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We report ground-level gamma and proton radiation tests of a passively mode-locked diode-pumped solid-state laser (DPSSL) with Yb:KYW gain medium. A total gamma dose of 170 krad(H2O) applied in 5 days generates minor changes in performances while maintaining solitonic regime. Pre-irradiation specifications are fully recovered over a day to a few weeks timescale. A proton fluence of 9.76·1010 cm−2 applied in few minutes shows no alteration of the laser performances. Furthermore, complete stabilization of the laser shows excellent noise properties. From our results, we claim that the investigated femtosecond DPSSL technology can be considered rad-hard and would be suitable for generating frequency combs compatible with long duration space missions.
© 2015 Optical Society of America
1. Introduction
Highly stable frequency combs generated by advanced mode-locked femtosecond lasers have led to several breakthroughs in the field of precision optical metrology and precision measurements [1–5]. Such femtosecond lasers also show a high potential for space-related applications like e.g. ultra-precise distance measurement for formation-flying satellite operation [6, 7], time and frequency transfer [8, 9], generation of ultra-low phase noise microwave [10], optical spectroscopy [11], etc... For this purpose, optical frequency combs from mode-locked femtosecond lasers must be capable of surviving challenging environmental conditions along the whole lifetime of a typical space mission, and especially high energy radiation.
In the framework of the ESA M3 mission candidate STE-QUEST (which aimed at precise measurements of general relativity effects [12]) standard ground-level radiation tests were performed on a femtosecond diode-pumped solid-state laser (DPSSL) with Yb:KYW gain medium. Such lasers can be built in a compact way and have shown properties like low cavity losses, high power and short pulse duration [13], which surpass those of fiber lasers usually being seen as a preferential choice for space applications [14, 15]. These properties, which are similar to those of Ti:sapphire lasers, can offer superior performance for specific space applications. Furthermore and as presented here, such laser architecture is not prone to radiation induced attenuation (RIA), as seen in fiber lasers [16].
Here, we show that an Yb:KYW DPSSL laser is capable of sustaining a radiation dose corresponding to more than five years in outer space on a highly elliptic orbit around Earth.
2. Lasers under test
Two Yb:KYW gain medium DPSSL passively mode-locked with a semiconductor saturable absorber mirror (SESAM) technology are investigated. The optical design is based on the architecture described in [13] with the difference that intracavity negative dispersion is achieved with GTI mirrors only. The system is packaged in a 1 mm thick stainless steel casing. The relevant parameters of both lasers are summarized in Table 1.
Table 1. Relevant laser parameters measured before the test campaign
For frequency metrology applications, the stability of the frequency comb is of paramount importance. This was investigated for Laser #1 by stabilizing the two degrees of freedom of the frequency comb: frep (comb spacing) and fCEO (offset frequency) using the setup shown in Fig. 1.
Fig. 1 Schematic of the stabilized optical frequency comb. PZT: piezoelectric transducer; PM: Polarization maintaining; SM: single-mode; HNLF: highly nonlinear fiber; H-maser: active hydrogen maser
The measurement setup is very close to the one reported in [17]. Here, the repetition rate signal frep is provided by a photodiode as an input for the stabilization loop which acts on an intracavity piezoelectric transducer. For the detection and stabilization of fCEO an f-2f interferometer was implemented. Compressed pulses with 100 fs duration were obtained via spectral broadening through self-phase modulation in a polarization maintaining single-mode fiber (~17 cm) followed by pulse compression using GTI mirrors. This allowed for sufficient peak power for the generation of an octave spanning coherent supercontinuum spectrum in a highly non-linear fiber (HNLF). In this way, the carrier envelope offset (CEO) beat note generated in the f-2f interferometer exhibited a signal-to-noise ratio (SNR) of 35 dB for a 91 kHz resolution bandwidth. The RF spectrum of the stabilized fCEO is shown in Fig. 2(a). A lock bandwidth of about 100 kHz leads to a SNR of about 36 dB above the servo bumps when measured with a resolution bandwidth of 30 Hz. For all measurements, fCEO and frep are simultaneously stabilized using the same reference synthesizer. In Fig. 2(b), the residual phase noise of the stabilized fCEO is plotted at a carrier frequency of 282 MHz (corresponding to the fCEO beat associated to a higher harmonic of frep). The in-loop root-mean-square (RMS) integrated phase noise is 1.1 rad [1 Hz; 1 MHz]. The in-loop Allan deviation (Fig. 2(c)), with a noise equivalent bandwidth of 500 Hz, shows a fCEO fractional instability of 3.5·10−12 at an averaging time of 1 s for the beat frequency of 282 MHz. The plot follows the expected τ−1 slope going down to 3.8·10−14 at 100 s averaging time. fCEO Allan deviation allows comparison of performance of frequency combs when its contribution to the optical carrier frequency instability is analyzed corresponding to the comb equation described in [18]. This contribution amounts to 3.3·10−18 at 1 second (see second y-axis) for the optical carrier frequency situated at 291 THz. Typical performance of diode-pumped solid-state lasers with similar repetition rate and output power is in the region between 10−15 [19] and 2∙10−18 [17] when stabilization via pump power modulation is used, like here. Relative Intensity Noise (RIN) measurements of the laser output have also been performed and are displayed for free running and locked fCEO in Fig. 2(d). The free running laser RIN exhibits a plateau-like behavior at −125 dBc/Hz. The plateau starts from about 100 Hz and extends until about 30 kHz, which frequency corresponds to the inverse of the upper state life time of the laser gain medium [20]. Noise from the pump laser is cut above this frequency by the low-pass filtering effect of the gain medium. When fCEO is phase-locked, the RIN diminishes by up to 25 dB for frequencies below 30 kHz. This behavior is due to the fact that fCEO fluctuations are strongly governed by intracavity power fluctuations and hence pump laser power fluctuations [17]. The ensemble of these measurements shows that this laser can be considered as a high performance frequency comb.
