OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 17894–17903
« Show journal navigation

Femtosecond laser writing of a flat-top interleaver via cascaded Mach-Zehnder interferometers

Jason C. Ng, Chengbo Li, Peter R. Herman, and Li Qian  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 17894-17903 (2012)
http://dx.doi.org/10.1364/OE.20.017894


View Full Text Article

Acrobat PDF (1150 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

A flat-top interleaver consisting of cascaded Mach-Zehnder interferometers (MZIs) was fabricated in bulk glass by femtosecond laser direct writing. Spectral contrast ratios of greater than 15 dB were demonstrated over a 30 nm bandwidth for 3 nm channel spacing. The observed spectral response agreed well with a standard transfer matrix model generated from responses of individual optical components, demonstrating the possibility for multi-component optical design as well as sufficient process accuracy and fabrication consistency for femtosecond laser writing of advanced optical circuits in three dimensions.

© 2012 OSA

1. Introduction

In recent years, much effort has been directed towards improving laser control of femtosecond waveguide writing for creating optical devices for applications in optical telecommunications. Femtosecond laser writing has advantages over traditional fabrication techniques because it is flexible enough to create and integrate multiple 3D photonic devices into a single transparent substrate [1

1. G. Della Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt. 11(1), 013001 (2009). [CrossRef]

3

3. S. Nolte, M. Will, J. Burghoff, and A. Tuennermann, “Femtosecond waveguide writing: a new avenue to three dimensional integrated optics,” Appl. Phys., A Mater. Sci. Process. 77(1), 109–111 (2003). [CrossRef]

]. A variety of passive optical devices have been demonstrated with femtosecond laser writing including waveguides [4

4. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

,5

5. C. Florea and K. A. Winick, “Fabrication and characterization of photonic devices directly written in glass using femtosecond laser pulses,” J. Lightwave Technol. 21(1), 246–253 (2003). [CrossRef]

], power splitters [6

6. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24(18), 1311–1313 (1999). [CrossRef] [PubMed]

], directional couplers [7

7. A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26(1), 42–43 (2001). [CrossRef] [PubMed]

], polarization splitters [8

8. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express 19(13), 11992–11999 (2011). [CrossRef] [PubMed]

] and Bragg gratings [9

9. H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Type II femtosecond laser writing of Bragg grating waveguides in bulk glass,” Electron. Lett. 42(21), 1223–1224 (2006). [CrossRef]

]. Active routing and switching has been demonstrated in fused silica [10

10. R. Keil, M. Heinrich, F. Dreisow, T. Pertsch, A. Tünnermann, S. Nolte, D. N. Christodoulides, and A. Szameit, “All-optical routing and switching for three-dimensional photonic circuitry,” Sci Rep 1, 94 (2011). [CrossRef] [PubMed]

] while waveguides written in active media have produced optical amplifiers [11

11. Y. Sikorski, A. A. Said, P. Bado, R. Maynard, C. Florea, and K. A. Winick, “Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses,” Electron. Lett. 36(3), 226–227 (2000). [CrossRef]

] and lasers [12

12. S. Taccheo, G. Della Valle, R. Osellame, G. Cerullo, N. Chiodo, P. Laporta, O. Svelto, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses,” Opt. Lett. 29(22), 2626–2628 (2004). [CrossRef] [PubMed]

] when integrated with fiber systems. These individual building blocks open up a wide range of possibilities for more highly functional integrated photonic devices, but so far, only simple interferometers [13

13. K. Minoshima, A. Kowalevicz, E. Ippen, and J. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10(15), 645–652 (2002). [PubMed]

,14

14. G. Li, K. A. Winick, A. A. Said, M. Dugan, and P. Bado, “Waveguide electro-optic modulator in fused silica fabricated by femtosecond laser direct writing and thermal poling,” Opt. Lett. 31(6), 739–741 (2006). [CrossRef] [PubMed]

] or directional couplers for quantum communication [15

15. G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. O’Brien, “Laser written waveguide photonic quantum circuits,” Opt. Express 17(15), 12546–12554 (2009). [CrossRef] [PubMed]

] have been demonstrated.

The integration of laser written optical components into functional optical microsystems presents a formidable challenge, owing to higher waveguide loss at 0.5 dB/cm [8

8. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express 19(13), 11992–11999 (2011). [CrossRef] [PubMed]

] compared with planar lightwave circuit (PLC) fabrication methods of < 0.1 dB/cm [16

16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

], together with limited process control to compensate for wavelength dependence, dispersion, phase offsets, and birefringence. Unlike the advanced design and manufacturing finesse available in today's fiber and PLC engineering, femtosecond laser writing poses its own unique challenges in managing optical loss in variously directed waveguide paths and in controlling waveguide mode size and birefringence through a myriad of laser exposure parameters such as pulse duration, polarization [17

17. S. M. Eaton, Contrasts in Thermal Diffusion and Heat Accumulation Effects in the Fabrication of Waveguides in Glasses using Variable Repetition Rate Femtosecond Laser (University of Toronto, 2008).

