Design of a low noise millimeter wave interferometer based on single sideband modulation Zhong Heng Tsinghua Univ. Gao Zhe, Professor Ling Bili, Professor

Actuality Motivation Principle System design Content Summary

Existing Density Diagnostics Michelson Type Extremely compact design and low cost Temporal resolution Phase resolution 0.5μs 0.02π focusing lens group Oblique glass window Zhong et al, Rev. Sci. Instrum. 87, 11E109 (2016) 1

Motivation Higher SNR and phase stability Higher temporal resolution Real time density signal Heterodyne Techniques Narrowly;besides;moreover 2

Principle 3 Heterodyne interferometer Density determination LO RF The frequency of the two arms differ slightly by a value of fIF ∆𝜑 will be induced by refraction index A balanced mixer will load ∆𝜑 on fIF I/Q demodulator will extract the ∆𝜑 IF Balanced mixer I/Q demodulation Density determination With a given wavelength, the line integrated density can be directly deduced from the phase difference. 3

Principle I/Q I/Q LO RF IF LO 4 Mixer Mixer SSBM Mixer 2X 1X 1X 2X 1X Modulated IF Mixer LO RF IF LO RF IF Modulated IF I/Q sin ∆𝜑 cos ∆𝜑 I/Q sin ∆𝜑 cos ∆𝜑 Plasma Plasma Frequency stability LO RF Reference IF IF RF LO Phase noise LO RF IF SSBM Mixer IF LO Reference IF 2X MMW source 1X MMW source 1X Mixer This configuration consists of 2X Mixer 1X IF source 1X 4 SSBM

Principle Higher frequency stability Lower Phase noise Difficulties in single sideband modulation with small IF Millitech: Minimum IF is 2GHz Filtering bandwidth ≥2% of center frequency IF for SSB range is 1~5GHz(typ) Quinstar: But you have to use a bandpass filter to keep only the 1st harmonics Difficult to filter when fIF is small fIF larger than expected if doing the modulation directly in W band 5

Principle Refraction problem 6 overlarge wavelength invalidates the approximation used when deducing the phase-density equation and suffers from refraction problems. Example of refraction problem. Figure from Tudisco et al., Rev. Sci. Instrum. 84, 033505 (2013) The beam will deviate from the expected trace 6

Frequency optimization Principle Frequency optimization Refraction problem Minimize power loss by higher frequency Density resolution Overlarge frequency corresponds to poor phase resolution Vibration resistance Important for small wavelength operation Mechanical vibration disturbance The optimal frequency should be carefully deliberated 7

Frequency Optimization Elongation Triangularity Equilibrium reconstruction Phase resolution Larger wavelength corresponds to higher sensibility Refraction problem Higher frequency diminishes power loss Involves triangularity and elongation obtained from equilibrium reconstruction. A comprehensive consideration about … suggests Zhong et al, Rev. Sci. Instrum.87, 083501 (2016) 8

1 2 System Design 9 Single Sideband Modulation Frequency Sextuple Maximum IF stability Improved phase measurement 2 Frequency Sextuple Narrower SSB achievable Easier phase demodulation Easier performance test Avoid application of waveguides Now the construction and frequency of… Another advantage is that in this low frequency band we can test the filter easily by certain spectrum analysers, Furthermore, 9

Test Result 10 Outstanding source & SSB Small frequency span Excellent SNR No harmonics observable 10

Summary MMW interferometer with single sideband modulation and frequency sextuple. Maximum reference IF stability 1 Suitable probing beam frequency guaranteed with proper calculation. Delicate narrowband SSBM without other harmonics. 2 Under construction and near completion. Promising to give subtle density evolution. 3 With proper calculation Clear & reliable 11

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