A temperature-tunable etalon for optical telecommunication wavelength - Revision history

Yicheng at 23:12, 30 April 2021

2021-04-30T23:12:54Z

← Older revision		Revision as of 07:12, 1 May 2021
Line 5:		Line 5:
	A Fabry-Perot interferometer (or an Etalon), being probably the simplest form of all interferometers, is found useful in various applications such as spectral filtering or frequency referencing.		A Fabry-Perot interferometer (or an Etalon), being probably the simplest form of all interferometers, is found useful in various applications such as spectral filtering or frequency referencing.

	An etalon is typically constructed out of the two parallel reflecting surfaces of a transparent plate. The plate needs to have low absorption loss for the desired working wavelengths to ensure a relatively high finesse of the etalon. The material choice for visible wavelengths is usually fused silica ~~with a relatively~~ low absorption ~~coefficient of [bla]~~ and a thermal expansion ~~coefficient of [bla]~~.		An etalon is typically constructed out of the two parallel reflecting surfaces of a transparent plate. The plate needs to have low absorption loss for the desired working wavelengths to ensure a relatively high finesse of the etalon. The material choice for visible wavelengths is usually fused silica given its low absorption and thermal expansion.

	For optical telecommunication wavelengths, which range from about 1260nm to 1625nm, pure silicon becomes a more practical choice. Polished silicon wafers can be easily purchased at different thickness (100μm to a few mm), which allows for various free spectral ranges. The absorption coefficient of silicon is about <math>6 \times 10^{-5}</math>/cm at 1310nm. Given this low absorption loss at the telecom wavelength, it is possible in principle to build etalons with high finesse by applying Highly Reflective coatings to the two surfaces of a silicon wafer.		For optical telecommunication wavelengths, which range from about 1260nm to 1625nm, pure silicon becomes a more practical choice. Polished silicon wafers can be easily purchased at different thickness (100μm to a few mm), which allows for various free spectral ranges. The absorption coefficient of silicon is about <math>6 \times 10^{-5}</math>/cm at 1310nm. Given this low absorption loss at the telecom wavelength, it is possible in principle to build etalons with high finesse by applying Highly Reflective coatings to the two surfaces of a silicon wafer.

Yicheng at 15:07, 30 April 2021

2021-04-30T15:07:06Z

← Older revision		Revision as of 23:07, 30 April 2021
Line 14:		Line 14:
	A very quick intro to Fabry-Perot interferometer is here: [[How does an etalon work?]]. Alternatively one can turn to literally any text books on interferometry.		A very quick intro to Fabry-Perot interferometer is here: [[How does an etalon work?]]. Alternatively one can turn to literally any text books on interferometry.

	==A ~~bare silicon wafer~~ as an ~~etalon~~==		==A Bare Silicon Wafer as an Etalon==
	Even without any coatings on the surfaces, a silicon-air interface has about 30% reflectance, hence one should be able to observe the etalon effect of the wafer. We hence proceed to test the transmission spectrum as well as the temperature tunability of such a bare piece of silicon wafer.		Even without any coatings on the surfaces, a silicon-air interface has about 30% reflectance, hence one should be able to observe the etalon effect of the wafer. We hence proceed to test the transmission spectrum as well as the temperature tunability of such a bare piece of silicon wafer.
	===Setup===		===Setup===
Line 25:		Line 25:
	To measure the transmission spectrum of the silicon etalon, we prepared a Super-luminescence LED to provide a wide-band light of 1320<math>\pm</math>50nm. The optical beam propagates through the etalon and the transmitted light spectrum is measured with a wave-meter (2GHz spectral resolution).<ref>https://www.bristol-inst.com/bristol-instruments-products/spectrum-analyzers/771-series-laser-spectrum-analyzer/</ref>		To measure the transmission spectrum of the silicon etalon, we prepared a Super-luminescence LED to provide a wide-band light of 1320<math>\pm</math>50nm. The optical beam propagates through the etalon and the transmitted light spectrum is measured with a wave-meter (2GHz spectral resolution).<ref>https://www.bristol-inst.com/bristol-instruments-products/spectrum-analyzers/771-series-laser-spectrum-analyzer/</ref>

	===Bare Silicon Wafer: Transmission ~~spectrum~~===		===Bare Silicon Wafer: Transmission Spectrum===
	The measured transmitted spectrum together with a zoom-in view are shown below.		The measured transmitted spectrum together with a zoom-in view are shown below.

