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Technical Insight

Magazine Feature
This article was originally featured in the edition:
Issue 3 2022

Hybrid Photonic Integration: Transferring concepts from the communications bands into short wavelengths

News

Photonic integration at wavelengths outside the classical communications bands in the infrared has the potential to enable new applications in sensing, analytics and quantum technologies. Hybrid integration approaches, such as Fraunhofer HHI’s PolyBoard, allow for the transfer of device concepts towards shorter wavelengths while at the same time leveraging the technology of mature PIC platforms.

By Moritz Kleinert, Fraunhofer HHI

Optical communications as a large market relying on mature fiber-optic infrastructure has been the main driver of the push towards photonic integration over the last decade. This resulted in the development of various photonic integration platforms that cater for classical fiber-optics communications bands in the infrared around 1300 nm and 1550 nm. These include monolithic semiconductor-based platforms such as InP and SOI as well as hybrid insulator-based ones such as Si3N4 and polymers. These technologies, developed for optical communications, have enabled the fabrication of PICs for applications with lower maturity in sensing, analytics, optical computing, and other areas. This approach has proven successful for applications that can operate in the infrared wavelength ranges covered by the established platforms. However, it excludes a broad range of photonic systems that must operate at other, especially shorter, wavelengths due to constraints imposed by physics, but would nevertheless greatly benefit from photonic integration.


Figure 1. Optical network unit fabricated in the PolyBoard platform for the C and L band. The functional building blocks of the PIC are highlighted. (Fraunhofer HHI / EOS Edition)

InP- and SOI-based platforms cannot address this issue, because they become absorbing at shorter wavelengths. Monolithic photonic integration in other material platforms suitable for these spectral regions, such as GaAs, would be possible but would require the development of new technologies, thus forfeiting the economies of scale associated with the use of mature platforms designed for communication.

In this context, hybrid photonic integration is preferable to the two aforementioned options, as it allows for the combination of individual active photonic components (laser diodes, photodetectors, and modulators) from various material systems with integrated passive waveguides with broad spectral transparency to form complex chips.

Figure 2. Schematic of hybrid tunable laser and optical spectra obtained from lasers around 1565 nm, 1064 nm and 785 nm.

In this way, the development of PICs for short wavelengths only requires the availability of suitable active components and the adaptation of the PIC building block designs to the operating wavelength of the respective application, but not the development of a new technology line. The PolyBoard platform developed by Fraunhofer HHI uses such a hybrid integration approach. It relies on polymer channel waveguides fabricated with standard wafer-scale clean room processes and comprises various functional building blocks developed for the infrared fiber-optical communications bands. Figure 1 shows an optical network unit (ONU) for passive optical networks in FTTx applications [1]. This ONU is a bidirectional device, receiving downstream data in the L band and transmitting upstream data in a selectable DWDM channel in the C band. The single optical fiber carrying the up- and downstream data is passively coupled to the PolyBoard PIC via an etched U groove structure [2]. The upstream in the C band is generated by a hybrid tunable DBR laser [2]. Bottom-illuminated InP photodiodes detect the downstream signal in the L band. They are placed on top of micro-machined, gold-coated 45° mirrors that deflect light from the waveguide plane into the detectors. Upstream and downstream are separated from the same integrated waveguide by a dichroic mirror, realized as a thin-film filter (TFF).

Figure 3. Thin-film filters in the PolyBoard platform and optical spectra of a C/L band splitter and a 785 nm pump light filter.

While the above-mentioned functional building blocks were developed for the C band around 1550 nm, the hybrid integration approach allows for the transfer of these functionalities to other wavelengths. This is facilitated by the broad spectral transparency of the passive PolyBoard waveguides from the infrared to the visible. Hence, it is possible to design transparent single-mode waveguides for the entire spectral band. Transferring signal detection to smaller wavelengths is straightforward because PD integration via mirrors allows for the use of planar PDs manufactured in material systems such as GaAs and Si. The approach to adapt the other functional building blocks is described in the following sections.

Tunable lasers as light sources
The hybrid integration concept of the tunable laser in the PolyBoard platform is presented in the top left of Figure 2. Photons are generated in a III/V semiconductor gain chip (GC) that has a highly reflective (HR) coating against air on one facet and an anti-reflective (AR) coating against the polymer waveguide on the other facet. The waveguide at the facet of the gain chip is angled to further reduce unwanted reflections and butt-joined coupled to the PolyBoard chip. The second facet of the laser cavity is formed by a Bragg grating in the integrated waveguide. Localized heating of the Bragg waveguide with the heating electrode allows for the tuning of the wavelength reflected by the grating and thus the wavelength of the laser emission. An additional thermo-optic phase shifter enables fine-tuning of the phase of the longitudinal laser mode.

The standard implementation of the hybrid tunable laser structure uses an InP gain chip, which provides optical gain in the C band, and a Bragg grating designed to reflect a wavelength in this range. The tuning of the emission spectrum of such a laser is shown in the top left of Figure 2. Various variants of this building block, e.g. for direct modulation and for a narrow optical linewidth, are available. The ONU of Figure 1 uses a tunable laser that allows direct modulation of the upstream signal at 10 Gbit/s. The transfer of this building block to shorter wavelengths involves two aspects. Firstly, the gain chip has to be fabricated in a material system that provides gain in the desired spectral bands. Secondly, the parameters of the Bragg grating must be re-designed to match the output wavelength. However, the basic layout, fabrication techniques, assembly processes, and control electronics do not need to be changed. The measurement results obtained from hybrid tunable lasers at 1064 nm and 785 nm with GaAs GCs shown in the lower row of Figure 2 prove the feasibility of this approach [4].

