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Abstract
Here, we present a compact, high-resolution, and ultrabroad-bandwidth arrayed waveguide grating (AWG) realized in a silicon nitride (Si3N4) platform. The AWG has a cascaded configuration with a 1×3 flat-passband AWG as the primary filter and three 1×70 AWGs as secondary filters (i.e. 210 output channels in total). The primary AWG has 0.5-dB bandwidth of 45 nm over 190 nm spectral range. The ultrabroad-bandwidth is achieved by using an innovative design that is based on a multiple-input multi-mode interference (MMI) coupler placed at the entrance of the first free propagation region of the primary AWG. The optical bandwidth of the cascaded AWG is 190 nm, and the spectral resolution is 1 nm. The overall device size is only 1.1 × 1.0 cm2. Optical loss at the central channel is 4 dB, which is 3 dB less than a conventional design with the same bandwidth and resolution values but using a primary filter with Gaussian transfer function. To the best of our knowledge, this is the first demonstration of an ultrabroad-bandwidth cascaded AWG on a small footprint. We also propose a novel low-loss (∼ 0.8 dB) design using a small AWG instead of an MMI coupler in the primary filter part, which can be used in applications where the light intensity is very weak, such as Raman spectroscopy.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compact, high-resolution and large-bandwidth spectrometers are in high demand in various applications including spectroscopy, medical imaging, astronomy, agriculture, the food industry, and many more [1–3]. While maintaining performance, the size of a bulky spectrometer can be reduced using integrated optics. The arrayed-waveguide grating (AWG) is the planar integrated optics version of a diffraction grating with high stability, small footprint, and easy fabrication [4]. It is mainly used in optical wavelength-division-multiplexed networks; however, recently it garnered quite some attention for developing miniaturized optical systems [5–7].
Combining high resolution and large free spectral range (FSR) in a single AWG is rather difficult to implement due to physical constraints and increased optical losses. However, by applying different approaches [8–12], the limitations on resolution and FSR of an AWG can be overcome. The commonly used method called cascading is based on the use of two stages of the conventional AWG structure (i.e. with Gaussian transfer function) with different resolution and FSR values [9,10]. The first stage, called the primary AWG, divides the bandwidth into a small number of large passbands and the second stage, called the secondary AWG, provides the intended narrow passbands. Usually, a primary filter has a much smaller size than a secondary filter. A conventional cascaded AWG has a large footprint, because it uses several big secondary AWGs. Moreover, due to Gaussian transfer function, the insertion loss increases significantly at the passband crossings, which also increases the non-uniformity [11]. One simple solution to this problem is to use fewer secondary AWGs with larger bandwidths, which also means that the primary filter has to have wider passbands preferably with a flat-top, i.e. rectangular, shape. Several approaches have been proposed to implement AWGs having such a flat-top passband shape [12–19]; however, most of them do not have wide passbands [14–19]. It is possible to enlarge the passband width of the primary filter by using novel MMI designs and still achieve a reasonable insertion loss.
Here, we present a compact cascaded AWG system based on an ultrabroad-spectral-range flat-top primary AWG [Fig. 1(a)]. The broad bandwidth operation was provided by using an innovative multiple-input MMI coupler design. As a proof-of-principle, a 3-channel, 65-nm-spacing primary AWG was designed and a 0.5-dB bandwidth of 45 nm over 190 nm spectral range was demonstrated with an overall device size of 0.8 × 2.5 mm2. A cascaded AWG system was realized in silicon nitride (Si3N4) platform using this flat-top 1×3 AWG as a primary filter and three 1×70 AWGs as secondary filters. The mask layout of the fabricated cascaded spectrometer is shown in Fig. 1(a). The overall device size is only 1.1 × 1.0 cm2. Compared to a conventional cascaded spectrometer using a primary filter with broad Gaussian transfer function, our design has 3 dB less optical losses. To the best of our knowledge, this is the smallest Si3N4-based cascaded AWG device with a bandwidth exceeding 190 nm. We also propose a new primary filter design with very low loss values.
