August 9, 2016

Laser Source

Laser Source CAD

Laser source

The Laser Source provides laser beams to the Caesium Tube. The Laser Source provides 14 different laser signals with very precise optical frequencies, and recombines them in order to deliver those laser beams through 10 optical fibres. SODERN is in charge of the Laser Source.

Laser Source Design Rationale

Laser diodes are used to deliver the laser signal, but a lot of other components (optical, opto-electronic, electronic or mechanical ones) are necessary to meet the performance requirements.
The high spectral purity is provided by 2 extended cavity laser diodes (ECLs) for the 2 main optical frequencies. ECLs have a spectral purity 10 to 100 times higher than simple laser diodes. Since an ECL delivers less laser power (about 50 mW maximum) than a single laser diode (about 150 mW), there is not enough laser power for the 4-5 frequency.
Semiconductor amplifiers are not reliable enough at 852 nm wavelength for a space application. So we use 2 slave laser sources (simple laser diodes delivering 150 mW each) to amplify the power of the 4-5 laser. They are locked in frequency by injecting in their cavity part of the Master Laser signal. They each deliver about 150 mW at the frequency emitted by the Master Laser when the injection is efficient.
In order to achieve high frequency stability at the precise value required, each ECL-emitted frequency is compared to a caesium reference (saturated absorption line of Caesium cell) and locked onto this frequency by a servo-loop. The laser frequency is modulated by a diode current modulation (at a few 100 kHz).
A frequency change induces a differential absorption of the signal arriving in a Caesium cell (used as reference), leading to a variation of the laser power detected by a photodiode located behind the cell. These changes are recorded in a synchronous detection system. Slow corrections are made on the optical cavity length, whereas fast corrections are made on the laser current.
The laser beams frequency is shifted using acousto-optical modulators (AOM) in order to optimize the interaction with the caesium atoms. The Slave Lasers follow the frequency shifts of the Master Laser.
The laser beam polarization is controlled in order to distribute the power towards the different parts of the Laser Source and finally towards the optical fibres delivering them to the Caesium Tube (TC). The relative difference between the powers of 2 capture laser beams must be lower than 2%.

Optical Bench

All the optical components are mounted on the two faces of a 400 mm x 330 mm optical bench. The main components are:

  • 2 ECLs delivering the 2 main frequencies, optics and frequency stabilization electronics
  • 2 slave lasers (SL) amplifying the laser power at the 4-5 frequency
  • 4 caesium cells: 2 for frequency stabilization of the ECLs, 2 to control the good frequency locking of the Slave Lasers
  • 6 photodiodes: 2 for approaching the right frequency for ECLs, 2 for locking the ECLs on the right frequencies, and 2 for checking the frequency locking of SLs
  • 4 optical isolators (OI): 1 for each laser source, in order to protect them from optical feedback which would induce perturbations on the laser frequency
  • 6 AOMs in order to obtain the different frequencies needed in the optical parts of the TC
  • 7 mechanical shutters (MS) in order to switch the laser power off
  • 10 polarization maintained fibres (PM) to deliver all the beams to the various TC zones with a good polarization ratio
  • 10 rotating mirror mechanisms preparing the injection in the optical fibres. They allow a correction of the differences between the power of the 6 capture laser beams after launch and during the flight, and a re-optimization of the laser power of the selection and detection beams when needed.

We must add to the list mentionned above a lot of optical components in order to adapt the laser beams and distribution ratios, for an accurate distribution of the power supplied by the 4 laser sources, towards the 10 different fibres—lenses, delay slats, polarizing cube beam splitters, mirrors, etc.

The electronics to generate and control the different parameters are located in the lower part of the laser source unit. Some temperature regulations are also accommodated on the optical bench (for example, laser diodes).
The ancillary equipment of the optical bench consists of the laser current, voltage and temperature control units, the laser frequency-locking unit, and the drive electronic for the AOMs and the mechanisms. This equipment is located on a plate below the optical bench inside the SL.

