October 24, 2023

LASER SOURCE (SL)

Laser Source CAD

Laser source

The Laser Source (SL) provides laser beams to the Caesium Tube. It provides 14 different laser signals with very precise optical frequencies, and recombines them for delivery through 10 optical fibres. SODERN built the SL.

Laser Source Design Rationale

Laser diodes are used to generate 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 two extended cavity laser diodes (ECLs) for the two 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 simple laser diode (about 150 mW), there is not enough laser power for the  4-5 frequency.

Semiconductor amplifiers are not reliable enough at the 852-nm wavelength for use in space. We therefore use two slave laser sources (simple laser diodes delivering 150 mW each) to amplify the power of the  4-5 laser. They are frequency-locked by injecting a Master Laser signal in their cavity. 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 a caesium cell) and locked onto this frequency by a servo-loop. The laser frequency is modulated by the diode current (at a few hundred 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 by a synchronous detection system. Slow corrections are made to the optical cavity length, whereas fast corrections are made to the laser current.

The laser beams’ frequency is shifted using acousto-optical modulators (AOM) in order to optimize interaction with the caesium atoms. The Slave Lasers follow the frequency shifts of the Master Laser.

The laser beam polarization is controlled to distribute the power to 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 the two 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:

  • two ECLs delivering the two main frequencies, optics and frequency stabilization electronics
  • two slave lasers amplifying the laser power at the  4-5 frequency
  • four caesium cells: two for frequency stabilization of the ECLs, two to ensure good frequency locking of the Slave Lasers
  • six photodiodes: two for approaching the right frequency for ECLs, two for locking the ECLs on the right frequencies, and two for checking frequency locking of the slave lasers
  • four optical isolators (OI): one for each laser source, to protect them from optical feedback which would interfere with the laser frequency
  • six AOMs to obtain the different frequencies needed in the optical parts of the TC
  • seven mechanical shutters (MS) to switch off the laser power 
  • ten polarization-maintained fibres (PM) to deliver all the beams to the various TC zones with a good polarization ratio
  • ten rotating mirror mechanisms preparing injection into the optical fibres. This enables a correction of the differences between the power of the six 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.

An additional set of optical components is required to adjust the laser beams and distribution ratios, to ensure accurate distribution of the power supplied by the four laser sources to the 10 different fibres—lenses, delay slats, polarizing cube beam splitters, mirrors, etc.

The electronics designed to generate and control the different parameters are located in the lower part of the laser source unit. Temperature control devices are also accommodated on the optical bench (for example for the 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 electronics for the AOMs and the mechanisms. This equipment is located on a plate below the optical bench inside the SL.

Main SL Unit

Laser Diodes
PHARAO uses the laser diodes qualified and flown on the previous SILEX and OICETS projects  (same wavelength but at higher power).
ECL - Extended Cavity Laser
The concept chosen consists of a linear cavity containing a laser diode, a collimating lens, anamorphic optics to make the laser beam circular, a specific intra-cavity filter to obtain a high spectral selectivity, and a cat’s-eye system for the output mirror to reduce sensitivity to mechanical instabilities. The length of the extended cavity is selected by the location of the mirror, which is moved by 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
The AOMs generate the required frequency shifts, but also losses in laser power, and they need a lot of electrical power (radiofrequency signal). They are optimized to save power, optimize diffraction efficiency, and reduce the angles between diffracted and incident beams to less than 1° in order to simplify the design, alignment and integration of the SL.
AOM module
AOM module
Optical Isolators
The optical isolators are commercial-off-the-shelf components. Smaller components than those generally used by scientific laboratories have been chosen to reduce the overall dimensions of the SL. However, they are adapted for use in space and their magnetic shielding is optimized to ensure optical isolation of the lasers and avoid magnetic disturbances on the TC.
Optical Polarization Maintaining Fibres (PM)
In order to obtain a high polarization ratio at the end of the fibre, and as polarizing fibres are no longer available, we eventually chose PM fibres, which are less selective in polarization but can fill the need combined with polarizing cube beam splitters 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 off the laser beams completely: an extinction of 120 dB is required compared to the maximum power level of a laser beam. They will be operated about 107 times during the PHARAO mission. Shutters using stepper motors have been designed to fulfil this need.
SL mechanical shutter
SL mechanical shutter (Cedrat)
Power Balancing Mechanisms
In order to balance the power of the six different capture laser beams with high precision (1%), we need to rotate the optical mirrors. Piezoelectric devices have been chosen for their performance. They also re-optimize 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 stem from the ACES payload accommodation constraints, which require a high level of compactness, low electric power consumption, a wide range of storage and operating temperatures, and the need to be air- and vacuum-operated with a sufficient level of performance to check out PHARAO and ACES.

The main constraints are:

  • dimensions: 529 mm*330 mm*180 mm (31 litres) and mass: 20 kg
  • electrical power consumption: 38 W
  • 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 stringent for the SL optical layout, considering the high performance and the number of functions required to ensure efficient interactions between laser beams and caesium atoms, for optimal operation of the PHARAO clock. This means minimizing component size and count, maximizing their efficiency, optimizing performance on the optical bench, performing as many functions with as few components as possible, and limiting power consumption and the potential degradation of performance.

The laser power requirement induces a risk for laser diodes and their lifetime because they have to be operated at maximum power for at least three years. In normal operating mode, the estimated lifetime is more than 240,000 hrs at 20°C and 42,000 hrs at 70°C, for space-rated diodes. Furthermore, the diodes are operated with an optical feedback system (at a quite high level for ECLs) that is known for sometimes leading to rapid mortality of laser diodes. Therefore, every ECL and slave laser is duplicated for redundancy.

The ECL master laser and the AOMs are the most critical components from a technological point of view. They have a specific qualification programme.