Fig. 2 (a) RF spectrum of the stabilized fCEO beat note at 282 MHz (resolution bandwidth of 30 Hz, 100 traces averaging). (b) Residual phase noise of the stabilized fCEO for a carrier frequency of 282 MHz. (c) fCEO beat note (282 MHz) Allan deviation. (d) RIN measurements for free running and fully-stabilized laser #1
3. Radiation tests
Radiation testing has been performed on the basis of the space environment effects to be expected in the context of the former M3 mission candidate STE-QUEST. These effects are described in details in [21] for the mission required highly elliptic (Apogee: 51018 km, Perigee: 700 km) and high inclination (72.1°) orbit around the Earth, thus continuously crossing the Van Allen radiation belts for a nominal duration of 5 years. Energetic particle radiation, especially from trapped electrons [21] will thus play a very important role and its effects must be anticipated carefully. Although it is not feasible to exactly recreate the space environment in a radiation test facility, the device degradation can approximately be characterized in terms of how much radiation energy is deposited, without regard to the type or energy of the incident radiation. In this work, total Ionizing Dose (TID) effects as well as Total Non-Ionizing Dose (TNID), or Displacement Damage Effects, have been investigated. Both TID and TNID are cumulative effects and typically result in slow degradation of EEE (Electronic, Electrical and Electromechanical) components as well as material degradation with increasing received dose [22]. TID effects have been assessed at the Co-60 facility at ESA-ESTEC (Noordwijk, Netherlands) and are described and discussed in Section 3.1. TNID effects have been assessed at the Proton Irradiation Facility (PIF) at the Paul Scherrer Institute (Villigen, Switzerland). These tests are described and discussed in Section 3.2.
3.1 Gamma irradiation tests
Exposure to 60Co radioactive sources with emission of gamma rays at 1.173 MeV and 1.333 MeV is an established and convenient way to simulate TID effects due to penetrating electrons and protons in the space radiation spectrum on electronic components and materials [22]. The important parameters to set for this facility are the TID in rad (1 rad = 0.01 J/kg) and the dose rate in rad/s. Note that these values are calibrated for energy absorption in water. For irradiation in usual materials (typically Si, GaAs, etc…), a conversion factor should be calculated, knowing the photon mass attenuation coefficient μ/ρ and the mass energy-absorption coefficient μen/ρ for these materials. Tables for elements Z = 1 to 92, as well as for 48 compounds and mixtures of radiological interest can be found in [23].
Most space flights environment dose rates are situated between 10−6 and 10−2 rad/s. Laboratory gamma sources can cover a large dose rate range from about 10−3 to 104 rad/s [22]. Hence, it is possible to deposit the TID within a test device specific to a given mission on a much shorter time scale. Note however that in outer space conditions, i.e. at much lower dose rates, increasing component degradation is observed. This enhanced low dose rate sensitivity (ELDRS) particularly affects bipolar technologies [30]. In order to estimate the TID for the mission’s duration, we refer to the dose-depth curve calculated with the SHIELDOSE simulation package in Fig. 3.
