PRECISON TIMING CONTROL FOR RADIO ASTRONOMY

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PRECISON TIMING CONTROL FOR RADIO ASTRONOMY
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GROUP MEMBERS

• SURENDRA KUMAR BATCHU
• SRIDHAR SONGOJU
• SAI SAMEER TADIMETI VENKATA
• AMARDEEP BADDAM
• DILEEP THAMMAIAHGARI

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ATACAMA LARGE MILLIMETER ARRAY

• An international radio astronomical facility
  under construction in Chile
• In collaboration with USA,Canada,Europe and Japan
• Consists of an array up to 64 12m parabolic
  antennas
• Detects radio waves in the frequency band 91-350
  ghz
• Located at an elevation of 5000m

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MAIN ADVANTAGES

• Able to reveal the structure of the cold regions
  of the universe with greater sensitivity and
  resolution
• And even the dark at visible wave lengths
• Moves is various directions

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BASIC SETUP
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THE HUBBLE TELESCOPE AND ITS IMAGE FROM NEW MEXICO
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Photonic Local Oscillator (LO) reference
distribution system
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Photonic Local Oscillator (LO) reference
distribution system

• Distribution system consists of Central Building
  and
• array of 64 Antennas .

• Central Building houses
• Master Laser (ML)
• Laser Synthesizer (LS)
• Line length corrector (LLC)

• Each Antenna has a
• Photo detector
• Local/Antenna Oscillators
• Mixers Digitization and Transmission
  circuit

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Photonic Local Oscillator (LO) reference
distribution system

• Distribution system aims at optical transmission
  of LO reference signal to the antennas with
  required stability(38 fs).
• LO signal consists of two optical waves -- Master
  Laser and Slave Laser-- of wavelength around
  1.556 µm.
• Slave Laser is phase locked with Master Laser
  with frequency offset ranging between 27-142 GHz.
• Lo reference is transmitted to each and every
  antenna through LLC.

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Photonic Local Oscillator (LO) reference
distribution system
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Photonic Local Oscillator (LO) reference
distribution system

• At each antenna LO signal is sent to photo
  detector which outputs Lo ref signal ranging
  between 27-142GHz.
• This 27-142 GHz LO ref is used to phase-lock the
  antenna oscillators which output LO ref at 27
  -938GHz.
• This LO ref is used to down convert sky signals
  (31-950 GHz ) to Intermediate frequency (IF) band
  of 4-12 GH z.
• IF signal is transmitted back to correlaters in
  Central Bldg to finally generate astronomical
  images.

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ALMA LASER SYNTHESIZER
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OPTICAL PHASE LOCK LOOP

• Laser can be thought of as a noisy oscillator
  whose output is not an ideal sinusoidal wave and
  the output is given by
• E(t) A(t) cos(2p fc t
  f(t))..(1)
• where E(t) is the amplitude of the
  electrical field of the optical wave, A(t) is the
  amplitude noise, fc is the average optical
  frequency of the wave and f(t) is the phase
  noise.
• The beat signal equation is given by
• ibeat(t) A1(t)A2(t) cos(2p( f1 - f2)t
  f1(t) - f2(t))

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LOOP BANDWIDTH

• The bandwidth of OPLL is computed by measuring
  the power spectral density (PSD) of the combined
  phase noise of the ML and slave Lasers and
  applying the transfer function correspo0nding to
  the closed loop response of the error signal
  relative to input changes.
• Low-frequency phase fluctuations are thus tracked
  almost perfectly, whereas fluctuations outside
  the locking bandwidth remain largely unaffected.
• The total residual phase noise power is given by

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LOOP BANDWIDTH

• In this case, the OPLL acts as a highpass filter
  and the OPLL cutoff frequency necessary for
  meeting a given phase noise level can be
  estimated.

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OPTICAL PHASE CONTROL

• To provide actuation for closed loop control, a
  mechanism for changing the optical phase of slave
  laser a bandwidth of at least 1MHz is needed.
• One mechanism uses the tuning of the frequency of
  the fiber which is achieved by applying a voltage
  to a piezo crystal, which stretches the Bragg
  grating.
• A "Bragg Grating" is a periodic or non-periodic
  perturbation of the effective absorption
  coefficient and/or the effective refractive index
  of an optical waveguide. More simply put, a Bragg
  Grating can reflect a predetermined narrow or
  broad range of wavelengths of light incident on
  the grating, while passing all other wavelengths
  of the light.

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OPTICAL PHASE CONTROL

• By increasing the frequency of the optical wave,
  the optical wave of the slave laser will
  oscillate more rapidly and consequently the phase
  also gets increased.
• In this way the frequency is increased until the
  slave laser catches up the phase with the ML and
  vice versa.
• The laser tuning is mechanical in nature and
  displays lightly damped mechanical resonances
  above 20KHz. These resonances introduce phase
  lags that prevent high loop bandwidth.

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OPTICAL PHASE CONTROL

• To prevent this we use an external optical
  frequency shifter (AOM-acousto-optic modulator
  which is used to apply small but fast frequency
  changes to the laser output.