], focusing power, or pulse front tilt [18

18. P. G. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]

] that collectively drive a unique material response.

Amongst various functions such as optical switching, power splitting, and multiplexing found in higher level integrated optical circuits, the interleaver serves as a passive optical filter in dense wavelength division multiplexing (DWDM) to spectrally combine multiple communication channels. A flat-top interleaver (FTI) is preferred for the widened passbands that improves signal-to-noise and reduces channel cross-talk compared with simple MZIs [16

16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

]. FTIs are characterized as finite (FIR) and infinite impulse response (IIR) filters, where the latter relies on resonance interference. FIR filters have been demonstrated with fiber gratings [19

19. F. Bilodeau, D. C. Johnson, S. Theriault, B. Malo, J. Albert, and K. O. Hill, “An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings,” IEEE Photon. Technol. Lett. 7(4), 388–390 (1995). [CrossRef]

] and cascaded MZIs [16

16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

], while IIR filters include Mach-Zehnder ring-resonator interferometers (MZIRRs) [20

20. Y. Zhang, W. Huang, X. Wang, H. Xu, and Z. Cai, “Design of flat-top interleaver and tunable dispersion compensator using cascaded Sagnac loop mirrors and ring resonators,” Appl. Opt. 48(32), 6213–6222 (2009). [CrossRef] [PubMed]

], and Michelson-Gires-Tournois interferometers (MGTIs) [21

21. L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express 15(10), 6439–6457 (2007). [CrossRef] [PubMed]

]. In general, IIR filters offer more attractive flat-top responses with higher contrast ratios [22

22. Q. J. Wang, Y. Zhang, and Y. C. Soh, “Design of 100/300 GHz optical interleaver with IIR architectures,” Opt. Express 13(7), 2643–2652 (2005). [CrossRef] [PubMed]

], while FIR filters are known for their reliability and simplicity, and have higher potential for integration into photonic circuits [16

16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

]. Cascaded MZI FTIs have been demonstrated in PLCs [16

16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

], but not in laser written devices.

Building upon existing laser recipes and preliminary work on FTI [23

23. J. Ng, C. Li, P. Herman, and L. Qian, “Flap-Top Interleaver by Femtosecond Laser Writing of Cascaded Mach-Zehnder Interferometers in Fused Silica,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper JTuI71.

], we extend femtosecond laser processing to create a more advanced integrated optical circuit in bulk glass. An FTI was designed and fabricated from a combination of various directional couplers (DCs) and MZIs. Loss, phase delay, coupling strength and wavelength dependence were characterized to facilitate an accurate modeling design that could balance trade-offs between loss, component size, and process variability and thereby deliver an optimized FTI design across a broad spectral window. Despite moderate variability in the laser process and component responses, the modeling permitted an FTI device to be fabricated by direct laser writing. Hence, the feasibility for laser writing to form highly functional integrated multi-component photonic systems has been demonstrated.

2. FTI modeling

An integrated four-port FTI circuit based on two cascaded MZIs was selected that combines several four-port optical devices including the three DCs as seen in Fig. 1
Fig. 1 Schematic design for the four-port FTI composed of two cascaded MZIs with input and output ports labelled as 1 and 2, and 3 and 4, respectively, together with a magnified (inset) view of a single DC with coupling distance, d = 8 µm. The optimized FTI design requires coupling lengths of Lc = 0.955, 0.000, and 0.236 mm (left to right for the 3 DCs) and the indicated path length differences, ΔL.
.

An optimized FTI design required the free spectral range (FSR) of the two resulting MZIs to differ by a factor of two. The FTI design following a standard transfer-matrix method based on a 2 × 2 system matrix, MFTI, to relate the output electric fields (E3and E4 at ports 3 and 4, respectively) to the input fields (E1 and E2 at ports 1 and 2, respectively):
(E3E4)=MFTI(E1E2)
(1)
The overall system transfer matrix for the present four-port FTI,
MFTI=MDC3(MΔθp2Mα2)MZI2MDC2(MΔθp1Mα1)MZI1MDC1,
(2)
accounts for DC coupling responses as represented by MDC1, MDC2and MDC3as well as the asymmetric propagation losses, Mα1and Mα2, and the phase delays, MΔθp1and MΔθp2, imposed by the two waveguide arms of each of MZI section. The power coupling ratio, rj, of the output ports, j = 3 or 4, were extracted from the square of the fields:

rj(λ)=PjP3+P4=Ej2E32+E42,j=3,4.
(3)