Fig. 3 Dose in Si as a function of spherical Al shielding as calculated by SHIELDOSE for the STE-QUEST mission (adapted from [21] p.36)
In this graph, the red curve gives the TID (rad) in silicon through a spherical aluminum shielding as a function of thickness. For a “low case” thickness of 3 mm, the accumulated TID in Si over the mission’s duration is about 200 krad(Si). In radiation hardness tests reported in literature, the TID value of 100 krad is often found (for example [24, 25]). For bulk optics consisting mainly of SiO2, some authors estimate that it can be considered sufficiently hard to radiation if 90% optical transmission is preserved after receiving a dose of 100 krad(SiO2) [25].
For these tests, we used Laser #2 (see specs in Table 1). The driving electronics, which is in a separate box and connected to the laser head with a cable, has not been irradiated. This is justified since the risks and criticalities are linked to the laser head and not to the driving electronics. A schematics of the test setup is shown in Fig. 4. The laser and optics are mounted on a breadboard positioned vertically and placed on a translation table, facing the 60Co source at a distance corresponding to a dose rate of 20.8 rad/s (effective dose rate received quasi-homogenously by the entire laser head, including the built-in pump diode). The laser beam is split at a polarizing beam splitter (PBS) and about 90% of the light is directed towards a powermeter. The remaining 10% are coupled into a single mode optical fiber from which both optical and microwave spectra are recorded. Lead bricks are disposed between the source and the setup in order to protect the optics of the test setup and especially the optical fiber which is sensitive to gamma irradiation [24]. These elements are not part of the laser itself, but are used to monitor the characteristics of the laser output in real time during the irradiation. Power and data cables as well as the optical fiber are pulled through tubes connecting the irradiation and control rooms. Two dosimeters were used to directly monitor the TID and dose rate at the laser position and behind the lead bricks. In order to minimize dosimetry errors, a 2 mm thick aluminum sheet was placed in front of the laser box [26]. Irradiation was switched on and off by simply removing and respectively covering the 60Co source with a shielding cylinder.
Fig. 4 Schematics of the test setup used at the ESA-ESTEC 60Co test facility. MSA: Microwave Spectrum Analyzer; PD: Photodiode; OSA; Optical Spectrum Analyzer; PBS: Polarizing beam splitter.
The TID measured at the laser position within 136 hours at a dose rate of 20.8 rad(H2O)/min was 169.8 krad(H2O) (~154 krad(Si)). The TID measured behind the shielding bricks protecting the optical elements was 606 rad(H2O). The average temperature, pressure and relative humidity measured in the radiation test room were 22°C, 1012.96 mbar and 44.26%, respectively. A summary of the most relevant laser parameters measured at different times relative to the irradiation period is given in Table 2. From this table, different effects can be observed:
Table 2. Repetition rate (frep), wavelength (λ), power at the laser head (Pout), room temperature (Tr), pulse duration (tp), spectral width (Δλ) and time-bandwidth product (Δτ·Δν) measured at different times over the whole test process.
- a) The repetition rate (frep) and the laser center wavelength (λ) remained stable along the whole process. Furthermore, broad λ (~350 nm span centered at 1030 nm) and frep (~2 MHz span centered at 40 MHz) spectra recorded every 10 seconds with an averaging over 10 spectra over the 136 hours of irradiation do not show any noticeable change.
- b) A net ~5% decrease in the output power of the laser during the irradiation process (i.e. from 141.2 mW measured in the control room before transferring the test setup in the test room and 134.6 mW measured in the control room ~1h30 after the 60Co source has been shut) has been observed. This effect is further illustrated in Fig. 5(a) where the laser power is measured on one arm of the PBS (see Fig. 4). During the irradiation period delimited by two vertical dashed blue lines, a monotonic decrease of the power takes place, with a total power loss of about 5% (the slight increase in power before the irradiation period is inherent to the intrinsic thermal stabilization in steady-state of the laser which was switched on at time 0s). About four days after the end of the irradiation run (laser OFF in between), the laser has been switched on for 21 hours and the total output power (Pout) has been logged. The result is shown in Fig. 5(b). The power starts at the value measured after the irradiation (3rd line in Table 2), and then increases gradually up to about 140 mW. The power measured five days later was 140.1 mW (4th line in Table 2), indicating that a stable maximum value very close to the initial value before the irradiation (141.2 mW) has been reached. This “self-healing” process taking place while the laser is running could be related to an effect observed in pump diodes where irradiation-induced damages can anneal as soon as current is applied and as a consequence the optical power can be restored close to its value before irradiation [27]. Such annealing can also be induced by the optical power applied to the poorly thermo-conducting gain medium during the laser operation. Further studies are necessary to confirm these hypotheses.