Fig. 1. (a) The schematic of the cascaded AWG system using the flat-top primary AWG and three secondary AWGs. (b) 4-input MMI configuration using three modified blunt Y couplers. (c) Modified blunt Y coupler with relevant design parameters. (d) Calculated loss values of modified blunt Y coupler shown in (c) for 1400 nm and 1600 nm wavelengths. Note that aspect ratio is not 1:1.
2. Working principle and design
2.1 Waveguide design
The AWGs were realized using a 200-nm-thick low-stress plasma enhanced chemical vapor deposition (PECVD) Si3N4 film, which was deposited on a 3.5-µm-thick thermally-oxidized silicon wafer (Rogue Valley Microdevices). A 4.0 µm thick SiO2 layer (n = 1.47) was deposited by PECVD to complete the waveguiding cross-section. The refractive indexes of the thermal oxide and Si3N4 layer were 1.464 and 2.0 at 1550 nm, respectively. Single-mode rib waveguides with 0.1 µm of slab height and 1.0 µm of waveguide width were designed. The cut-off wavelength of these waveguides was 1200 nm. The effective refractive index of the fundamental transverse electric (TE) mode in the rib waveguide was calculated to be 1.54 by using BPM simulations. The propagation loss of the Si3N4 waveguides was ∼3 dB/cm at 1550 nm due to low-quality of the PECVD film. In the future, Si3N4 wafers deposited by low pressure chemical vapor deposition (LPCVD) method will be preferred [20].
2.2 Working principle, simulations and device design
The flat passband is achieved by creating a flat electric field distribution at the input waveguide of the AWG, which is reproduced at the output plane. Here, we created the flat electric field distribution by placing an MMI coupler with 4-input ports at the entrance of the first FPR of the primary AWG as seen in Fig. 1(b). The length and width of the MMI was 40 µm, and 15 µm, respectively. Both the width of the input waveguides and the spacing between them were 1 µm. Wide passbands with minimum loss were achieved by applying a certain phase delay (Δφ) between the inner and outer input waveguides of the MMI. Here, loss refers to decrease in transmission intensity caused by the MMI coupler compared to a single mode input waveguide [see Fig. 2(a)]. In order to find the optimum Δφ, the 0.5-dB bandwidth and loss values of the output channels of the primary AWG were calculated between Δφ = 0 and Δφ = 40°. As it can be seen from Figs. 2(b) and 2(c), there is a tradeoff between passband width and loss; therefore, an optimum Δφ value was obtained as 35°. In the fabricated devices, the phase difference was provided by employing optical path length difference between inner and outer waveguides of the MMI coupler [see Fig. 1(b)]. The phase difference of 35° corresponds to an optical path length difference of 94.6 µm, 98.5 µm, and 102.6 µm at 1485 nm, 1550 nm, and 1615 nm, respectively. For the fabricated devices, an average optical path length difference value of 98.5 µm was used.
Fig. 2. (a) Schematic definition of simulation parameters; i.e. loss and 0.5 dB bandwidth for a Gaussian-type output response (black line) versus flat-top output response (red line). Effect of phase difference between MMI inputs on (b) 0.5 dB bandwidth and (c) loss. Different colors represent three transmission peaks of the primary filter centered at 1485 nm, 1550 nm, and 1615 nm.
In order to split the input light into the four input ports of the MMI coupler, we used modified blunt Y couplers with an input waveguide width of w1 = 1 µm, and gap between the output waveguides of g = 0.8 µm [5]. By scanning the output width of the tapered region, i.e. w2, at two wavelengths, i.e. 1400 nm and 1600 nm, the minimum transmission loss value was obtained at w2 = 3 µm [Fig. 1(d)]. The length of the adiabatic region was L = 90 µm. The effect of material dispersion was included in the device design and simulations. Figure 3 shows the electric field distribution at the output of the MMI coupler at λ = 1485 nm, 1550 nm, and 1615 nm. As expected, a rectangular electric field distribution is obtained at the end of the MMI coupler for all three wavelengths. The primary AWG has three 3-µm-wide output waveguides with a 9-µm separation between each in order to assure the optimum adjacent crosstalk value between them. The design parameters (i.e. central wavelength (λc), FSR, spectral resolution (Δλ), path length increment between adjacent arrayed waveguides (ΔL), grating order (m), number of arrayed waveguides (M), and number of output channels (N)) of the primary and the secondary AWGs are given in Table 1.