2.1 Directional couplers

The DC layout as shown in the inset of Fig. 1 consists of circular waveguide segments to form symmetric S-bends that converge to parallel, symmetric waveguides, closely separated by coupling distance, d, over a coupling length, Lc. Following Eq. (3), the power coupling ratios for laser formed DCs were found to be well described by coupled mode theory (CMT) [24

24. W. J. Chen, S. M. Eaton, H. Zhang, and P. R. Herman, “Broadband directional couplers fabricated in bulk glass with high repetition rate femtosecond laser pulses,” Opt. Express 16(15), 11470–11480 (2008). [CrossRef] [PubMed]

], according to the sinusoidal response:
rj(λ)=Asin2[θDC(LC,λ)],j=3,4,whereθDC(LC,λ)=κ(λ)Lc+ϕ(λ).
(4)
Here, κ(λ), is the coupling strength between the waveguides that depends on the waveguide separation distance and mode overlap, ϕ(λ)is an additional phase shift to account for coupling in the S-bends, andAis the maximum power transfer ratio that is expected to equal one for the present case of symmetric waveguides [25

25. S. M. Eaton, W.-J. Chen, H. Zhang, R. Iyer, J. Li, M. L. Ng, S. Ho, J. S. Aitchison, and P. R. Herman, “Spectral loss characterization of femtosecond laser written waveguides in glass with application to demultiplexing of 1300 and 1550 nm wavelengths,” J. Lightwave Technol. 27(9), 1079–1085 (2009). [CrossRef]

]. Although Lccan be easily varied in the circuit design to provide an optimum DC splitting ratio, rj, both the magnitude and the wavelength dependence in κ(λ)and ϕ(λ) are highly varied with the waveguide layout and laser exposure conditions [24

24. W. J. Chen, S. M. Eaton, H. Zhang, and P. R. Herman, “Broadband directional couplers fabricated in bulk glass with high repetition rate femtosecond laser pulses,” Opt. Express 16(15), 11470–11480 (2008). [CrossRef] [PubMed]

,25

25. S. M. Eaton, W.-J. Chen, H. Zhang, R. Iyer, J. Li, M. L. Ng, S. Ho, J. S. Aitchison, and P. R. Herman, “Spectral loss characterization of femtosecond laser written waveguides in glass with application to demultiplexing of 1300 and 1550 nm wavelengths,” J. Lightwave Technol. 27(9), 1079–1085 (2009). [CrossRef]

] that can dramatically limit the useful spectral bandwidth of the FTI. Finally, since propagation losses through the DC were symmetric in both waveguide arms and small (< 0.1 dB) relative to the other FTI losses, these symmetric losses could be ignored in the overall FTI response, yielding the following transfer matrix for the lossless DC:

MDC=(cosθDCisinθDCisinθDCcosθDC)
(5)

2.2 MZI response

The MZI spectral response was separated into a phase response, MΔθp, and a propagation loss response, Mα, shown below:
MΔθp=(exp(iΔθp2)00exp(iΔθp2)),whereΔθp=2πneff(λ)ΔLλ,and
(6)
Mα=(10110[αs(λ)Ls1+αc(R,λ)Lc1]0010110[αs(λ)Ls2+αc(R,λ)Lc2])
(7)
The path difference, ΔL, between the waveguide arms combined with the effective index, neff(λ), of the device introduces a relative phase delay, Δθp, between the waveguide arms, which results in a spectral fringe pattern. The propagation losses depend on the straight and curved decay constantsαs(λ)and αc(λ)and the lengths of these sections, Lsand Lc, respectively.

The MZI arms differed greatly in the proportion of straight and curved sections, which results in strongly asymmetric losses between the arms. This dramatically alters the power ratio in waveguides entering subsequent DCs, reducing the channel contrast.

3. Laser writing and characterization of waveguides

The laser-written photonic devices were probed with a broadband light source (Agilent 83437A) over the 1250 to 1650 nm spectral range by free space launching (0.3 NA lens) into the entrance waveguide while the light exiting the device was end coupled into a single mode fiber for capturing transmission spectra with an optical spectrum analyzer (Ando AQ6317B). Index matching oil was used at the fiber-to-waveguide interfaces to reduce Fresnel reflection loss. A free-space linearly polarizer permitted excitation of pure vertically or horizontally polarized modes that was necessary to separate polarization dependent responses associated with an expected waveguide birefringence of Δn ~5 × 10−5 [8

8. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express 19(13), 11992–11999 (2011). [CrossRef] [PubMed]

]. All spectral responses were normalized in the same way against a reference straight waveguide with known insertion loss.