- c) About 17 days after the end of the irradiation run (laser ON after the 21h logging only for a couple of hours in between), a decrease of about 1.5 nm (corresponding to 21%) in the spectral width is measured along with an increase in the pulse duration by about 55 fs. The corresponding time-bandwidth product remains very close (within ~3%) to the one measured before the irradiation, i.e. very close to the transform limit. The laser calibration data show a linear relation between the average output power and the spectral width with a negative slope of ~60pm/mW around the working setpoint. Thus the 6.6 mW power loss naturally induces a decrease in spectral width of ~0.4 nm. This only represents 26% of the total measured spectral width decrease of ~1.5 nm. About 7 weeks after this measurement (laser switched OFF during this time), new measurements showed a pulse duration of 180 fs and a spectral width of 6.2 nm, with a time-bandwidth product (0.316) even closer to the transform-limit for sech2 pulses (~0.315). This apparent recovery happens over a large time scale and is not dominated by the application of a current in the laser. Although the solitonic regime seems to be conserved along the observed variations in pulse duration and spectral width, further tests are needed in order to understand the cause of the observed changes, especially the ~74% part in the decrease of the spectral width which cannot be explained solely with the irradiation-induced power decrease.
In any case what clearly results from this experiment is that the DPSSL suffers no permanent loss of its performance upon gamma irradiation up to 170 krad(H2O). According to the rate of performance-loss during the high-flux gamma irradiation and the recovery rate during normal operation, we can expect that the effects of the low-flux cosmic irradiation during the mission are continuously counterbalanced by the observed self-healing effect with no effective loss of performances over time. See Fig. 5.
Fig. 5 a) Power measured on one arm of the PBS in Fig. 4(a) as a function of time. The irradiation period is delimited by the two vertical dashed blue lines. The data gap after about 1 hour is due to the transfer of the setup from the control to the irradiation room. The one around 40 hours is due to a logging failure which happened over night. b) Power at the output of the laser logged for about 21 hours, ~5 days after 170 krad of TID had been deposited in the laser.
3.2 Proton irradiation tests
Exposure to energetic protons is an established way to assess displacement damage effects (or TNID effect) in devices. The total proton fluence spectrum obtained from the addition of the trapped and solar protons spectra for the STE-QUEST orbit is presented in Fig. 6 (black curve). This spectrum is clearly dominated by low-energy particles. In practice however, incident protons with energies lower than 20 MeV are effectively shielded by the satellite walls [28]. Modeling the proton spectrum behind a 5 mm thick aluminum shield [29] shows that the contribution of low-energy protons is negligible as can be seen in Fig. 6 (blue curve). The Proton Irradiation Facility (PIF) at Paul Scherrer Institute (PSI) in Villigen (Switzerland) offers a mono-energetic proton source with particles energy of 100 MeV. The proton energy can be reduced by introducing calibrated copper degraders in the beam path. Considering the proton energies available using this technique, the test was designed to reproduce the spectral distribution of protons using four fixed energy values distributed between 18 and 100 MeV. To ensure that the dose represents a worst-case scenario, the target fluencies were set for 7.5 years on the STE-QUEST orbit, i.e. 50% higher than the actual requirement. The corresponding spectrum is plotted in red in Fig. 6. A copper aperture is placed at the beam exit to limit its section down to 58 mm in diameter. This allows selective irradiation of specific parts of the device under test. The same measurement setup and laser (Laser #2) as for the gamma irradiation tests were used and the laser surface was placed 1 cm away from the aperture.
Fig. 6 Spectrum (adapted from [21]) of integral proton fluence averaged over the STE-QUEST orbit (black line) for 5 years mission. Corresponding spectrum behind 5 mm Aluminum shield (blue line), integral fluence applied in our irradiation experiment (red line).
At this distance, the beam has a top-hat profile with no significant divergence. Four different zones of the laser frame were selectively irradiated, each receiving the equivalent proton fluence of 7.5 years on the STE QUEST orbit. The critical components addressed in the four irradiated zones are listed in Table 3.
Table 3. Laser components present in the irradiated zones
Each zone was irradiated with protons at the four defined energies in the following chronological order: ~100, 60, 30 and 18 MeV. The measured energies and corresponding fluences applied at each irradiation run are summarized in Table 4. For each zone, a first cycle of irradiations was performed with fluences equivalent to 2.5 years on the STE-QUEST orbit. After assessing that no loss of performance resulted from this irradiation, a second cycle equivalent to another 5 years in orbit was applied. The total amount of irradiation directly applied to each critical component of the laser corresponds to 7.5 years (red line in Fig. 6). It is worth mentioning that in spite of a selective application of the direct proton beam, all the components of the laser were subjected to a strong field of secondary radiation during the whole experiment. Our irradiation test constitutes thus a worst-case scenario with accumulated doses that largely exceeds the requirements of the STE-QUEST mission.