Thin-film filters for wavelength and polarization handling
In addition to the usual waveguide-integrated filters such as Bragg gratings or arrayed waveguide gratings, the PolyBoard platform offers thin-film filters as a means of spectral filtering and polarization handling. These TFFs consist of a µm-thin polymer carrier film and a dielectric layer stack that realizes the intended filter functionality. Hence, filtering properties of coated free-space optics can be transferred directly to integrated optics for all relevant optical wavelengths. The TFFs are fabricated on wafer-scale and are therefore compatible with the scaling of PIC fabrication. The top row of Figure 3 shows TFFs after removal from the wafer and after insertion into the slots etched perpendicularly into the waveguide layer of the PolyBoard chip.

The measured spectral characteristics of the TFF used in the ONU are shown in the lower left of Figure 3. Here, the dielectric layer stack is designed to reflect the C band emission from the tunable laser and transmit the L band downstream from the network. Since the etched slots on the PolyBoard chip are the same for all spectral bands, transferring this functionality to shorter wavelengths requires only the design of an appropriate dielectric filter stack. An example is shown in the lower right of Figure 3. This filter was designed specifically for quantum technology applications that use non-linear crystals pumped at 785 nm to create a photon pair in the C band. After the non-linear process, the pump light must be filtered out of the integrated waveguide to avoid contaminating the generated stream of photon pairs. Using the TFF approach, the high pump suppression achieved with dielectric coatings in free-space optics can be applied to integrated optics. In this case, a pump suppression of 68 dB inside the PIC waveguide was confirmed, meaning that only one in 6.3 million pump photons unintentionally passes through the filter [4]. In addition to the two examples shown here, there are many other TFFs for spectral and polarization filtering from the visible to the infrared available.

U Grooves for fiber coupling and micro-optical benches
Reliable optical coupling between single-mode fibers and integrated chips is one of the greatest challenges in manufacturing PIC-based optical assemblies and modules. In contrast to the usually required active alignment, in the PolyBoard platform U grooves allow for a passive single-mode coupling with sub-µm precision. A cross section of a U groove is presented in the top left of Figure 4. The fiber is held in place by a U-shaped trench etched into the polymer layer of the PIC. The width of the trench corresponds to the 125-µm diameter of standard optical fibers. As the PolyBoard waveguides and the U groove are defined in the same lithographic step, the horizontal alignment between the fiber core and the PIC waveguide is nearly perfect. Vertical alignment is controlled by a precise wafer-scale dry etching process so that the depth of the etched trench is half the fiber diameter. This approach is suitable for all wavelengths at which single-mode optical fibers with a standard diameter of 125 µm are available.

Figure 4. Cross section of a U groove and PolyBoard chip for non-linear frequency conversion with corresponding spectrum.

A chip using U grooves for coupling a SMF-28 fiber for the C band and a 780HP for 780 nm is presented in the top right of Figure 4. Despite the two U grooves for fiber coupling, this chip features two additional U grooves for the insertion 125-µm diameter graded-index (GRIN) lenses. These lenses are designed to create a collimated or focused beam within the etched free-space section on the chip. This approach allows for the combination of bulk crystals, e.g. for non-linear or non-reciprocal optical crystals, with PICs. Here, a periodically poled lithium niobate (ppLN) crystal is inserted into the PIC. When pumped with light around 1550 nm, the non-linear optical properties of the crystal enables the generation of second harmonic (SHG) light around 775 nm. Despite the second harmonic with an optical power of up to 8.2 dBm, higher harmonics at 517 nm and 388 nm are also coupled into the fiber [5]. The broad transparency of the passive polymer waveguides in this hybrid integration approach combined with the on-chip micro-optical bench therefore enables PICs for efficient on-chip optical frequency conversion with applications in sensing and quantum technologies.

Conclusion and outlook
The flexibility of hybrid photonic integration in selecting optimal active components and independently optimizing on-chip functional building blocks shows great potential to meet the market needs for PICs at wavelengths shorter than standard communications bands. Although essential PIC building blocks have already been transferred and successfully demonstrated, challenges remain on the path towards complete PIC platforms for short wavelengths. One of these is the still limited availability of active components, especially lasers, for some spectral regions in the visible range.

To some degree, this can be circumvented by using on-chip non-linear processes for frequency doubling of infrared light, as shown in Figure 4. Another challenge is long-term stable optical coupling at short wavelengths, such as green or blue. Due to the high optical power densities at the facets of the single-mode waveguides and the high photon energies, the design of these interfaces is significantly more demanding than for infrared wavelengths. By further addressing these challenges and building upon the extensive experience gained in the design, fabrication, assembly, and control of PICs for communications bands, hybrid integration is one of most promising photonic technologies for short wavelengths.


References

[1] Zhang, Ziyang, et al. “C/L-band colorless ONU based on polymer bidirectional optical subassembly.” Journal of Lightwave Technology 33.6 (2015): 1230-1234.

[2] Kleinert, Moritz, et al. “Recent progress in InP/polymer-based devices for telecom and data center applications.” Integrated Optics: Devices, Materials, and Technologies XIX 9365 (2015): 92-105.

[3] de Felipe, David, et al. “Polymer-based external cavity lasers: Tuning efficiency, reliability, and polarization diversity.” IEEE Photonics Technology Letters 26.14 (2014):1391-1394.

[4] Kleinert, Moritz, et al. “Hybrid Polymer Integration for Communications, Sensing and Quantum Technologies from the Visible to the Infrared.” 2021 European Conference on Optical Communication (ECOC). IEEE, 2021.

[5] Conradi, Hauke, et al. “Second Harmonic Generation in Polymer Photonic Integrated Circuits.” Journal of Lightwave Technology 39.7 (2020): 2123-2129.

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