4. FTI design

The FTI was developed through successive stages of laser fabrication and characterization of individual waveguides and four-port DC and MZI devices, to construct the transfer matrix response functions in Section 2. A range of laser exposure conditions was explored to minimize waveguide loss while also seeking a small bend radius for compactness of the back-to-back MZI design to fit over a 3” wide glass substrate. With these constraints, a channel spacing of 3 nm was targeted together with DC coupling ratios of 50.0%, 72.8%, and 92.6% that were previously optimized [27

27. Q. Wang and S. He, “Optimal design of a flat-top interleaver based on cascaded MZ interferometers by using a genetic algorithm,” Opt. Commun. 224(4-6), 229–236 (2003). [CrossRef]

] to generate wide flat-top passbands with deep contrast ratios. A convergence of the measured and simulated spectral response was finally sought to provide a balanced interleaver response over the broadest spectral window.

4.1 Optical losses

The FTI design was focused in the 1250 to 1325 nm range of Fig. 2, where propagation loss of all the straight and curved waveguides was lowest (< 1.00 dB/cm). Design wavelengths of 1310 nm and 1391 nm were identified for the 35 mm and 75 mm radius waveguides, respectively, that would provide nearly identical propagation losses of 0.71 dB/cm and 0.94 dB/cm, respectively, for balancing losses in the both arms of each MZI. While the lower loss and flatter wavelength response of the larger 75 mm radius waveguides would provide a broader interleaver spectrum, the lower 35 mm radius waveguides were selected for both the DC and MZI curved sections to provide a five-fold denser channel spacing and more compact design. The propagation loss data in Fig. 2 provided the αs(R=,λ)and αc(R=35mm,λ) matrix elements required in Eq. (7).

4.2 Directional couplers

To characterize the directional coupler response under the current laser writing conditions, a series of symmetric DCs were written with the minimum waveguide separation fixed atd = 8 µm and coupling lengths varied from Lc = 0 to 2 mm in 0.2-mm steps. Figure 3
Fig. 3 The measured coupling ratio,r3 and r4, at 1310 nm for DCs of various coupling lengths and the representation of Eq. (4) (solid lines) with DC matrix parameters:
κ = 1.398 mm−1,φ = 1.021, andA = 0.944 and a coefficient of determination,R2 = 0.99956.
shows the measured coupling ratios calculated from Eq. (3) for 1310 nm and fitted according to Eq. (4) to yield κ(λ), φ(λ), and A(λ) values of 1.398 mm−1, 1.021, and 0.994, respectively, for port 3. This corresponds to a beat length of 2.247 mm that permitted selection of any DC coupling ratio (r = 0 to 1) up to a high 25-dB contrast ratio for coupling lengths under 1.124 mm. Interpolation of the present data offered coupling lengths of Lc = 0.955, 0, and 0.230 mm to meet the respective 50.0%, 72.8%, and 92.6% coupling ratios required in the present FTI design for operation around 1310 nm.

The DC spectral response, r(λ), for a 3-dB coupler design at 1310 nm was assessed as shown in Fig. 4
Fig. 4 Spectral response of a single DC with coupling length, Lc = 0.955 mm, yielding a nearly flat 3.0 ± 0.5 dB coupling ratio over a 30-nm bandwidth between 1296 and 1326 nm.
, yielding a relatively flat wavelength dependence of 3.0 ± 0.5 dB (50 ± 5%) between 1296 to 1326 nm that predicts a 30-nm spectral window for favorable FTI response.

4.3 MZI response

For waveguides limited to 35 mm bend radius, the maximum path differences that could be generated over the present substrate was ΔL1 = 192.92 µm and ΔL2 = 385.84 µm for the cascaded MZI to yield a minimum FTI channel bandwidth of 3 nm at 1310 nm. Larger channel separation can be arbitrarily designed with smaller path differences, up to the symmetric ΔL=0 case. The phase delay between the MZI arms was precisely modeled by extracting a linear estimate of neff(λ)=1.47361.7891×105λ from the fringe spacing in an MZI over the 1250 to 1350 nm range. Given the small refractive index modification induced by laser writing, the waveguide contribution to the chromatic dispersion will be much less than that of the material dispersion. We therefore expect the chromatic dispersion of the device in the region of its flat passbands to follow that of fused silica.