Table 4. Proton fluences applied at each irradiated zone of the laser
A difference with the gamma experiment was the discontinuous nature of the proton irradiation. Each irradiation run, depending on the selected energy and fluence is performed within 1 to 12 minutes. The laser performance was only recorded at the end of each irradiation run (8 runs per position) and after each manipulation of the setup to position the different zones of the laser in front of the proton beam. The results are presented in Fig. 7, where the main laser parameters: optical power, spectral width, central wavelength and repetition rate are plotted.
Fig. 7 Main laser parameters monitoring. Large rhombic points indicate the performance immediately after a position change and before any further irradiation is applied. The series of 8 small filled circular points represent the performance after each of the 8 irradiation runs applied to each zone of the laser. After the last irradiation, the laser setup was removed from the beam line and its performance was recorded 0, 15, 30 and 60 minutes afterwards.
It is observed that the main changes in power output, spectral FWHM and repetition rate are not associated with the irradiation, but rather with the manipulations of the setup. Handling the setup exposes the laser to shocks and can induce mechanical constrains that slightly shift the operation points. This is particularly pronounced when the setup was changed from a horizontal to a vertical position just before starting the first irradiation run (see change after the first two points of Fig. 7). The apparent shifts are marginal and do not support a loss of laser performance. The observed variations of all the parameters are lower than what is normally observed during a day of work in a laboratory with controlled conditions. We can conclude therefore that no critical issues have been identified during the proton irradiation test. The tested technology seems to be totally robust to proton irradiation as it would be subject to in the framework of the STE-QUEST mission.
4. Conclusions
Extensive gamma irradiation and proton irradiation tests have been conducted on a femtosecond passively mode-locked diode-pumped solid-state laser based on Yb:KYW gain medium. It has been demonstrated that this technology is:
- 1. Capable to sustain a total gamma dose of 170 krad(H2O) applied in 5 days and corresponding to 5 years dose in STE-QUEST orbit with a 3.5 mm spherical Al-shield. A 5% power loss and a 20% pulse length increase have been observed at the end of the irradiation session. The laser power has quasi fully recovered (only 1 mW below initial value) within about a day by applying a pump current and about 40 days later the laser had roughly recovered its pre-irradiation pulse width and even improved its time-bandwidth product.
- 2. Capable to sustain a total proton fluence of 9.76∙1010 cm−2 with energies distributed between 18 and 100 MeV, equivalent to 7.5 years in the highly elliptical STE-QUEST orbit with no laser parameters degradation.
According to the rate of performance-loss during the high-flux gamma irradiation and the recovery rate during normal operation, we can expect that the effects of the low-flux cosmic irradiation during the mission are continuously counterbalanced by the observed self-healing effect with no or very low effective loss of performances over time. A similar self-healing effect has been observed in laser diodes through annealing of irradiation-induced damages when applying a pump current.
From these observations, we claim that the investigated femtosecond DPSSL technology is compatible with the STE-QUEST environmental requirements and can be considered rad-hard.
In the context of STE-QUEST, the femtosecond laser is used as an optical-to-microwave frequency divider. This requires full laser stabilization (repetition rate frep and carrier-envelope offset frequency fCEO). While frep stabilization is straight-forward, fCEO stabilization requires an additional f-2f nonlinear Michelson interferometer with components like a highly nonlinear fiber and a frequency-doubling nonlinear crystal. Such a sub-system is not expected to be as critical as the laser head but would nevertheless require radiation testing to assess the suitability of some key components and to confirm the radiation hardness of the whole frequency comb. Furthermore the use of a similar laser to generate low-phase noise microwave signals has been demonstrated with performances surpassing the STE-QUEST specifications [13]. This task could be part of future work to further develop this high potential technology with several different applications in space environments (ultra-precise distance measurement, time and frequency transfer, optical spectroscopy, optical-to-microwave frequency divider in an optical atomic clock, etc.).
Acknowledgments
The authors acknowledge Jonathan Bennès for the electronics, L. Balet for help in data acquisition, Alessandra Costantino and Michele Muschitiello for experimental support with the Co-60 facility at ESA-ESTEC, Ilia Britvitch for experimental support with the proton facility at PSI and Qun Feng Chen from University of Düsseldorf for proton irradiation shielding simulations. This work was funded by the Prodex program and the Canton of Neuchâtel. Support from the Swiss Space Office is gratefully acknowledged.
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