Line 32:		Line 32:
	The observed free spectral range is about 2.3nm as opposed to the estimated value of 2.45nm for a wafer with 100μm thickness. The cause of this 0.1nm difference is yet unclear, but could be attributed to the thickness variation of the wafer itself, or the possibility that the wafer is tilted against the incidence beam. As anticipated, the reflectance of the silicon-air interface is only about 30%, which causes a low visibility (~0.44) of the transmitted spectrum. The finesse of this etalon is estimated to be about 2.6, which is considered low for typical etalon performances.		The observed free spectral range is about 2.3nm as opposed to the estimated value of 2.45nm for a wafer with 100μm thickness. The cause of this 0.1nm difference is yet unclear, but could be attributed to the thickness variation of the wafer itself, or the possibility that the wafer is tilted against the incidence beam. As anticipated, the reflectance of the silicon-air interface is only about 30%, which causes a low visibility (~0.44) of the transmitted spectrum. The finesse of this etalon is estimated to be about 2.6, which is considered low for typical etalon performances.

	===Bare ~~silicon wafer~~: ~~temperature tuning~~===		===Bare Silicon Wafer: Temperature Tuning===

	The temperature of the setup is adjusted and stabilized with a TEC controller. A few different temperatures were tried. The two graphs below shows the position of different transmission peaks around 1320nm at 4 different temperatures.		The temperature of the setup is adjusted and stabilized with a TEC controller. A few different temperatures were tried. The two graphs below shows the position of different transmission peaks around 1320nm at 4 different temperatures.
Line 58:		Line 58:
	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[here]]'''.		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[here]]'''.

	==Stage ~~summary~~ for the ~~project~~==		==Stage Summary for the Project==

	In summary, we show that a thin piece of polished silicon wafer can behave as an etalon with low finesse at telecom wavelength. One can quite easily tune the transmission wavelengths of this etalon by simply varying the temperature. This wavelength tuning has a temperature dependence of about 0.1nm/K, and this effect is mainly due to the change in refractive index of the silicon material itself.		In summary, we show that a thin piece of polished silicon wafer can behave as an etalon with low finesse at telecom wavelength. One can quite easily tune the transmission wavelengths of this etalon by simply varying the temperature. This wavelength tuning has a temperature dependence of about 0.1nm/K, and this effect is mainly due to the change in refractive index of the silicon material itself.

	In principle, one can increase the finesse of the silicon etalon by applying HR coatings to the surfaces. This temperature tunable silicon etalon can be used as a variable spectral filter or frequency reference for telecom wavelength.		In principle, one can increase the finesse of the silicon etalon by applying HR coatings to the surfaces. This temperature tunable silicon etalon can be used as a variable spectral filter or frequency reference for telecom wavelength.

Yicheng: /* Stage summary for the project */

2021-04-30T15:05:21Z

Stage summary for the project

← Older revision		Revision as of 23:05, 30 April 2021
Line 59:		Line 59:

	==Stage summary for the project==		==Stage summary for the project==

			In summary, we show that a thin piece of polished silicon wafer can behave as an etalon with low finesse at telecom wavelength. One can quite easily tune the transmission wavelengths of this etalon by simply varying the temperature. This wavelength tuning has a temperature dependence of about 0.1nm/K, and this effect is mainly due to the change in refractive index of the silicon material itself.

			In principle, one can increase the finesse of the silicon etalon by applying HR coatings to the surfaces. This temperature tunable silicon etalon can be used as a variable spectral filter or frequency reference for telecom wavelength.

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:28:49Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:28, 30 April 2021
Line 56:		Line 56:
	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[~~Deposition rate of silicon\|~~here]]'''.		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[here]]'''.

	==Stage summary for the project==		==Stage summary for the project==

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:28:27Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:28, 30 April 2021
Line 56:		Line 56:
	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[~~Wikipedia:~~Deposition rate of silicon\|here]]'''.		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[Deposition rate of silicon\|here]]'''.

	==Stage summary for the project==		==Stage summary for the project==

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:27:58Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:27, 30 April 2021
Line 56:		Line 56:
	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[here]]'''.		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[Wikipedia:Deposition rate of silicon\|here]]'''.

	==Stage summary for the project==		==Stage summary for the project==

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:25:56Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:25, 30 April 2021
Line 56:		Line 56:
	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented [['''here''']].		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented '''[[here]]'''.

	==Stage summary for the project==		==Stage summary for the project==

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:25:25Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:25, 30 April 2021
Line 56:		Line 56:
	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented [[here]].		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented [['''here''']].


