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Abstract
This paper presents on-chip free beam optics on polymer-based photonic components. Due to the circumstance that waveguide-based optics allows no direct beam access we use Gradient index (GRIN) lenses assembled into the chip to collimate the beam from the waveguides. This enables low loss power transmission over a length of 1432 µm. Even though the beam propagates through air it is possible to create a resonator with a wavelength shift of 0.002 nm/°C, hence the allowed deviations from the ITU-T grid (100 GHz) are met for ± 20 °C. In order to guarantee reliable laser stability, it is necessary to implement optical isolators at the output of the laser. This requires the insertion of bulk material into the chip and is realized by a 1050 µm thick coated glass. Due to the large gap of the free-space section, it is possible to combine different resonators together. This demonstrates the feasibility of an integrated wavelength-meter.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The development of integrated optics has provided a spectrum of wavelength manipulating devices such as arrayed waveguide gratings (AWG), fiber Bragg gratings (FBG), ring resonators [1] and directional couplers [2]. However, all these structures share the following two disadvantages: First, they are build out of materials with non-negligible temperature dependencies, leading to thermal expansion and a change in the refractive index which both negatively impacts long term stability. Secondly, the inherent waveguide nature makes a direct access to the beam impossible. In this paper we present an approach that enables free-space optics on a polymer platform.
It has been demonstrated how to implement a Gaussian beam on a silicon-on-insulator chip by a pair of planar-convex lenses in silicon photonics [3]. The manufacturing process by electron-beam lithography is rather complex and the tolerances of the structures are quite low due to the high refractive index contrast between silicon and air, but also due to the geometry of the structure. Furthermore, it is challenging to integrate additional bulk material into the parallel beam since the silicon is too brittle. Another disadvantage is the lack of coating. The lenses are directly etched into the chip, thus applying homogeneous thin films on the surface is difficult. Using GRIN lenses in a polymer platform enables integrated temperature tolerant wavelength locking like shown in [4]. The filter-system is placed into the passive polymer part of the hybrid InP-Polymer laser. The integrated locker avoids the usage of additional space and cost consuming external components. Such a device is designed in such a way that the material constants have less influence, making its operation more stable with respect to the temperature. Furthermore, it is possible to prefabricate the GRIN lens components before inserting them into the chip, hence a homogeneous coating on the planar GRIN lens outputs is possible. The polymer is not too brittle and mechanical mounting becomes possible. In order to give additional information we will describe the assembly of such a device in this paper.
The coupling of a GRIN lens at the output facet of a chip is shown in [5]. The assembly might be challenging due to missing mechanical fixation and the alignment has to be monitored actively. The approach used in this paper uses a U-groove [2] that makes it possible to insert the GRIN lenses passively. Furthermore, expensive assembly and measurement equipment is not necessary because the etched vertical and horizontal boundaries enable accurate positioning and fix the lenses also during the curing of the glue.
A small difference of the refractive index of the polymer and the lenses makes an anti-reflective coating at the coupling point unnecessary. On the other hand, the lenses can be preprocessed with reflective coating to achieve a certain reflectivity at the facets [6]. This opens the possibility for an athermal Fabry-Pérot resonator.
2. Concept of the on-chip free space section
Figure 1 presents a view of an on-chip free-space section including a U-groove to integrate the GRIN lenses into a polymer platform. The first GRIN Lens collimates the incoming beam of the waveguide, creating a parallel beam at the GRIN lens output, forming a free beam region. The second GRIN lens focuses the light to the output-waveguide. The measured spectra are normalized by the reference waveguides next to the free beam region. In this setup we use polymer based waveguides. The cladding material has a refractive index ncl = 1.45 while the core of the waveguide has nco = 1.48. The height and width of the core are 3.2 μm. In order to achieve a better mode overlap the waveguides are laterally tapered up to 0.8 µm to the coupling point of the GRIN lenses. The GRIN lenses we used for the setup are commercially available and produced by GRINTECH GmbH. The effective refractive index has a sech-shape with a gradient constant of g = 1.8 1/mm. The refractive index at the center of the profile is n0 = 1.515. The diameter is 120 µm, just like the standard single mode fiber. The length of these lenses depends on the gradient constant, but is typically 870 µm to achieve a ¼ pitch.
Fig. 1 Scheme of the Polymer-platform with inserted GRIN lenses to achieve gap for free-space optics.
2.1 Assembly instruction
The assembly process starts with the placement of the lens into the U-groove. Since the lens has a slightly larger diameter than the width of the groove, parts of it stay above. Using a curved dispenser-needle which is mounted on a xyz-positioner, the lens was moved until contact with the waveguide occurred and secondly then pressed into the U-groove. Due to the slightly narrower groove, the lens is mechanically fixed by its vertical and horizontal limitations. In addition, the coupling point between the waveguide and the lens was filled with index matching glue and then cured.
2.2 Error analysis for lens displacement
In order to estimate the losses due to displacement at the coupling point, which is a possibility due to manufacturing tolerances, we simulated a vertical and horizontal displacement between the tapered waveguide and the GRIN lens in FIMMWAVE. Figure 2 shows that for production tolerances of ± 1 µm the losses are about 0.8 dB. The initial losses for 0 µm offsets are similar to the waveguide to single-mode-fiber coupling losses.
Fig. 2 Coupling loss for a vertical and horizontal offset between the waveguide and the GRIN lens, simulated in FIMMWAVE.