Main SL Unit Description

Laser Diodes
Pharao uses the laser diodes qualified and flown onto previous projects SILEX and OICETS (same wavelength but at higher power).
ECL - Extended Cavity Laser
The concept chosen is made of a linear cavity containing a laser diode, a collimating lens, an anamorphic optics to make the laser beam circular, an intra-cavity specific filter to obtain a high spectral selectivity, and a cat-eye system for the output mirror in order to have less sensitivity to mechanical instabilities. The length of the extended cavity is selected by the location of the mirror which moves with a piezoelectric motor. This design has been demonstrated by BNM-SYRTE and simulated at CNES by the engineering model; its industrial version for space use is made by EADS-SODERN.
Laser Source ECL
Laser Source ECL (EADS Sodern)
Laser Source ECL layout
Laser Source ECL layout (EADS Sodern)
AOMs - Acousto-Optical Modulators
They generate the frequency shifts needed, but also losses in laser power, and they need a high electric power (radiofrequency signal). They are optimized in order to obtain a reduction of the power consumption, an optimization of the diffraction efficiency, and a reduction of the angles between diffracted and incident beams to less than 1° in order to simplify designing, alignment and integration of the SL.
AOM module
AOM module
Optical Isolators
These components are commercial ones. Smaller components than those usually used by scientific laboratories have been chosen in order to reduce the overall dimensions of the SL. However, they are adapted for space utilization and their magnetic shielding are optimized to ensure optical isolation of the lasers and avoid magnetic perturbation on the TC.
Optical Polarization Maintaining Fibres
In order to obtain a high polarization ratio at the end of the fibre, and polarizing fibres being not available anymore, we eventually chose PM fibres, which are less selective in polarization but can fill the need combined with polarizing cube beam splitters used at their outputs, in the Caesium Tube.

SL Mechanisms

Three kinds of mechanisms are needed in the SL—ECL translators, Mechanical Shutters and Power Balancing Mechanisms.

ECL Mechanisms
Each ECL has a piezoelectric translator to move the end mirror of the extended cavity and select or correct the laser frequency.
Mechanical Shutters
These mechanisms have to shut the laser beams completely: an extinction of 120 dB is required compared to the maximum level of power of a laser beam. They will be operated about 107 times during the PHARAO mission. Shutters using step-to-step motors have been designed to fulfil the need.
SL mechanical shutter
SL mechanical shutter (Cedrat)
Power Balancing Mechanisms
In order to balance the powers of the 6 different capture laser beams with a high precision (1%), we need to rotate optical mirrors. Piezoelectric devices have been chosen for their performances. They also re-optimize the injection of the laser beams into the optical fibres after launch and during the mission if necessary.
SL fiber optics injection mechanism
SL fibre optics injection mechanism(Cedrat)

Main Challenges

The main challenges of the SL design are induced from the ACES payload accommodation constraints, which impose a high level of compactness, low electric power consumption, a wide range of storage temperature and of operating temperature, and the need to be air and vacuum operated with a sufficient level of performances for the checking of PHARAO and ACES.

The main constraints are:

  • the dimensions: 529 mm*330 mm*180 mm (31 liters) and mass: 20 kg;
  • the electrical power consumption: 38 W;
  • the operating temperature: 10/33.5°C and non-operating temperature: -50/+75°C.

EM model complete during tests
EM model complete during tests

These requirements are very constraining for the SL optical layout, considering the high performances and the number of functions required to reach efficient interactions between laser beams and Caesium atoms, for an optimal operation of the PHARAO clock. This requires minimizing the number and size of the components, maximizing their efficiency, optimizing their performances on the optical bench, doing as much functions as possible while minimizing the number of components, and limiting the power consumption and the potential degradations of the performances.
The laser power requirement induces a risk for laser diodes and their lifetime because they have to be operated at maximum power during at least 3 years. In normal operation mode, the estimated lifetime is higher 240,000 h at 20°C and 42,000 at 70°C, for space-qualified diodes. Furthermore, the diodes are operated with an optical feedback (at a rather high level for ECLs) known for sometimes leading to rapid laser diode death. Therefore, every ECL and slave laser is duplicated for redundancy.
The ECL master laser and the different AOMs are the most critical components from a technological point of view. They have a specific qualification programme.