Performance of the Filter

Figure 5.19Photograph of the final filter used to tune an External Cavity QCL.

The filter presented in this section with the design described in the last section has lead to publication [193]. The resulting filter, pictured in Fig. 5.19 appears mirror-like with no visible cracks. There is no change in appearance and performance after 18 months exposure to ambient humidity. It fulfills the requirements for adhesion and abrasion as stated in MIL- C-48497. A cross-sectional SEM micrograph can be seen in Fig. 5.20. As is clear from the image, the layers are very homogenous and the interfaces are smooth and abrupt. The filters were deposited onto substrates that are 10 mm by 20 mm in size.

Fig. 5.21shows the transmission spectrum as measured with an FTIR (top) and the trans- mission spectrum as calculated using the design parameters (bottom) at normal incidence which are both in very good agreement with each other. The central peak has a FWHM of 6 cm−1_{, as can be seen from the inset, which equals approximately 0.3% of the central wave-}

length of approximately 2280 cm−1_{. The peak hight in this measurement is approximately}

0.4. Both the width and the hight of the central peak in this measurement are not correct. This is due to two reasons. One is that the FTIR’s probe beam is not collimated, thus the

Figure 5.20 Cross-sectional SEM micrograph of filter structure (substrate towards the top of the

image).

peak is “smeared out” as is the result of the many incident angles in conjunction with the fact that the peak position tunes with angle. This makes the peak both smaller and wider. The second is due to the fact that the backside of the filter is not AR coated in conjunction with the fact that the FTIR’s globar source is not coherent. Thus, the peak is expected to be lowered by the amount that is lost to the Fresnel reflection at the backside, which for the high refractive index of silicon is expected to be approximately 28%. All of these effects are avoided in the following by performing subsequent measurements using our S-ECQCL Spec- trometer presented in Chapter 4 as a tunable coherent MIR light source with a collimated probe beam. This leads to other effects that are also discussed shortly.

Fig. 5.22 shows a zoomed-in plot of the central transmission peak of the filter obtained using our Littrow ECQCL Spectrometer instead of an FTIR (black dots). Also plotted in Fig. 5.22 are the calculated transmission of the design (dashed red line) with the targeted layer thicknesses. The solid blue line is the calculated transmission using the actual layer thicknesses. The fitted values of the layer thicknesses are dH = 250.0 nm and dL= 761.3 nm

and differ from the targeted values (dH = 252.5 nm, and dL = 763.8 nm) by less than

3 nm. Assuming the transparency of the low-index material is the limiting factor of the peak tranmission, the extinction coefficient was calculated to be approximately k ≈ 4.5 × 10−4_.

The transmission peak of the filter at six different incident angles can be seen in Fig. 5.23 also obtained using our tunable Littrow ECQCL. At normal incidence the transmission is located at 2276 cm−1 _{or 4.40 µm, has an amplitude of 55% and a FWHM of 3.2 cm}−1 _or

6 nm, which is 0.14% of the central wavelength.

The ripples superimposed on the peaks seen in Figs. 5.22 and 5.23 are the Fabry-Perot modes of the substrate due to the backside of the filter not being AR-coated. These tune at a different rate with varying angle than the main peak. This effect deteriorates the tuning behavior and is simply avoided by coating the backside. It can on the other hand be increased by coating the backside with a highly-reflective coating, producing a filter with a 1-2 orders of magnitude smaller bandwidth with comparable peak transmission. This is because the substrate then acts as a second high-finesse cavity with a very small free spectral range and consequently much smaller peak width. Since the peaks can always be tuned with angle so

!"#$%&"' $(!%)#*"'+,'"$(-.+ */(01."$$"$2 $(!%)#*"'+,#0*%#)+ */(01."$$"$2 3 &#.$!($$(4. 5 567 568 569 56: 56; 56< =#>".%!?"&+@0!A7B 88<5 88<; 88C5 88C; 88D5 88D; 88E5

Figure 5.22 Transmission characteristic of the filter. Dashed red line: Calculated transmission using

the discussed design and design layer thicknesses. Black dots: Measured transmission, using our tunable Littrow ECQCL. Solid blue line: Calculated transmission using same design and actual layer thicknesses that differ from the targeted ones by less than 3 nm. The ripples superimposed on the peak are the Fabry-Perot modes of the substrate due to the backside of the filter not being AR-coated. This is explained later in this section.

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that one coincides with the central wavelength of the main peak (red and black lines in Fig. 5.23), this produces a high-transmission central peak and small satellite peaks in the tails of the transmission range of the main peak, which can be ignored in most cases. This effect can be used to produce miniature ECQCLs with fixed wavelengths or small tuning ranges with highly stable and predictable wavelengths comparable to DFB (Distributed Feedback) QCLs, but without the need to actively stabilize the laser’s temperature. Hitting a particular wavelength when developing a new production process is also much easier than with DFB due to the residual tunability.

The coupled-cavity effect only becomes visible when the beam has a coherence length longer than the effective optical path through filter and substrate. The intensity of the probe beam as a function of frequency is also plotted in Fig.5.23for reference. Clearly visible are the CO2

lines of the ambient air and the decreasing intensity towards higher frequencies limiting the useful spectral range at about 2350 cm−1_{. Consequently, there is considerable noise imposed}

on the transmission peak at 25◦_{, unfortunately making it impossible to use this particular}

laser to take spectra at higher angles. But it is to be noted, that the peak amplitude and width do not vary appreciably up until 20◦_{. The smaller maximum transmission at 10}◦_{, 15}◦_,

and 20◦ _{are not due to lower transmission of the Fabry-Perot cavity at more oblique angles,}

but due to more unfavorable coincidence of the filter and substrate cavities. If we arbitrarily set 50◦ _{as a conservative estimate of the upper limit of the incident angle at which the filter}

works without performance degradation, the tuning range is approximately 300 cm−1 _{or 13%}

of the central wavelength.

The thickness variations of the deposited layers across one filter is negligible, i.e. in the sub-nanometer range. This can be seen from the following argument. The theoretical and experimental performance in Fig. 5.22 agree well upon illumination of the entire filter area with the probe beam. But an absolute layer thickness shift as small as 2.5 nm shifts the transmission peak by 12 cm−1 _{as is also clear from Fig. 5.22 (difference between red and}

blue lines). Thus if a variation of similar magnitude was present on a single filter, the effect on the transmission would be catastrophic. Since adjacent filters are only 10 mm apart on the substrate holder in the deposition system, the thickness variation is also marginal across different filters of the same coating run.

The peak position as a function of angle (relative to normal) is plotted in Fig. 5.28. The line is a fit of Eq. (5.20) to the points resulting in an effective index of refraction neff = 1.66,

Peak Position Fit with n*=1.657

ν(θ)=

Tuning Angle θ[deg]

0 5 10 15 20

Figure 5.24 Peak position as a function of angle (relative to normal).

5.5 Alignment-stabilized filter-tuned External-Cavity QCL