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

2. Actuality
Motivation
Principle
System design
Content
Summary

3. 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

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

5. Principle 3 Heterodyne interferometer Density determinationLO 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

6. 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

7. 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

8. 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, (2013)
The beam will deviate from the expected trace 6

9. 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

10. 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, (2016) 8

11. 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

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

13. 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

14. Thank you!