A.

Light Emitting Diodes

LEDs used in the experiments were commercially available devices emitting at different wavelengths. The details concerning manufacturer, basic parameters, and number of irradiated samples are given in the Table 1.

Table 1.

Data of tested LEDs.

B.

Laser Diodes

LDs used in the experiments were commercially available devices emitting in the blue and NIR. The details concerning manufacturer, basic parameters, and number of irradiated samples are given in the Table 2. All the samples were allocated to the displacement damage test.

Table 2.

Data of the COTS LDs.

C.

Monolithically integrated Master-Oscillator Power-Amplifier

During the EU FP7 BRITESPACE project a monolithically integrated MOPA was developed. The full description of the devices is given in [Faugeron2015] and here we just give a brief description. The device consists of three independently driven sections. First section, master-oscillator (MO), is the distributed feedback laser (DFB) which is a seed laser, the second is the modulator, and the third one is the power amplifier (PA). Due to the fact that very low reflectivity of the PA facet was demanded, the modulator section was bended, and tilted tapered PA was proposed, which allows obtaining very stable optical spectrum showing high side mode suppression ratio which is necessary for green house gases spectroscopy and LIDAR applications. Front facet was anti-reflective coated (AR<10^–3) whereas the rear (DFB) facet was high-reflective coated (HR = 90%). The scheme is shown in Fig.1. The laser dice is mounted on AlN submount. The epitaxial design is based on an asymmetrical cladding structure which has previously demonstrated very good performances for DFB and Fabry-Perot (FP) lasers [Faugeron2013]. This method allows decreasing drastically the internal losses due to the low overlap of the enlarged optical mode with the p-doped layers. The dilute waveguide was composed of 14 periods of InGaAsP (λ_g = 1.17 μm)/InP layers. The total waveguide was 1.53 μm thick. More details about the principle of dilute waveguide are given in [Faugeron2013a]. The epitaxial structure was grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on n-type InP substrates. It includes six 8 nm thick compressively strained (0.85%) InGaAsP quantum wells and five 10 nm thick InGaAsP (λg = 1.17 μm) barriers. The measured photoluminescence peak was 1.57 μm. For the electric isolation of the different sections the intersection regions were defined by removing the metallization and the p-doped contact layer followed by the ionic implantation of the region. A measured inter-section resistance is of about 2 kΩ. A 4 μm thick gold layer was deposited onto the surface to improve heat dissipation and to allow electrical bonding with minimum damage. The MOPA is emitting at 1572 nm and the light is coming out from both, rear and front facets. The light from the rear facet is used for frequency stabilization and the light from PA facet is used in the LIDAR application.

Fig. 1.

Scheme of the monolithically integrated MOPA.

A.

Characterization of LEDs and LDs

Schematic of the measurement setup for COTS devices is shown in Fig. 2(a). The setup consists of the integration sphere where the device under test (DUT) is placed. The outputs of the sphere are connected to the power meter and optical spectrum analyzer (OSA) or spectrometer. The power meter is the Newport 2936-C, and due to the fact that sources with different wavelengths were tested, following detectors were used: Newport 818-UV for UV light, Newport 883-SL for visible and NIR, Newport 818-IG for NIR, and Newport 818P-001-02 for high power lasers. The OSA is the Antritsu MS9710C and the spectrometer Ocean Optics NIRQ512. To drive the LED and commercial LDs a Thorlabs LDC 8010 was used or precision power supply Keithley 2602B. In case of the three sections MOPA, one device was mounted and the rest were not mounted and measured in the setup with probes shown in Fig. 2(b). The DFB section was driven by LaseLock 19, the modulator and PA section were driven by means of HP Agilent 6624A when driven the mounted device and by means of Arroyo and high precision power supply Keithely 236 and HP E3631A when driven in setup with probes. The temperature of the devices was controlled by means of Peltier element and read 25°C if not explicitly stated otherwise. In case of some LEDs there was no possibility to control the temperature so they were operated in the room temperature which read 22±3°C. During the measurements the light-current-voltage (L-I-V) characteristics and spectra were measured. During the radiation the devices were placed in the custom holders which fit to the radiation beam dimensions. Due to the fact that devices were not operated or operated at low currents (below threshold in case of LDs) there was no necessity to control the temperature of the DUTs during radiation and the tests were performed at room temperature.

Fig. 2.

The scheme of the measurement systems for COTS devices (a) and MOPA (b).

B.

Radiation facilities and beam parameters

TID tests on LEDs were performed in the RADLAB in Centro Nacional de Aceleradores (CNA) in Seville, Spain. The laboratory is equipped with ⁶⁰Co gamma source with nominal activity of 444 TBq (12 kCi) with range of dose rate: 36 rad(Si)/h to 36 krad(Si)/h (0.36 Gy/h – 360 Gy/h). The dose rate during the test was of about 220 rad(Si)/h. The uncertainty of the dose level is below 10% and the non-uniformity of the radiation field is also less than 10%. The LEDs were irradiated up to 100 krad(Si) with several intermediate measurement steps at 10, 20, 50, and 75 krad(Si). After the irradiation they were annealed for 24 hours at the room temperature (denoted as A1) following with the second annealing at higher temperature (denoted as A2) for 168 hours.