5. Interleaver prototype

The spectral response of the FTI design for the optimized waveguide device parameters in Section 4 were calculated from the FTI matrix response in Eq. (2) and plotted in Fig. 5
Fig. 5 The measured (Exp) and simulated (Model) transmission spectra for the FTI showing good agreement between the design and the device spectra. A 15-dB contrast ratio is observed over a 30-nm spectral band of 1287 to 1317 nm.
(solid lines) together with the spectral response of the associated laser-fabricated FTI prototype (dashed lines). An accurate spectral alignment was found between the design and prototype channels for both output ports (3 and 4), coinciding within ± 0.01 nm ( ± 0.3% of FSR) for the 1310 nm channel and increasing to ± 0.1 nm ( ± 3%) at 1269 nm and 1341 nm. A moderately strong contrast ratio of > 15 dB was maintained between the output ports across a 30-nm operating band from 1287 and 1317 nm, attesting to the high precision and reproducibility of laser fabrication without the need for phase trimming of the MZI arms. These contrast ratios fell ~10 dB short of the 25 dB design values expected near 1310 nm, potentially arising from small variations in the laser exposure control that lead to varied waveguide properties (neff(λ) and α(λ)), detuned DCs, or phase errors in the MZI arms. Moving away from the design wavelength, the coupling ratios for both measured and predicted responses became increasingly unbalanced due to the limited 30-nm bandwidth found in Fig. 4 for the present 3-dB coupler design. The contrast degrades further to < 10 dB for longer wavelengths, λ > 1340 nm, due to the strongly increased bend loss anticipated in the longer MZI waveguide arms by Fig. 2 (R = 35 mm) that unbalance the power ratios at the DCs. Nevertheless, a 10-dB contrast ratio was maintained on all FTI output ports over a large 70-nm range from 1270 to 1340 nm.

The flat-top response of the FTI in Fig. 5 was characterized by a 0.5-dB passband width of 1.85 nm for the 3-nm channel spacing at 1308 nm. An average 0.5-dB passband of 1.75 nm was found for the five channels in the 1290 to 1317 nm band, representing a 46% wider passband that meets closely with the 50% widening expected over a single MZI interleaver design. Outside of this spectral window, the 0.5-dB passband narrowed to values as low as 1.2 nm (λ> 1338 nm) and became double peaked (λ< 1269 nm) for the port 4 channels, owing to imbalance in the MZI arm loss, MZI phases, and DCs splitting ratio.

The propagation loss of the FTI shown in Fig. 2 was inferred from the insertion loss spectrum that gives a value of 9.3-dB loss at the 1310 nm design wavelength. The minimum propagation loss of 1.5 ± 0.1 dB/cm found in the 1250 to 1310 nm aligns well with the 1287 to 1317 nm operating range of the optimized interleaver response in Fig. 5. The interleaver loss increased strongly to > 3.6 dB/cm with longer wavelength, λ> 1430 nm, that followed the trend of losses for the R = 35 mm curved waveguide in Fig. 2.

Immersion of the FTI into ice water yielded a small 0.5 nm spectral shift from room temperature, illustrating a very small temperature dependence of ~0.03 nm/°C or 1% FSR/°C around 1310 nm.

6. Discussion and conclusion

Several directions are available for further improving the spectral response of the femtosecond laser written interleaver. A reduction of total insertion loss particularly towards longer wavelengths into the C- and L- telecom bands requires development of stronger guiding waveguides with bend loss falling well below the 5.0 dB/cm values seen in Fig. 2 for λ > 1430 nm. Femtosecond laser writing with high NA oil immersion lenses is one promising approach in this direction that offers two-fold smaller mode sizes (i.e. 7.2 µm mode field diameter) and two-fold higher refractive index contrast [30

30. S. M. Eaton, M. L. Ng, R. Osellame, and P. R. Herman, “High refractive index contrast in fused silica waveguides by tightly focused high-repetition rate femtosecond laser,” J. Non-Cryst. Solids 357(11-13), 2387–2391 (2011). [CrossRef]

] such that higher curvature waveguides could be exploited for creating more compact MZI and FTI devices with higher channel isolation and denser channel spacing. The FTI bandwidth was restricted by the narrow 30-nm bandwidth in the present symmetric DC design (Fig. 4), while a 10-fold bandwidth improvement has been found previously with asymmetric DCs laser-written in borosilicate glass [24

24. W. J. Chen, S. M. Eaton, H. Zhang, and P. R. Herman, “Broadband directional couplers fabricated in bulk glass with high repetition rate femtosecond laser pulses,” Opt. Express 16(15), 11470–11480 (2008). [CrossRef] [PubMed]

].

The FTI was analyzed for a single linear polarization to avoid birefringence (Δn~5×105) [8

8. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express 19(13), 11992–11999 (2011). [CrossRef] [PubMed]

] effects in the laser written waveguides. Harnessing heat-accumulation effects such as observed in waveguides formed in borosilicate glass [9

9. H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Type II femtosecond laser writing of Bragg grating waveguides in bulk glass,” Electron. Lett. 42(21), 1223–1224 (2006). [CrossRef]

] can offer much lower birefringence to aid in developing a polarization insensitive interleaver.

