A.

Global laser architecture

As commonly seen, our fiber laser consists of a pump laser diode, a Wavelength Division Multiplexer (WDM), an Yb doped Fiber Bragg Grating (FBG), an optical isolator and a splitter to allow a feedback control.

In this architecture, the noise reduction loop acts on the pump diode current by detecting the low frequency fluctuations of the output optical power from the dedicated photodiode located in the 20 % splitter output.

The corresponding block diagram is presented in Fig. 1.

Fig. 1.

Laser architecture

B.

Athermal design of the Laser Fiber Assembly (LFA)

The ASM laser has to be able to pump the ⁴He at 1082.908 nm (in air std) within the temperature range of [-5 °C; +50 °C] which is the specified qualification operating temperature range of the ASM electronics on the Swarm satellites. One of the main challenges to take up for the LFA conception was to reduce the thermal wavelength drift as much as possible in a passive way. We could have used, for example, a 1083 nm laser diode with a thermo-electric Peltier device but the laser consumption would have been highly increased.

In order to detail the achieved passive athermal LFA design we remind here first the theoretical principle behind the operation of the FBG. The reflected wavelength λ, also called the Bragg wavelength, is given by the following equation:

where n is the effective refractive index of the fiber and Λ is the grating period.

When differentiating this equation (and only considering the fluctuations due to the temperature), we obtain:

The grating period Λ is modified by adapting the Bragg grating length L (which is in our case about 50 mm). Thus, we deduce:

where the 0.78 coefficient accounts for the photo-elastic effect of fiber.

Equation (3) describes the wavelength behavior with respect to the temperature. The first term is commonly called “thermo-optic constant” and is characteristic of the fiber (about +7 pm/°C for our Yb doped fiber). The second term corresponds to the thermal wavelength drift in relation to the fiber thermal expansion.

The aim of the athermal LFA design is to obtain a wavelength thermal drift close to 0 pm/°C by achieving the compensation of the first term by the second one in equation (3). Solving this equation has led to the final design where a global negative thermal expansion coefficient has been achieved due to a specific assembly of aluminium, titanium, Zerodur^® (zero expansion glass-ceramic) pieces and a piezoelectric actuator. The role of the piezoelectric actuator (made by the CEDRAT Group) [3] is to modulate and to allow a fine tuning of the laser wavelength around the ⁴He D₀ transition. A picture of the final LFA design is given in Fig. 2.

Fig. 2.

Photography of a LFA: the pre-stressed piezoelectric actuator on the right is glued on the Zerodur^®; the LFA is then fixed in the ASM electronics box with the titanium bridge put upon the Zerodur^®

C.

Fixation system

Another challenge to take up was the design of a suitable fixation system of the LFA allowing it to comply with the shocks and vibrations specifications. A solution has been developed using a titanium bridge, coned-disc springs (also known as Belleville washers) and elastomers (with suitable thickness and hardness, and a low outgassing characteristic).

The pieces of elastomer are located under the LFA and between the titanium bridge and the LFA in order to dampen any tri-axis vibrations or shocks. This fixation system has successfully passed the vibrations and shocks qualification tests. The final titanium fixation bridge is shown on the LFA of Fig. 2.

A.

RIN performance

For both consumption and simplicity reasons, in the ASM instrument it has been chosen to detect the magnetic resonance signals coming from the ⁴He cell around 1 kHz, which corresponds to the 1 kHz modulation of the continuous part of the LA0 magnetic resonance signals between the Zeeman sub-levels [4]. This signal is high enough to allow the measurement of the magnetic field with a low noise laser: the corresponding RIN measurement of the ASM laser is shown in Fig. 3 (obtained from an electronic spectrum analyzer and an InGaAs photodiode). As shown, the low frequency feedback control loop presents the opportunity to reduce the RIN from about -105 dB/√Hz at 2 mW of output power around 1 kHz down to -140 dB/√Hz. The low noise laser specifications are thus met.

Fig. 3.

RIN measured at 1 kHz with a SWARM flight model laser

B.

Wavelength and power performance in thermal vacuum environment

Given that an active medium is present in the fiber laser (Yb doping), there are energy losses during the wavelength conversion. These thermal losses induce an increase of the local temperature which is amplified under vacuum conditions. Several tests have demonstrated that this increase of temperature make the laser unstable, the power drop and the wavelength rapidly increase even with low pump powers of about 10 mW and a fiber diameter of 250µm.

A suitable way to enhance the thermal dissipation and thus to properly operate the laser under vacuum was eventually found and implemented. In addition, this solution has allowed the reduction of the acoustic disruption as well as the improvement of power and wavelength performance. A few results are given in Fig. 4.

Fig. 4.

Power and wavelength measurements with and without our thermal dissipation solution in air (on the left) and wavelength behavior during a thermal cycle in vacuum (on the right)

The first graph in Fig. 4 shows that this thermal dissipation solution increases the output power as well as reduces the wavelength fluctuations when the pump power is increased in air environment. The same results are obtained when this test is carried out under vacuum (with the thermal dissipation solution).

On the second graph in Fig. 4, the thermal wavelength drift of the LFA has been demonstrated lower than 1 pm/°C (typically close to 0.5 pm/°C) when operated over the qualification temperature range of [-5 °C; +50 °C].