Fig. 3. Simulated electric field distribution in the MMI coupler configuration at central wavelengths of (a) λ = 1485 nm, (b) λ = 1550 nm, and (c) λ = 1615 nm. A rectangular electric field distribution at the end of the MMI coupler is obtained for all cases (indicated by red color).
Table 1. Design Parameters of the Primary and the Secondary AWGs.
Results of the beam propagation method simulations (BeamProp, RSoft Inc.) of the primary AWG using 18-µm-wide input and multiple-input MMI design are given in Figs. 4(a) and 4(b), respectively. The overall loss of the primary AWG was around 4 dB at 1550 nm including 3 dB of MMI intrinsic loss due to field mismatch between input (i.e. rectangular) and output waveguides (i.e. Gaussian) of the AWG and 1 dB of excess loss due to gaps between waveguides at arrayed waveguide/FPR interfaces. This design has 3 dB less loss compared to a conventional AWG with a larger passband provided by a wider input waveguide (i.e. 18 µm). Since this work aims to prove the feasibility of the proposed design, the resolution and the FSR values were chosen arbitrarily; however, the proposed idea is applicable to high-resolution systems as well.
Fig. 4. Beam propagation simulation results of the (a) conventional primary AWG with 18-µm-wide input waveguide, and (b) ultrabroad-bandwidth flat-top primary AWG.
3. Experimental results and discussions
Optical transmission measurements were performed by coupling TE-polarized light from a supercontinuum light source (NKT SuperK EXTREME, EXR4) into the input waveguide with a single-mode fiber. The output signal was sent to an optical spectrum analyzer (Yokogawa, AQ6370B) through a butt-coupled single-mode fiber. Results were reproducible within ± 10%, the inaccuracy being mainly due to fiber-chip alignment errors. The transmission spectra measured at the output channels of the cascaded AWG were normalized with respect to the transmission spectrum of a rib waveguide with a propagation length equal to the length of the cascaded AWG device.
On the same wafer, both the flat-top primary AWG and the cascaded AWG were fabricated to be able to characterize their performances separately. The measured transmission spectra of the flat-top primary AWG and the cascaded AWG are given in Figs. 5(a) and 6(a), respectively. The measured values of resolution are consistent with the simulation results. However, the wavelength range of the optical fibers used in the experiments was only up to 1640 nm; therefore, the range between 1640 nm and 1650 nm could not be measured which caused some uncertainty in the FSR values. From current results, these are expected to be larger than 190 nm. As expected, flat-top AWG based on the 4-input MMI design resulted in a large bandwidth. However, we have observed significant dips at three main passband crossings of the cascaded AWG device [Fig. 6(a)], which we did not observe for the separate flat-top primary AWG [Fig. 5(a)]. The main reason is the PECVD film non-homogeneity. Only 65 channels out of 70 channels in the secondary AWGs were needed to resolve the complete spectral content of the cascaded system. The crosstalk and loss values of the primary and the middle AWG of the cascaded system (i.e. AWG 2) are given in Fig. 5(b) and Fig. 6(c), respectively. The crosstalk values are much higher than predicted for both devices, which we attribute to two main problems; a poor deposition control over PEVCD film homogeneity and quality, and phase errors induced by fabrication imperfections and waveguide geometry. The refractive indices of core and cladding layers can have nonuniformities of up to ± 1 × 10−3 in refractive index, and the core layer can show thickness variations up to ± 5% over the wafer. The waveguide width can vary by ± 0.1 µm. The standard deviation of the phase errors were calculated to be σ(δϕ) ∼0.11 rad for 1 cm-long waveguide [21]. Fiber-to-chip coupling losses were simulated to be 7 dB per chip facet. The excess loss value of the cascaded AWG was >-7 dB, including 3 dB of MMI intrinsic loss, 1 dB of excess loss due to gaps between waveguides at arrayed waveguide/FPR interfaces, and 3 dB of propagation loss. The loss non-uniformity was ∼ 6 dB for AWG 2 [see Fig. 6(c)]. The ripples observed in the transmission responses of the primary AWG are reproducible, which correspond to the maxima points in the passband as a result of the image mismatching. The overall device size is 1.1 × 1.0 cm2, which is half of the size of a conventional cascaded spectrometer consisting of a 1×7 primary AWG with Gaussian transfer function and 30 nm resolution, and seven 1 × 30 secondary AWGs with 1 nm resolution (2.1 × 1.0 cm2) as shown in Fig. 7.