The flat-topped FTI response in Fig. 5 demonstrates the precision of matrix modeling as a design tool that can now be reliability applied to femtosecond laser writing of waveguides to develop more functional optical circuits. Design approaches were applied here to account for various loss, phase and power splitting imbalances and enable femtosecond laser writing in a single step process without complex trimming procedures. As the capabilities of femtosecond laser fabrication improve, this design approach will facilitate more practical device applications into areas such as spectral shaping and polarization control or phase-array waveguide gratings to exploit the 3D advantage of the laser writing process.

In summary, the design optimization and fabrication of an integrated five-component flat-top interleaver was demonstrated by using femtosecond laser writing of optical waveguides in bulk glass. The laser processes and design were sufficiently robust to produce 46% widened 0.5-dB flat-top response with channel contrast ratios above 15 dB over a 30 nm bandwidth. Overall, the transfer matrix model successfully captured the phase and loss responses through multiple photonic components for predicting the spectral response of the FTI, with spectral alignment within ± 0.01 nm or 0.3% of the 3 nm channel spacing. The results demonstrate the accuracy and reproducibility of the femtosecond laser writing of more functional 3D optical circuits inside bulk transparent glass.

Acknowledgments

The authors gratefully acknowledge support from Canadian Institute for Photonic Innovations and the Natural Sciences and Engineering Research Council of Canada.

References and links

1.

G. Della Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt. 11(1), 013001 (2009). [CrossRef]

2.

R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2(4), 219–225 (2008). [CrossRef]

3.

S. Nolte, M. Will, J. Burghoff, and A. Tuennermann, “Femtosecond waveguide writing: a new avenue to three dimensional integrated optics,” Appl. Phys., A Mater. Sci. Process. 77(1), 109–111 (2003). [CrossRef]

4.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

5.

C. Florea and K. A. Winick, “Fabrication and characterization of photonic devices directly written in glass using femtosecond laser pulses,” J. Lightwave Technol. 21(1), 246–253 (2003). [CrossRef]

6.

D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett. 24(18), 1311–1313 (1999). [CrossRef] [PubMed]

7.

A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett. 26(1), 42–43 (2001). [CrossRef] [PubMed]

8.

L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express 19(13), 11992–11999 (2011). [CrossRef] [PubMed]

9.

H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Type II femtosecond laser writing of Bragg grating waveguides in bulk glass,” Electron. Lett. 42(21), 1223–1224 (2006). [CrossRef]

10.

R. Keil, M. Heinrich, F. Dreisow, T. Pertsch, A. Tünnermann, S. Nolte, D. N. Christodoulides, and A. Szameit, “All-optical routing and switching for three-dimensional photonic circuitry,” Sci Rep 1, 94 (2011). [CrossRef] [PubMed]

11.

Y. Sikorski, A. A. Said, P. Bado, R. Maynard, C. Florea, and K. A. Winick, “Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses,” Electron. Lett. 36(3), 226–227 (2000). [CrossRef]

12.

S. Taccheo, G. Della Valle, R. Osellame, G. Cerullo, N. Chiodo, P. Laporta, O. Svelto, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses,” Opt. Lett. 29(22), 2626–2628 (2004). [CrossRef] [PubMed]

13.

K. Minoshima, A. Kowalevicz, E. Ippen, and J. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express 10(15), 645–652 (2002). [PubMed]

14.

G. Li, K. A. Winick, A. A. Said, M. Dugan, and P. Bado, “Waveguide electro-optic modulator in fused silica fabricated by femtosecond laser direct writing and thermal poling,” Opt. Lett. 31(6), 739–741 (2006). [CrossRef] [PubMed]

15.

G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. O’Brien, “Laser written waveguide photonic quantum circuits,” Opt. Express 17(15), 12546–12554 (2009). [CrossRef] [PubMed]

16.

M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol. 22(3), 895–902 (2004). [CrossRef]

17.

S. M. Eaton, Contrasts in Thermal Diffusion and Heat Accumulation Effects in the Fabrication of Waveguides in Glasses using Variable Repetition Rate Femtosecond Laser (University of Toronto, 2008).

18.

P. G. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]

19.

F. Bilodeau, D. C. Johnson, S. Theriault, B. Malo, J. Albert, and K. O. Hill, “An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings,” IEEE Photon. Technol. Lett. 7(4), 388–390 (1995). [CrossRef]

20.

Y. Zhang, W. Huang, X. Wang, H. Xu, and Z. Cai, “Design of flat-top interleaver and tunable dispersion compensator using cascaded Sagnac loop mirrors and ring resonators,” Appl. Opt. 48(32), 6213–6222 (2009). [CrossRef] [PubMed]

21.

L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express 15(10), 6439–6457 (2007). [CrossRef] [PubMed]

22.

Q. J. Wang, Y. Zhang, and Y. C. Soh, “Design of 100/300 GHz optical interleaver with IIR architectures,” Opt. Express 13(7), 2643–2652 (2005). [CrossRef] [PubMed]

23.