3. Measurement results
In order to show the general feasibility of the integrated free beam-optic we firstly confirm the low loss power transmission. Secondly, we create a free-space-etalon by coating the GRIN lenses and analyze its temperature dependency. Next, we demonstrate the insertion of a mm-scaled bulk material by a coated glass. Then we demonstrate the benefit of a large free-space section by a combination of a free-space-etalon and a bulk-etalon and show the feasibility for an integrated wavelength-meter.
3.1 Low loss power transmission
Figure 3 shows the measured spectra of the approach shown in Fig. 1. The GRIN lenses have an anti-reflective (AR) coating at their air interface, hence the beam gets transmitted parallel to the output lens. The transmission length is 1432 ± 1 µm and the losses at 1570 nm are less than 0.5 ± 0.1 dB. The spectra of the reference waveguides next to the tapered input/output waveguides show waveguide-length dependent oscillating losses over all wavelengths. This points to faulty manufacturing. Therefore, the tapered input and output waveguides also show wavelength dependent loss and the transmission spectra in Fig. 3 show an increase of the loss up to 1.7 ± 0.1 dB at 1530 nm.
Fig. 3 Measured spectrum of the on-chip free beam transmission between the two GRIN lenses with a propagation length of 1432 µm.
3.2 Athermal free-space-etalon
The approach of Fig. 1 was extended by reflective coating (RC) of the GRIN lenses at the air interface. Thus, a free-space-etalon with air as its medium was created like it is commonly used in external athermal wavelength lockers. The lenses have been coated before they were mounted into the polymer platform. Figure 4(a) shows the spectra for the free-space-etalon with a transmission length of 1571 ± 4 µm, a FSRλ of 0.76 ± 0.01 nm, and an extinction ratio of 5.98 ± 0.05 dB. Figure 4(b) shows the ROI with a temperature induced wavelength shift of 0.05 nm for a temperature difference of 25 °C. With a wavelength shift of 0.002 nm/°C the allowed deviation of ± 5% for the ITU-grid [7] with 100 GHz channel spacing would still be met for a temperature change of ± 20 °C.
Fig. 4 (a) Measured spectra of the on-chip free-space-etalon at 25 and 50 °C. (b) The region of interest (ROI) will make it possible to identify the temperature shift.
3.3 Bulk-etalon
The success of the integration of the GRIN lenses and its extension by a coating motivates further investigation like shown in Fig. 5(a). Here, a coated 1150 µm Quartz glass is inserted into the gap of the free-space-optic. The edge of the gap was used to insert the etalon with no tilt. Nevertheless, an accurate tilt insertion might be possible by using a design with an angled edge. The possibility of inserting other big devices into the free beam optic opens up even more ways to use the device, e.g. by inserting Faraday rotators with two polarizers to realize an optical isolator. The two coated facets create a bulk-etalon. The output of these etalon is collected by the output GRIN lens and measured. Figure 5(b) shows the measured transmission spectra of the bulk-etalon. The average transmission loss is about 6.6 dB. The extinction ratio of the filter is 2.19 ± 0.17 dB with a FSRλ of 0.72 ± 0.02 nm.
Fig. 5 (a) Inserted Grin lenses into the polymer platform with a bulk material in the free space gap. (b) Measured spectra of the on-chip free beam transmission and a coated glass, 1150 µm thick, in the optical path with a propagation length of 1350 µm.
3.4 Wavelength-meter
In order to demonstrate the benefit of such a big free-space section we combined two Fabry-Pérot interferometer in free-space section. The gap between the input lens and the bulk-etalon with resonator length of 1150 ± 5 µm was not filled with index matching glue. The input lens was coated with a RC, hence a free-space-etalon with resonator length of 50 ± 7 µm was created. The measured spectrum in Fig. 6(a) is a superposition of the two etalons. Figure 6(b) shows the band-pass-separated spectra of the ROI to explain the method of a wavelength-meter in one free-space section: point 1. is used to roughly estimate the wavelength and point 2. is used to determine the wavelength channel accurately. Under the assumption that every peak, valley, and maximum-slope stands for a certain wavelength, the lowest accuracy is ± 0.18 nm and was determined with minimal efforts. If one additionally used a look-up-table or linear interpolation the accuracy would be even higher. The range of the wavelength-meter is about 3 nm. Furthermore, the spectrum of the bulk-etalon can be used as a feedback signal for wavelength locking.
Fig. 6 (a) Measured spectra of the superposition of the bulk-etalon and the free-space-etalon. (b) Separated spectra with (1.) rough wavelength and (2.) accurate wavelength determination.
4. Conclusions
In this paper we presented on-chip free beam optics on a polymer platform using the GRIN lens technology and described the assembly process. We successfully demonstrated low loss power transmission within a range of 1432 ± 1 µm and 0.73 dB loss. Additionally, we showed the temperature stability of the free-space-etalon with its resonator length of 1571 ± 4 µm, extinction ratio of 5.98 ± 0.05 dB and a FSRλ of 0.76 ± 0.01 nm that is stable within a temperature change of ± 20 °C for the ITU-grid with 100 GHz channel spacing. This enables sensors with an open cavity, e.g. for gas or liquid analyzers, but also wavelength lockers. Furthermore, we demonstrated the feasibility of insertion of material into the parallel beam and showed power transmission through a 1150 µm thick glass with 6.6 dB losses. This technology does not only enable further incorporation of external materials into photonic components such as liquid crystals or magneto-optic elements, but also for depositing the analyte in photonic sensors. Additionally, we demonstrated the advantage of a large free-space section by the combination of a free-space-etalon and a bulk-etalon and give a motivation for further investigations of an integrated wavelength-meter.
References and links
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