Programmable photonic integrated circuits (PICs), which dynamically reconfigure spectral and time delay responses of optical signals, are a rapidly growing field within integrated photonics [1–6]. Programmable PIC’s advantages over traditional PIC designs include a drastic reduction of custom design of devices that often need multiple iterations of fabrication to optimize their performances. Current implementations, albeit powerful, rely on dense meshes of waveguides and couplers to accurately control signal responses. A common drawback of this method is a lack of an intuitive connection between the target output spectrum and configuration of phase shifters, which can total hundreds in quantity. As a result, building a library of desired functions relies on recording the collective response of the phase shifters, which can require hundreds of Monte Carlo simulations [7]. We propose a novel method of creating an arbitrary response circuit, where the connection between phase shifters and wavelength response is more direct. Modular components, which can tune the transmission coefficient at a single predetermined wavelength, can be concatenated to allow arbitrary response over a larger spectral bandwidth. However, to our knowledge, a building block with this response for a narrow spectral range does not exist amongst current silicon photonic designs. As such, we experimentally demonstrate a Mach–Zehnder interferometer (MZI) based tunable notch filter that replaces an MZI power splitter/combiner with an apodized four-port Bragg add/drop filter to provide a large free spectral range (FSR) and wavelength selectivity. Although combinations of MZI and Bragg-based devices exist in the literature [8–10], these devices are intended for add/drop filtering and modulation applications and do not offer tunable waveform-shaping capability. Additionally, this device is useful for various applications, including single-mode modulation by incorporating a high-speed phase shifter, sensing with a broad spectral range, and microwave filtering exploiting an inherently low spectral crosstalk.

The device depicted schematically in Fig. 1 is composed of two four-port Bragg add/drop filters similar to those discussed in the literature [11]. This device heavily suppresses the Bragg sidelobe response by apodization via bending the drop waveguide in a Gaussian profile. These structures are connected in an MZI configuration such that the through-port of the first Bragg filter is routed to the input of the second Bragg filter, forming the first arm of the MZI. The second arm is created by routing the drop-port of the first Bragg filter to the add-port of the second Bragg filter. Both arms are composed of a standard 500 nm wide waveguide. Both Bragg filters are identical in design with an extinction ratio of 3 dB to provide a high extinction ratio for the output of the MZI.

To ensure spectral bandwidths (∼3nm) useful for wavelength division multiplexing (WDM) and compatibility with fully etched fabrication processes, the Bragg add/drop filter, shown in Fig. 2(a), is realized by implementing periodic cladding perturbations. The FSR of the MZI is designed to be 10.5 nm, which allows the Bragg filter to operate completely within a single MZI FSR, suppress the other MZI resonances, and enable a notch filter response. The spectral bandwidth is determined by the magnitude of the modal perturbation in the cladding, which is minimized by its distance from the input waveguide (e.g., ∼75nm). The period of the perturbations, Λ, is chosen to be 310 nm to ensure operation within the C-band. Each perturbation has an oval shape to implement adiabatic changes to the optical mode to perform a smooth apodization and increase side-mode suppression. The apodization is performed with a Gaussian profile where the gap between the two waveguides is defined as where Gmin is the minimum distance between the two waveguides, H is the height of the drop-port waveguide curve, a is the apodization coefficient, and L is the length of the filter. Additionally, a second set of perturbations below the waveguide is implemented with an offset equal to Λ/2 to reduce the undesired backreflection of the incident light [12]. Simulated performance of the filter, created with Lumerical MODE, is shown in Fig. 2(b).

The devices are fabricated at Applied Nanotools under multi-project-wafer (MPW) service. Waveguides are patterned using electron beam lithography on a silicon-on-insulator wafer with a 220 nm thick device layer. TiW alloy heaters are deposited above the MZI arms and Bragg add/drop filters for tuning purposes. To verify filter performance, the through-port and drop-port of four concatenated filters are measured. Each filter is identical in design and only differs in Λ, which is increased by 3 nm in each successive channel to shift the center of the operating spectral band. In this configuration, the through-port is connected to the input of the successive filter allowing the drop-ports to be freely measured [see Fig. 3(a)]. Measured port responses, excited by a tunable laser (Keysight 81600B) and collected by a detector (Keysight 81635A), are normalized to the maximum value of the through-port and are shown in Fig. 3(b).

Each passband, as measured from the drop-port, supports side-mode suppression of at least 20 dB due to the apodization of the waveguide. Although the apodization is symmetric in space, the spectral sidelobe suppression can be asymmetric due to dispersion, as confirmed by the simulated spectra. Typically, for efficient system operation, crosstalk between adjacent WDM channels is desired to be 18 dB or less, which these filters meet for dense WDM applications. Although electron beam lithography is used, the apodization method is robust enough to be reproduced in photolithography, enabling the potential for cost-effective large-scale CMOS manufacturing. Each passband successfully has a maximum power of ∼3dB, allowing ∼3dB of power to remain on the through-port for the corresponding wavelength, effectively functioning as a 50/50 power splitter. Negligible power is measured from the add-port of each filter, affirming low insertion loss for the filter realizing an equivalent of a spectral beam splitter.

Next, simulated performance of the MZI is investigated using Lumerical Interconnect. For simplicity, a Bragg add/drop filter is only used for MZI power splitting, and an ideal Y-junction is used to combine the power from each respective MZI arm. As seen in Fig. 4(a), when the phase delay between the two arms is π/2, a full extinction of 65 dB at the center wavelength of the MZI is achieved. Full reversal of the extinction is achieved by removing the phase difference between the two arms and returning to an untuned state. The extinction ratio is arbitrarily tuned by tuning the phase between these two extremes.