Fig. 5. Transmission measurement results of the (a) ultra-broadband flat-top primary AWG. (b) The crosstalk and loss values for all three output waveguides of the primary AWG device.
Fig. 6. Transmission measurement results of the (a) cascaded AWG and (b) a zoomed spectrum of AWG 2. (c) The crosstalk and loss values of the central AWG device (AWG 2).
Fig. 7. Schematic of cascaded AWG configurations with same FSR and resolution values using (a) a conventional design with 1×7 primary AWG with Gaussian response, and seven 1×30 secondary AWGs, and (b) our proposed design with an ultrabroad-spectral-range flat-top 1×3 AWG, and three 1×70 secondary AWGs. The size of the cascaded spectrometer in (b) is half of (a).
In order to eliminate the intrinsic losses introduced by the MMI coupler, we propose a new broad-passband primary AWG design using the synchronized grating concept introduced by Dragone [16]. Figure 8(a) shows the schematic of the new design. A small AWG (i.e. 1.5 × 0.5 mm2) with four closely spaced output waveguides is connected to the first FPR of the primary AWG in order to generate an input light spot that shifts with wavelength. The FSR of the small AWG, i.e. FSRm, is equal to the wavelength-channel spacing of the primary AWG, i.e. Δλn, so that the input light spot for the primary AWG makes a full ‘sweep in position along the x-direction for each wavelength channel (i.e. synchronized) [17]. In this way the dispersion of the primary AWG for the wavelength range contained in one wavelength channel will be compensated, and thereby all these wavelengths will be imaged at the same output waveguide. Since FSRm = Δλn, the resulting device has a flattened passband for all wavelength channels. Figure 8(b) shows the simulation results of the proposed design for the central channel. Here, FSRm is 60 nm, and spacing between its output waveguides is 0.8 µm to be within the fabrication limits. As expected, loss values (∼0.8 dB) were significantly lower than the MMI-based design. Lower loss values can be achieved by reducing spacing between output waveguides of the small AWG using 193-nm lithography, which can achieve feature sizes on the order of 80 nm. These low-loss cascaded AWG devices are very attractive for applications where the measured light intensity is very weak, such as Raman spectroscopy [22].
Fig. 8. (a) Schematic of the proposed low-loss flat-top primary AWG design. Here FSRm is the FSR of the small AWG, Δλn is the wavelength-channel spacing of the primary AWG. (b) Comparison of the simulation results of MMI based design given in Fig. 4(b) (center channel) and small AWG based design.
4. Conclusions
In summary, we designed and realized a compact and very large bandwidth cascaded AWG system. An innovative multiple-input MMI configuration was implemented at the entrance of the first FPR of the primary AWG in order to obtain an ultra-broad passband. By using this method, we realized a flat-top AWG with 190 nm of bandwidth and 65 nm of channel spacing as a proof-of-principle. A cascaded AWG device with 195 channels was realized by using this flat-top AWG as the primary filter. The cascaded AWG has an overall device size of 1.1 cm × 1.0 cm2, which is half of the size of the cascaded configuration using a conventional primary AWG with wide Gaussian passband. We also proposed a new flat-top primary filter design with significantly low losses (∼0.8 dB), which can be used in weak-signal detection applications such as Raman spectroscopy [22]. In this work, we have addressed the biggest bottleneck of cascaded AWG systems for large bandwidths; i.e. large footprint. We have demonstrated the smallest cascaded AWG design with the largest bandwidth for Si3N4 platform; however, this idea can be applied to any material system and it will still provide the smallest design for a very large bandwidth range. Therefore, the proposed design is universal and valid for all material platforms. Spectrometers based on these compact, low loss and broadband cascaded AWGs will be very attractive for many different applications such as optical coherence tomography [23], astronomical telescopes [24], spectral tissue sensing [25] and so on.
Funding
Technology Foundation STW, Innovational Research Incentives Scheme Veni (SH302031).
Disclosures
The authors declare no conflicts of interest.
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