spectral and spatial filtering performed by a waveguide grating mirror (Fig. 5).
830
820
λ, nm 810
800
790 -1 DD
-0.5
0
0.5
θ, deg Fig. 5. (Color) Measured 1-T (λ, θ)
288
1
Diffractive Optics and Micro-Optics
In our opinion, the spatial filtering by waveguide gratings has a bright future, basically, due to the compactness of such filters. Essentially, it is only a few thin layers and a grating, which could be fabricated directly on a laser’s facet. Any other spatial filtering setup is enormously bulky compared to the waveguide grating mirror. Summary
In conclusion, we report experimental study of spectral and spatial filtering by waveguide grating mirror and show that the spatial filtering might be very useful for lateral mode control in laser resonators. This project has been supported by NSF, grant #ECS 0096800. This work was performed in part at the Cornell Nanfabrication Facility (Project 975-01). References 1. A. Hesel, A. Oliner, “A new theory of Wood’s anomalies on optical gratings,” Appl. Optics, 4, 1275-1297 (1965). 2. L. Mashev and E. Popov, “Zero order anomaly of dielectric coated gratings,” Optical Communications, 55 377-380 (1985). 3. I. A. Avrutskii (Avrutsky), G. A. Golubenko, V. A. Sychugov, and A. V. Tishchenko, “Light reflection from the surface of a corrugated waveguide,” Soviet Technical Physics Letters 11(8), 401-402 (1985). 4. D. Rosenblatt, A. Sharon, A. A. Friesem, “Resonant grating waveguide structures,” IEEE-QE, 33, 2038-2059 (1997). 5. I. A. Avrutsky, A. S. Svakhin, and V. A. Sychugov, “Interference phenomena in waveguide with two corrugated boundaries,” Journal of Modern Optics, 36(10), 1303-1320 (1989). 6. S. S. Wang and R. Magnusson, “Multilayer waveguide-grating filters,” Appl. Optics, 34, 2414-2420 (1995). 7. S. Tibuleac, P. P. Young, R. Magnusson, T. R. Holzheimer, “Experimental verification of waveguide-mode resonant transmission filters,” IEEE Microvawe and Guided Wave Lett., 9, 19-21 (1999). 8. S. Tibuleac, R. Magnusson, “Narrow-linewidth bandpass filters with diffractive thin-film layers,” Opt. Lett., 26, 584-586 (2001). 9. F. Pigeon, I. F. Salakhutdinov, A. V. Tishchenko, “Identity of long-range surface plasmons along asymmetric structures and their potential for refractometric sensors,” J. Appl. Phys., 90, 852-859 (2001). 10. F. Pigeon, O. Parriaux, Y. Ouerdane, A. V. Tishchenko, “Polarizing grating mirror for CW Nd:YAG microchip lasers,” IEEE PTL, 12, 648-650 (2000). 11. I. Avrutsky, R. Rabady, “Waveguide grating mirror for large area semiconductor lasers,” Opt. Lett., 26, 989991 (2001). 12. D. L. Brundrett, E. N. Glytsis, and T. K. Gaylord, “Normal-incidence guided–mode resonant grating filters: design and experimental demonstration,” Optics Letters, 23 (9), 700-702 (1998). 13. S. Peng and G. M. Morris, “Resonant scattering from two-dimensional gratings,” JOSA-A, 13 (5), 993-1005 (1996). 14. I. Avrutsky, Y. Zhao, V. Kochergin, “Surface plasmon assisted resonant tunneling of light through periodically corrugated thin metal film,” Optics Letters, 25(9), 595-597 (2000). 15. I. A. Avrutsky, and V. A. Sychugov, “Reflection of a beam of finite size from a corrugated waveguide,” Journal of Modern Optics, 36(11), 1527-1539 (1989).
289