Although using a Y-junction offers excellent simulated performance, practically it increases design difficulty due to the Bragg add/drop filter giving nontrivial arm length differences between light emitted from the through-port and drop-port. These path differences result in a small MZI FSR, introducing undesired extinctions for a given device. For fabricated design simplicity, two Bragg add/drop filters are then used to create a symmetric spatial design, improving the FSR of the device. This approach reduces the extinction ratio since attempting 50/50 power combining with an add/drop filter results in an additional insertion loss, limiting the effectiveness of the device. Simulated performance of using a Bragg add/drop filter as a combiner with Lumerical MODE is presented in Fig. 4(b).

Using a filter as a combiner introduces a second valley in the spectra of the extinction due to resonance splitting within the filter. As light enters the filter from the input and the add-port, which propagates in opposite directions and experiences asymmetry due to different waveguide widths, it excites a counterpropagating mode within the filter giving rise to a second Bragg center wavelength slightly offset from the first. However, this wavelength splitting is only detectable at high extinction, reducing the effect on potential applications since the output would contain relatively low power.

As such, two Bragg add/drop filters are used to realize the experimental notch filter. The MZI arms are designed to be equal with an arm length of 2100 μm. A heater is placed over a 300 μm section of a single arm and the two Bragg add/drop filters. First, the response of a single MZI versus phase change due to increase of temperature in one of the arms is investigated in Fig. 5. At full extinction, a 3 dB bandwidth of 2 nm, which is narrower than the response of a single filter since a second recombination filter continues to filter the spectral response, is observed. When the heater controlling the MZI arm is inactive, there is an extinction ratio of 10 dB. As the heater power is increased to 7 mW, the extinction reaches its maximum of 15 dB. Increasing the heater power further to 53 mW completely removes the extinction, allowing full input power to be detected at the output. As expected, continuing to increase heater power returns the extinction to the spectra, confirming the well-known cosine behavior of MZI structures [see Fig. 5(b)]. Although in Fig. 4(b) the filter transmission can be tuned to be higher than the rest of the spectra, this is not observed experimentally due to unequal power between the MZI arms as well as Fabry–Perot reflections between the chip and the edge coupled fibers.

As the heater is tuned, the MZI retains low spectral crosstalk between other wavelengths. As a result, concatenating multiple MZIs operating at different wavelengths allows arbitrary control over a broad spectral bandwidth. To demonstrate this effect, four MZIs, composed of filters characterized in Fig. 3, are concatenated and experimentally measured. The spectra of these devices are summarized in Fig. 6. Each device is measured with varying power applied to the heater controlling the MZI arm, allowing full phase reversal to the tuned device. To confirm independent tuning to each MZI, each heater is activated in succession. Each device is shown to operate with an extinction ratio of at least 15 dB, while maintaining minimal crosstalk between adjacent spectral channels/devices. The crosstalk presents potential results from thermal interactions between nearby devices. Altering the MZI arm phase shifter to a high-speed p-n junction in both arms could potentially remove thermal crosstalk while offering a drastic increase in modulation speed. Although this increases absorption in each arm, optical interference performance would remain consistent and has been accomplished in prior literature [10].

It should be noted that, although each MZI can be adjusted to have a full extinction, as fabricated each passive MZI due to fabrication tolerance has an arbitrary extinction ratio. Once tuned, the output intensity of MZIs will follow cosine behavior. To confirm this behavior, the heaters above the Bragg add/drop filters are equally tuned, redshifting the center wavelength of each filter. This behavior is confirmed experimentally in Fig. 7, which shows that tuning the filters has the effect of redshifting the MZI spectral response, allowing spectral reconfiguration of the device by 13 nm. As the response is tuned in wavelength, the extinction ratio first reaches a maximum, is extinguished, and returns to a maximum once again. Although the response varies in transmitted power while being tuned in wavelength, each filter retains the ability to be simultaneously tuned in extinction ratio by activating the phase-shifting heater. At higher applied heater powers to the Bragg add/drop filters, thermal expansion of the chip introduces instability between the input waveguide and fiber, increasing insertion loss appearing in the measurement. This effect can be negated by affixing the fiber to the chip using standard packaging services.

Although the described MZI uses a heater-based phase shifter, it is straightforward to replace the heater with a high-speed p-n junction provided by standard MPW services. Consequently, such MZI could operate at speeds of 30 Gb/s or higher using thoroughly tested process design kits [13]. This improvement would allow a device that combines high-speed modulation and WDM filtering by using the add-port of the second Bragg add/drop filter, which contains the apodized passband when the extinction ratio is high, to route data encoded light. Although an extinction ratio of 15 dB and a 3 dB bandwidth of 2 nm are nonideal, it is acceptable for controlling the amplitude of individual generated wavelengths in frequency combs, as well as WDM applications. The bandwidth of the filter can be improved to at least 0.5 nm by reducing the strength of the coupling coefficient [14]. Additionally, the extinction ratio can be improved by implementing a Y-junction as the MZI power combiner by strategically choosing arm lengths of the corresponding MZI. The extinction ratio can also be improved by reducing the insertion loss to the add/drop filters, which can be accomplished by using partially etched structures and further optimizing the apodization of the filters [11].

In conclusion, we have experimentally demonstrated a notch filter capable of tuning its extinction ratio by about 15 dB and center wavelength by 13 nm. Each filter is composed of highly modular four-port Bragg add/drop filters that are concatenated to provide arbitrary spectral shaping with minimal crosstalk between adjacent devices. This novel demonstrated PIC design is an excellent candidate for programmable PICs due to its ability to concatenate multiple devices while maintaining selective tuning of individual devices.