	==Stage summary for the project==		==Stage summary for the project==

Yicheng at 14:24, 30 April 2021

2021-04-30T14:24:05Z

← Older revision		Revision as of 22:24, 30 April 2021
Line 54:		Line 54:
	[[File:png_Coating.png\|500px]] [[File:simulation_result.png\|500px]]		[[File:png_Coating.png\|500px]] [[File:simulation_result.png\|500px]]

	One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve similar reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	~~==Silicon Coating Speed Calibrating==~~		Unfortunately this small project stops at this stage where creating HR coatings on silicon surface becomes a problem. We have in fact, gotten access to a RF sputtering machine is capable of creating such coatings. However, this particular machine needs certain amount of calibration work to ensure an accurate deposition rate such that one can create coated layers with the correct thickness. Some efforts have been made towards this direction and is briefly documented [[here]].
	~~Sputtering speed depends not only~~ on ~~the input RF power but also related to the distance between the target and the sample. and the~~ surface ~~area of the sample~~. We have to ~~calibrate the~~ sputtering ~~speed to make sure the thickness~~ of ~~the film we coated is correctly equal to a quarter lambda or a half lambda~~. ~~We sputtered silicon film on the optical window~~ to ~~measure~~ the thickness ~~of different sputtering time~~. In this ~~calibration experiment, we are using 100w power for RF signal,~~ and ~~the distance between the target and the sample is about 8cm, while the diameter of the glass (Optical Window)~~ is ~~25.4mm. We tried for 6000s, 8000s, 10000s, 12000s, 20000s~~. ~~</p><p>~~

	~~[[File:Silicon-thickness.png\|800px]] </p><p>~~

	~~The estimated speed is drawn in~~ the ~~graph with dashed lines.</p><p>~~		==Stage summary for the project==
	~~[[File:Substrate-BK7.png\|720px]]~~

Yicheng: /* Highly Reflective (HR) Coating on Silicon Surface */

2021-04-30T14:12:17Z

Highly Reflective (HR) Coating on Silicon Surface

← Older revision		Revision as of 22:12, 30 April 2021
Line 50:		Line 50:

	For HR coating materials on silicon at telecom wavelength, typical choices are Silicon Dioxide (SiO2), Silicon Nitride (Si3N4) or even just use silicon itself.		For HR coating materials on silicon at telecom wavelength, typical choices are Silicon Dioxide (SiO2), Silicon Nitride (Si3N4) or even just use silicon itself.
	An example of a six-layer coating is shown below using Si3N4 and SiO2, with layer thickness of 163nm and 226nm respectively.		An example of a six-layer coating is shown below using Si3N4 and SiO2, with layer thickness of 163nm and 226nm respectively. This can in theory achieve about 90% reflectance centered at 1310nm over a bandwidth of about 200nm.

	[[File:png_Coating.png\|500px]] [[File:simulation_result.png\|500px]]		[[File:png_Coating.png\|500px]] [[File:simulation_result.png\|500px]]

	~~When the thickness~~ of ~~all the layers is correct, the~~ reflection ~~spectrum should have a peak at a wavelength around 1310nm~~. The ~~reflection peak should reach 90%~~. ~~If so~~, ~~our etalon would have high finesse and can act as a very useful filter~~ for ~~high bandwidth lasers~~.		One can also use layers of Si3N4 (163nm) and Si (93nm) to achieve reflection behavior. The usage of silicon is slightly more preferable as its much higher refractive index (n=3.5) corresponds to much thinner coating layers, thus making it easier for manufacturing.

	==Silicon Coating Speed Calibrating==		==Silicon Coating Speed Calibrating==
	Sputtering speed depends not only on the input RF power but also related to the distance between the target and the sample. and the surface area of the sample. We have to calibrate the sputtering speed to make sure the thickness of the film we coated is correctly equal to a quarter lambda or a half lambda. We sputtered silicon film on the optical window to measure the thickness of different sputtering time. In this calibration experiment, we are using 100w power for RF signal, and the distance between the target and the sample is about 8cm, while the diameter of the glass (Optical Window) is 25.4mm. We tried for 6000s, 8000s, 10000s, 12000s, 20000s. </p><p>		Sputtering speed depends not only on the input RF power but also related to the distance between the target and the sample. and the surface area of the sample. We have to calibrate the sputtering speed to make sure the thickness of the film we coated is correctly equal to a quarter lambda or a half lambda. We sputtered silicon film on the optical window to measure the thickness of different sputtering time. In this calibration experiment, we are using 100w power for RF signal, and the distance between the target and the sample is about 8cm, while the diameter of the glass (Optical Window) is 25.4mm. We tried for 6000s, 8000s, 10000s, 12000s, 20000s. </p><p>