Besides, CNA was used for irradiation of MOPAs with low energy protons of 15.168 MeV, with the flux of 1.48·10⁹ protons/(s·cm²), and the diameter of the beam was of about 15 mm. The MOPAs had maturity problem of the inter-electrode processing, so the duration of the standard TID is beyond the reliability of the MOPA. Therefore a solution was found connected with the fact that irradiation with low energy protons, besides displacement damage, introduces also significant TID effect. The relationship between TID and proton fluence is given by the equation [McMahan2008]:

where the fluence is in units of p/cm² and the linear energy transfer (LET) in MeV·cm²/mg. LET is a measure of the energy deposited per unit length as an energetic particle travels through a material. The following Table 3 presents the LET, the penetration range in InP and the maximum fluence in p/cm² necessary for reaching an equivalent total dose of 26, 50, and 100 krad.

Table 3.

Equivalent TID for irradiation of MOPAs.

For DD test proton beams at UCL and KVI were used. At UCL a CYCLONE110 (CYClotron de LOuvain-la-NEuve) is a multiparticle, variable energy, isochronous cyclotron capable of accelerating protons up to 65 MeV. The beam diameter is of about 25 mm and the flux can be adjusted to the needs of applications. At KVI the standard proton beams used for irradiations produced by the AGOR cyclotron have primary energies in the range from 40 to 190 MeV, but at the DUT they are typically 5-10 MeV lower depending on the scatter system used. Using an energy degrader virtually all energies between 10 MeV to 184 MeV can be provided. The energy resolution of the beam when leaving the cyclotron is typically better than 0.25%. However, at the DUT position the resolution is in the order of a few MeV or more due to scatter in air, the scatter system and, when used, the energy degrader. Both circular and rectangular fields are available. The standard irradiation field has a diameter of 70 mm and homogeneity of better than ±3%. Typical fluxes are in the order of 10⁸ to 10⁹ p/(cm²·s) and fluences up to 10¹³ p/cm² can be delivered.

A.

Total ionizing dose

The TID test was performed on two types of LEDs: one emitting at 850 nm and the second at 255 nm. The radiation steps were the same for both types and read: 10, 20, 50, 75, and 100 krad. The samples were measured before and after the radiation test. Besides, all of the samples were annealed at 24°C for 24 h. Then the 850 nm LEDs were annealed at 100°C for 168 h, whereas the UV ones at 55°C for 168 h, according to the maximum operation temperatures given in manufacturer datasheets. In case of 850 nm LEDs 12 samples were irradiated under the following driving conditions: 4 samples biased at 10 mA, 4 samples biased at 70 mA, and 4 unbiased. In the radiation test of 255 nm LED 3 samples were biased at 5 mA and 3 were unbiased. There was always a control sample to check the setup.

The measurements were taken at 5 mA and 10 mA. The samples showed almost no degradation in power (below 1%) and the emission wavelength was changing independently on radiation level about 1.5 nm as can be seen from Fig. 3. Similar measurements were performed for UV LEDs. The results for both types of LEDs are shown in Table 4. The degradation in the output power is below 1% and 1.43% for 850 nm and 255 nm LEDs, respectively. There was no significant difference between the obtained results for biased and unbiased samples.

Fig. 3.

Optical power (a) and wavelength (b) at peak emission for 850 nm LED at different steps: INIT – initial, S1 – 10 krad, S2 – 20 krad, S3 – 50 krad, S4 – 75 krad, S5 – 100 krad, A1 – annealing 24 hours, and A2 – annealing 168 hours.

Table 4.

Results of TID test for 850 nm and 255 nm LEDs.

B.

Displacement damage

Displacement damage test was performed on four types of LEDs emitting at 850 nm, 830 nm, 470 nm, and 255 nm by means of protons with energy of 60 MeV. The final fluence was 8·10¹¹ p/cm² with the intermediate steps at 1·10¹¹, 4·10¹¹, 6·10¹¹ p/cm² for samples at 850 nm, 470 nm, and 255 nm. 830 nm LED was tested at 2.5·10¹⁰, 5·10¹⁰, 7.5·10¹⁰ 10·10¹⁰, 20·10¹⁰ p/cm². The examples of output power level and wavelength peak during the radiation are shown in Fig. 4. It can be seen that the wavelength is changing within ±1 nm independently of the fluence whereas the output power degrades with increasing fluence. Finally, about 25% drop of the output power after the last step was measured for the sample which was unbiased during the radiation test. The data for other LEDs after the final fluence are gathered in Table 5. In all cases the wavelength change is within the measurement error, but the output power is degrading with increasing fluence. All unbiased LEDs have shown higher percentage degradation of the output power than the biased devices. This can be attributed to the fact that besides the DD effect also the TID occurs which is less pronounced in the biased devices due to the annealing effect. The other observation is that the degradation is more visible at lower current excluding the 830 nm LED.

Fig. 4.

Optical power (a) and wavelength at peak emission (b) during the DD test for 850 nm LED.

Table 5.

Results of the DD test for COTS LEDs.