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FEEDBACK NETWORK

• The feedback network circuit of the OPLL
  is split into two parallel branches
• One with the fast frequency shifter
  handling the high frequency, low-amplitude
  compensation.
• The remaining low-frequency phase error is
  cancelled by a second feedback network circuit
  driving the laser-piezo.
  

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AUTOMATION OF PHASE LOCKING

• The phase locking control system is based on
  analog electronics to achieve high accuracy and
  high bandwidth
• The LS (Laser Synthesizer) includes an embedded
  computer that receives frequency tuning requests
  from the ALMA central controller through a CAN
  (controller area network) Bus link.
• The controller then sets the RF frequency
  synthesizer to a particular offset and tunes the
  slave laser until the frequency difference
  between ML and slave lasers is close enough to be
  within the phase-locking range.

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LINE LENGTH CORRECTOR

• The reference LO signal arriving at antenna must
  be stable.
• Change in the length of Fiber due to Optical and
  Electrical components, antenna rotation.
• Introduces the propagation delay.
• So a technique to stabilize the fiber length is
  needed.

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LINE LENGTH CORRECTOR

• Line Length Correction (LLC) is used to correct
  the length
• LLC is used to correct the length of the fiber to
  maintain the constant phase difference.
• The Optical wave shifts by 2p when length
  changes by 1.5 µm.

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LINE LENGTH CORRECTOR
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LINE LENGTH CORRECTOR

• Gain of each loop must be set to prevent the
  destabilizing of system.
• Maximum Bandwidth of LLC is limited by
  propagation delay of light in the fiber.

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LINE LENGTH CORRECTION RESULTS
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THE MASTER LASER

• INTRODUCTION
• LASER SOURCE
• THE OPTICAL REFERANCE
• THE FREQUENCY CONTROL LOOP
• AUTOMATION OF FREQUENCY LOCKING

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INTRODUCTION

• The interferometric measurement of the fiber
  length performed
• by the LLC uses the optical beam originating
  in the ML as a
• reference.

• This measurement method imposes two important
• requirements on the ML.

• The coherence length of the ML must be
  sufficiently long (gt30 km in fiber)
  
• (The distance over which
  interference will occur )
• 2. The optical frequency of the ML must possess
  a relative frequency stability that is better
  than the required relative fiber length accuracy

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The laser source

• A 1,556-nm distributed feedback (DFB) fiber
  laser is
• used as the source for the ML.

• This laser provides a compact and robust device
  with a
• narrow line width and, therefore, a long
  coherence
• length.

• The frequency stability of such lasers is
  insufficient for the
• LLC system since their frequency can fluctuate
  by tens of
• MHz as a function of ambient temperature and
  time.

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Optical Reference

• The optical reference selected for the ML is a
  transparent
• cell containing low-pressure gas of rubidium
  (Rb) atoms.

• Rb has the property that when a 778-nm (red)
  laser beam
• with the exact right frequency passes through
  the gas, a
• faint 420-nm (blue) fluorescence signal is
  emitted by the
• excited Rb atoms

• Since this two photon transition is highly
  stable and occurs
• only in a narrow band of optical frequencies
  (theoretically less
• than 200 kHz at 1,556 nm), the transition can
  be used as a
• sensitive frequency discriminator signal to
  which the laser
• can be locked to improve its frequency
  stability

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THE MASTER LASER PROTOTYPE
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• The 1,556-nm DFB fiber laser output is converted
  into
• 778-nm light by using an optical frequency
  doubler.

• The frequency doubling is performed by a
  periodically
• poled lithium niobate (PPLN) crystal, which
  is an
• optically nonlinear element.

• The temperature set point of the oven is
  continuously
• optimized by the ML controller using a slow
  locking loop
• algorithm implemented in software.

• The 778-nm light is then sent through the Rb
  cell
• and reflected back on itself to stimulate
  the two-
• photon transition of Rb

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The Frequency Control Loop

• To facilitate laser locking at the frequency
  where
• fluorescence is maximum, the 778-nm beam is
  phase
• modulated using an electro-optic modulator
  (EOM).

• This phase modulation creates a sideband of
  opposite
• phase on each side of the optical carrier

• If the optical frequency of the laser is on the
  center
• of the symmetrical two-photon transition, both
  sidebands
• are absorbed equally by the narrow line width,
  cancelling
• out completely.

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Automation of the Frequency Locking

• An embedded controller plays the crucial role of
  automating
• the locking of the ML on the desired
  two-photon transition of
• Rb.

• It is not possible to tune the laser with
  sufficient accuracy to
• place its frequency within the locking range
  of the desired
• transition.

• To solve this problem, the ML embedded software
  implements
• an automatic frequency calibration and locking
  algorithm

• Once the correct transition is found, the
  frequency of the
• laser is adjusted close to the targeted value
  and the locking
• loop is activated.

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CONCLUSION

• Dealt with three important subsystems namely
  1)Laser Synthesizer
• 2)Line Length Corrector
• 3)Master Laser
• ALMA is built around more control systems and
  feedback loops and antenna positioning systems.
• Gives what was invisible to previous instruments

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THANK YOU