J. Ng, C. Li, P. Herman, and L. Qian, “Flap-Top Interleaver by Femtosecond Laser Writing of Cascaded Mach-Zehnder Interferometers in Fused Silica,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper JTuI71.

24.

W. J. Chen, S. M. Eaton, H. Zhang, and P. R. Herman, “Broadband directional couplers fabricated in bulk glass with high repetition rate femtosecond laser pulses,” Opt. Express 16(15), 11470–11480 (2008). [CrossRef] [PubMed]

25.

S. M. Eaton, W.-J. Chen, H. Zhang, R. Iyer, J. Li, M. L. Ng, S. Ho, J. S. Aitchison, and P. R. Herman, “Spectral loss characterization of femtosecond laser written waveguides in glass with application to demultiplexing of 1300 and 1550 nm wavelengths,” J. Lightwave Technol. 27(9), 1079–1085 (2009). [CrossRef]

26.

H. Zhang, S. Ho, S. M. Eaton, J. Li, and P. R. Herman, “Three-dimensional optical sensing network written in fused silica glass with femtosecond laser,” Opt. Express 16(18), 14015–14023 (2008). [CrossRef] [PubMed]

27.

Q. Wang and S. He, “Optimal design of a flat-top interleaver based on cascaded MZ interferometers by using a genetic algorithm,” Opt. Commun. 224(4-6), 229–236 (2003). [CrossRef]

28.

L. Faustini and G. Martini, “Bend loss in single-mode fibers,” J. Lightwave Technol. 15(4), 671–679 (1997). [CrossRef]

29.

D. Marcuse, “Curvature loss formula for optical fibers,” J. Opt. Soc. Am. 66(3), 216–220 (1976). [CrossRef]

30.

S. M. Eaton, M. L. Ng, R. Osellame, and P. R. Herman, “High refractive index contrast in fused silica waveguides by tightly focused high-repetition rate femtosecond laser,” J. Non-Cryst. Solids 357(11-13), 2387–2391 (2011). [CrossRef]

31.

F. Landouceur and J. D. Love, Silica-Based Buried Channel Waveguides and Devices (Chapman & Hall, 1996).

OCIS Codes
(320.2250) Ultrafast optics : Femtosecond phenomena
(350.2460) Other areas of optics : Filters, interference

ToC Category:
Laser Microfabrication

History
Original Manuscript: June 5, 2012
Revised Manuscript: July 13, 2012
Manuscript Accepted: July 13, 2012
Published: July 20, 2012

Citation
Jason C. Ng, Chengbo Li, Peter R. Herman, and Li Qian, "Femtosecond laser writing of a flat-top interleaver via cascaded Mach-Zehnder interferometers," Opt. Express 20, 17894-17903 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-17894


