Application Note: Characterizing The Optical Properties Of Planar Waveguides, Optical Chips And Planar Light Circuits
Overview
Planar optical waveguide technologies are the key elements in the modern, high speed optical network. Recent, broad deployment of optical and hybrid optoelectronic chips and planar light circuits (PLCs) has been driven by the cost, size and operational benefits that these architectures offer.
From a measurement perspective, planar optical waveguide architectures offer several unique challenges. Even the highest quality Photonic chips are far more lossy per unit length than optical fiber due to higher absorption and scattering coefficients. Planar systems are also often much more polarization dependent, and planar systems are often multi-path and have strong wavelength dependencies (e.g. modulators, arrayed waveguide gratings). Nonetheless, rapid, accurate and thorough characterization of planar waveguides is key to functionality and operability.
Optical vector analysis offers several distinct measurement advantages that make it particularly well suited for characterization and analysis of planar waveguides and circuits.
High Speed Parametric Measurement
For most applications, optical waveguides must be characterized in terms of standard parameters required as input into network designs and maps. Those parameters include loss (both insertion, IL, and return, RL), group delay and/or chromatic dispersion (GD, CD), and polarization dependencies like polarization dependent loss (PDL) and differential group delay or polarization mode dispersion (DGD, PMD). Note: For discrete components, like planar waveguide chips, DGD=PMD. For most modern networking applications support multiple operational wavelengths and these quantities must therefore be measured by both designer and manufacturer over multiple input wavelengths. This is commonly achieved by employing a tunable laser source (TLS) performing multiple, time consuming measurements.
OVA provides the industries only single-scan, single connection, high-speed turn-key measurement solution that provides nearly instant access to all parametric quantities of interest such as IL, RL, GD, CD, PMD, PDL, etc. Figure 1 below shows the IL, PDL, GD and PMD of an interleaving device fabricated in a PLC.
Figure 1. Insertion loss (IL) and polarization mode dispersion (PMD) of three channels of an arrayed waveguide grating in a planar optical chip. This data was taken with 2.5 pm wavelength steps. Using OVA, full band characterization is completed in about three seconds.
High speed measurement is essential to PLC development and manufacturing as PLCs often require alignment to optical fiber. This step can be greatly simplified using the high speed nature of OVA as both alignment and final test can be performed simultaneously at a single test station.
Time Domain Mapping and Measurement of Distributed Loss
The complex (in the reiq sense) measurement of the OVA allows the user to not only perform high speed spectral analysis, but also, by use of the Fourier transform, allows the user to image an optical waveguide in the time-domain.
By virtue of its operating principle—measurements of frequency-domain amplitude and phase—OVA allows the user to interrogate optical waveguides in both the frequency- and time-domains. That is, the frequency domain data for a given waveguide can be Fourier transformed into a high-resolution time domain picture of that component's spatial response.
Figure 2 below shows the time domain response of a planar waveguide chip with facet reflections, waveguide scatter and discrete elements.
Figure 2. Time domain response in reflection of a silica-on-silicon waveguide measured using an OVA and showing multiple discrete reflective events and areas of distributed waveguide scatter and loss.
Figure 3 below displays the time domain impulse response of a highly birefringent PLC waveguide in transmission. The multiple impulses are due to light propagating back and forth between the waveguide facets (the Fabry-Perot effect).
Figure 3. (a) Graphical depiction of light propagating in a planar waveguide showing the primary and secondary (1st echo) transmission, and (b) time domain impulse response of a highly birefringent optical waveguide in transmission. The TE and TM response are clearly visible in the splitting of the transmission peaks as are the multiple round trip "echoes" cause by light propagating multiple times in the waveguide area.
Polarization Effects
Due to the geometry of planar devices, they typical suffer from stronger polarization dependence than the cylindrical geometry encountered in many other fiber systems. As polarization effects such as PMD can be the limiting factor in high speed optical data transmission.
OVA is unique in that it is capable of characterizing polarization effects such as PDL and PMD by scanning a tunable laser only once. This is achieved by conditioning the optical signal in the fiber before it propagates through the waveguide under test. The single scan nature of the technique leads to greatly enhanced measurement speeds and higher levels of accuracy and repeatability.
A result of this method of device characterization is that it relieves precise alignment requirements often associated with testing planar waveguides.
Figure 4 below highlights several measurement results showing various polarization inputs.
Figure 4. A screen capture showing TE (Red) and TM (green) transmission through a polarizing waveguide. The blue curve represents the wavelength dependant transmission of a mixture of the TE and TM states caused by errors in the launch condition. Using optical vector analysis, this level of polarization analysis can be extracted instantly from measured data requiring no prior alignment.
Summary
Planar optical waveguide technologies are the key elements in the modern, high speed optical network but present unique challenges from a measurement perspective. Even the highest quality Photonic chips are far more lossy per unit length than optical fiber due to higher absorption and scattering coefficients; they are often substantially more polarization dependent, and frequently have multi-path and strong wavelength dependencies (e.g. modulators, arrayed waveguide gratings). Optical vector analysis offers several distinct measurement advantages that make it particularly well suited for characterization and analysis of planar waveguides and circuits.
SOURCE: Luna Technologies, a Division of Luna Innovations Incorporated