It is well-known that distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers include gratings along their waveguides to provide wavelength selective feedback. Usually, two-step process is used, where gratings are first selectively etched into the waveguide core, this is followed by an epitaxial regrowth to bury the gratings such that the refractive index contrast in the grating is an effective refractive index difference between the etched and nonetched portion of the waveguide core. I am curious to know how to measure the reflectivity spectrum of such gratings experimentally. The spectrum should look like a reflectivity spectrum (stopband with interference fringes) of DBRs in VCSELs, which is very easy to experimentally measure. How to perform the similar measurement for the case of gratings in DFB or DBR lasers? Please note that scientific literature often reports the theoretical reflectivity data, not the experimental one.
Experimentally, it is very difficult to measure the reflection spectra of gratings and therefore the transmission spectrum is measured. This is sufficient since it's the compliment of the reflection spectrum.
If the grating is in passive material like silicon, then it's rather straightforward to build cutbacks and couple light to/from the chip with lensed fibers. With gratings in absorbing material (as in the case for lasers), measuring the transmission is further complicated by the optical loss. However, this can be calculated/measured pretty easily as well. So you'd need a few cutbacks to quantify 1) coupling loss 2) absorption/propagation loss and then you'll finally be able to make a comment about the spectra of your gratings.
For reflective dielectric layers as used in VCSELs, you could also use an ellipsometer (with a calibrated substrate) to measure the reflection
I agree that it is very difficult to measure reflection spectra of Bragg grating integrated on the waveguide laser. It is very useful to prepare Bragg grating test sample and then the transmission spectrum on each Bragg grating design can be measured.
Thank you both. Do you think the test sample should look like the the one shown in the attached file? If this is correct, why would the measurement of the reflection spectrum be challenging in this configuration? Can't we use 3-port optical circulators to measure the reflected beam? Another concern in this configuration is that the input and output facets will provide some undesired reflection, leading to a resonance. How can we then extract the transmission spectrum accutrately out of it? Any ideas?
Previously, I have used test sample design as shown in the attached file. You can eliminate/minimise the Fabry-Perot resonance measured on your Bragg grating test sample by subtracting the measured transmission spectrum with the transmission spectrum of the reference waveguide. Please be aware that the quality of your waveguide end facet and light coupling alignment may slightly affect the accuracy of your measurement.
I used the free-space coupling setup to measure the transmission spectrum of Bragg grating response. For my case, it is very challenging to collect the reflected light from small waveguide end facet using free-space setup.
I do not have experience with the 3-port optical circulators, maybe it can improve the collection of the reflected spectrum of Bragg grating sample.
Thanks. Just wonder whether you published any results relating to this.work Or you recommend any literature to understand better?
I am preparing a manuscript relating to this work.
Please refer attached files regarding the Bragg grating waveguide for your reference.
Hopefully, it helps!
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