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. G. Della Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt.11(1), 013001 (2009). [CrossRef]
  2. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics2(4), 219–225 (2008). [CrossRef]
  3. S. Nolte, M. Will, J. Burghoff, and A. Tuennermann, “Femtosecond waveguide writing: a new avenue to three dimensional integrated optics,” Appl. Phys., A Mater. Sci. Process.77(1), 109–111 (2003). [CrossRef]
  4. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett.21(21), 1729–1731 (1996). [CrossRef] [PubMed]
  5. C. Florea and K. A. Winick, “Fabrication and characterization of photonic devices directly written in glass using femtosecond laser pulses,” J. Lightwave Technol.21(1), 246–253 (2003). [CrossRef]
  6. D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses,” Opt. Lett.24(18), 1311–1313 (1999). [CrossRef] [PubMed]
  7. A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses,” Opt. Lett.26(1), 42–43 (2001). [CrossRef] [PubMed]
  8. L. A. Fernandes, J. R. Grenier, P. R. Herman, J. S. Aitchison, and P. V. S. Marques, “Femtosecond laser fabrication of birefringent directional couplers as polarization beam splitters in fused silica,” Opt. Express19(13), 11992–11999 (2011). [CrossRef] [PubMed]
  9. H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Type II femtosecond laser writing of Bragg grating waveguides in bulk glass,” Electron. Lett.42(21), 1223–1224 (2006). [CrossRef]
  10. R. Keil, M. Heinrich, F. Dreisow, T. Pertsch, A. Tünnermann, S. Nolte, D. N. Christodoulides, and A. Szameit, “All-optical routing and switching for three-dimensional photonic circuitry,” Sci Rep1, 94 (2011). [CrossRef] [PubMed]
  11. Y. Sikorski, A. A. Said, P. Bado, R. Maynard, C. Florea, and K. A. Winick, “Optical waveguide amplifier in Nd-doped glass written with near-IR femtosecond laser pulses,” Electron. Lett.36(3), 226–227 (2000). [CrossRef]
  12. S. Taccheo, G. Della Valle, R. Osellame, G. Cerullo, N. Chiodo, P. Laporta, O. Svelto, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses,” Opt. Lett.29(22), 2626–2628 (2004). [CrossRef] [PubMed]
  13. K. Minoshima, A. Kowalevicz, E. Ippen, and J. Fujimoto, “Fabrication of coupled mode photonic devices in glass by nonlinear femtosecond laser materials processing,” Opt. Express10(15), 645–652 (2002). [PubMed]
  14. G. Li, K. A. Winick, A. A. Said, M. Dugan, and P. Bado, “Waveguide electro-optic modulator in fused silica fabricated by femtosecond laser direct writing and thermal poling,” Opt. Lett.31(6), 739–741 (2006). [CrossRef] [PubMed]
  15. G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. O’Brien, “Laser written waveguide photonic quantum circuits,” Opt. Express17(15), 12546–12554 (2009). [CrossRef] [PubMed]
  16. M. Oguma, T. Kitoh, Y. Inoue, T. Mizuno, T. Shibata, M. Kohtoku, and Y. Hibino, “Compact and low-loss interleave filter employing lattice-form structure and silica-based waveguide,” J. Lightwave Technol.22(3), 895–902 (2004). [CrossRef]
  17. S. M. Eaton, Contrasts in Thermal Diffusion and Heat Accumulation Effects in the Fabrication of Waveguides in Glasses using Variable Repetition Rate Femtosecond Laser (University of Toronto, 2008).
  18. P. G. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett.90(15), 151120 (2007). [CrossRef]
  19. F. Bilodeau, D. C. Johnson, S. Theriault, B. Malo, J. Albert, and K. O. Hill, “An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings,” IEEE Photon. Technol. Lett.7(4), 388–390 (1995). [CrossRef]
  20. Y. Zhang, W. Huang, X. Wang, H. Xu, and Z. Cai, “Design of flat-top interleaver and tunable dispersion compensator using cascaded Sagnac loop mirrors and ring resonators,” Appl. Opt.48(32), 6213–6222 (2009). [CrossRef] [PubMed]
  21. L. Wei and J. W. Y. Lit, “Design optimization of flattop interleaver and its dispersion compensation,” Opt. Express15(10), 6439–6457 (2007). [CrossRef] [PubMed]
  22. Q. J. Wang, Y. Zhang, and Y. C. Soh, “Design of 100/300 GHz optical interleaver with IIR architectures,” Opt. Express13(7), 2643–2652 (2005). [CrossRef] [PubMed]
  23. J. Ng, C. Li, P. Herman, and L. Qian, “Flap-Top Interleaver by Femtosecond Laser Writing of Cascaded Mach-Zehnder Interferometers in Fused Silica,” in Quantum Electronics and Laser Science Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper JTuI71.
  24. W. J. Chen, S. M. Eaton, H. Zhang, and P. R. Herman, “Broadband directional couplers fabricated in bulk glass with high repetition rate femtosecond laser pulses,” Opt. Express16(15), 11470–11480 (2008). [CrossRef] [PubMed]
  25. S. M. Eaton, W.-J. Chen, H. Zhang, R. Iyer, J. Li, M. L. Ng, S. Ho, J. S. Aitchison, and P. R. Herman, “Spectral loss characterization of femtosecond laser written waveguides in glass with application to demultiplexing of 1300 and 1550 nm wavelengths,” J. Lightwave Technol.27(9), 1079–1085 (2009). [CrossRef]
  26. H. Zhang, S. Ho, S. M. Eaton, J. Li, and P. R. Herman, “Three-dimensional optical sensing network written in fused silica glass with femtosecond laser,” Opt. Express16(18), 14015–14023 (2008). [CrossRef] [PubMed]
  27. Q. Wang and S. He, “Optimal design of a flat-top interleaver based on cascaded MZ interferometers by using a genetic algorithm,” Opt. Commun.224(4-6), 229–236 (2003). [CrossRef]
  28. L. Faustini and G. Martini, “Bend loss in single-mode fibers,” J. Lightwave Technol.15(4), 671–679 (1997). [CrossRef]
  29. D. Marcuse, “Curvature loss formula for optical fibers,” J. Opt. Soc. Am.66(3), 216–220 (1976). [CrossRef]
  30. S. M. Eaton, M. L. Ng, R. Osellame, and P. R. Herman, “High refractive index contrast in fused silica waveguides by tightly focused high-repetition rate femtosecond laser,” J. Non-Cryst. Solids357(11-13), 2387–2391 (2011). [CrossRef]
  31. F. Landouceur and J. D. Love, Silica-Based Buried Channel Waveguides and Devices (Chapman & Hall, 1996).

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4 Fig. 5
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited