A frequency stabilization unit was developed to stabilize high power laser diodes for differential absorption lidar. Three distributed-feedback (DFB) lasers were used in this approach. To achieve the required frequency stability, two opto-electrical feedback loops for on- and off-line channels were coupled to the third opto-electrical loop for CO2 locking. A master DFB laser is locked to the selected CO2 absorption line using a single pass reference cell and a custom feedback loop based on a commercially available equipment (LaseLock 4 Channel). The master DFB stabilization range is around 18 kHz at the interval of 23 s, measured with a frequency comb. The light emitted from the master laser is then fed into the on- and off-line frequency locking loops (phase-lock loops) and it is used to stabilize the beat note of the on-line and off-line channels with respect to the master laser frequency. The offsets read 350 MHz with tunable range of 150 MHz and fixed 10 GHz for ON- and OFF-line signals, respectively.
A radiation hard image sensor HAS2 was chosen for the Juice Monitoring Camera, an important instrument to monitor the status of the JUpiter ICy moons Explorer (JUICE) spacecraft and to take wide-angle images of the Jovian system. To improve the perception, a radiation-hardened, non-organic color filter array (CFA) was deposited on the sensor silicon, with four colors: red, green, blue, and yellow. In this paper we present the results of the radiation tests on these customized image sensors. First, results on the total ionizing dose (TID) test up to 57 krad confirm that both the image sensor and the deposited CFA are resistant to gamma radiation in the measured range. Also, single event effects (SEE) measurements up to LET of 100 MeV·cm2/mg were performed. Three types of events were considered: single event latch-up (SEL), single event upset (SEU), and finally single event functional interruption (SEFI). Since only one SEL was registered on one of the samples at 62.5 MeV·cm2/mg there was no further analysis on this event. Based on the experimental results, the cross sections have been calculated for SEU and SEFI. These results were used as input for the SEE analysis with the CRÈME96 software. The rates of about 10-4 and 10-5 events/device/day were obtained for SEU and SEFI for quiet environment, respectively.
Near-infrared, visible and ultraviolet (NIR-VIS-UV) lidar echo emulator (LEE) was developed to emulate the returns of a spatial lidar at three wavelengths (355 nm, 532 nm and 1064 nm) within the framework of the HOLDON project. It is used to characterize the detection chain formed by a HgCdTe avalanche photodiode (MCT APD), read-out integrated circuit (ROIC), and surrounding electronics, which will be used in a lidar remote sensing of earth's atmosphere. It emulates two echoes in the three wavelengths. A short echo (pulse width <10 ns) with optical power dynamic range between 0.2 nW and 200 nW and a long echo (pulse width of about 270 μs) in range from 0.1 pW to 25 pW. The long echo has two adjustable parameters which allow changing the shape of the pulse (from a ramp to a rectangular pulse). The combination of both echoes allows characterizing the detector over the full 60 dB dynamic range, the target for the detection chain. The system consists of only one laser diode at 1064 nm, a driver, and an arbitrary wave generator (AWG) as seed to create the three waveleghts. The second harmonic generation (SHG) technique was used to generate the beam at 532 nm from the 1064nm laser diode. In this case, a non-linear KTP crystal was used. The sum frequency generation (SFG) technique was used to generate the beam at 355 nm from both previous beams. For the SFG, two Periodic Poled Lithium Niobate (PPLN) crystals were used. To optimize the output at 355 nm the temperature of the PPLN crystals was controlled by means of Peltier elements. A LabView program was designed to control all the configuration parameters of both echoes (long and short) and the laser. The software controls also the optimal temperatures where the highest power is reached at 355 nm. The overall system was assembled as an elegant breadboard for easy transportation. The NIR-VIS-UV LEE offers all the necessary requirements for exhaustive characterization of the lidar detection chains at three wavelengths and high dynamic range.
The LIDAR Echo Emulator (LEE) emulates the returned signals from a Lidar system. As the return signal of a Lidar system depends on the target, this implies the capability of shaping the lasers returns. In brief, long echo corresponds to a natural or diffusive object (canopy, clouds) and short echoes to a hard object like roofs, ground, etc. Such signals are necessary to be simulated to study and validate new detectors and detection systems without developing/procuring entire Lidar system. LEE consists of pulse shaping electronics to drive independently the lasers for each echo and internal detection system to monitor the pulses. The short echoes can be in the range from 5 ns to hundreds of ns and the long echoes from 1 μs up to hundreds of μs. The repetition rates of the developed emulator are from 100 Hz to 10 kHz with limitation that the longest pulse does not exceed 10% of the duty cycle. The power difference between both echoes can be set and is as high as 60 dB. The dynamics of the echoes is better than 50 dB within the 8 ns in the rising/falling edge of the pulse (echo). The output power can be tuned by means of variable attenuators giving a range of the incoming echoes from -35 dBm to -100 dBm. The LEE can also emulate multiple returns with aforementioned dynamics.
KEYWORDS: Image sensors, Commercial off the shelf technology, Cameras, Imaging systems, Optical filters, Image filtering, Signal to noise ratio, Annealing, Image acquisition, Space operations
12 Mpix color commercial-of-the-shelf (COTS) image sensor from CMOSIS was tested with proton radiation. The target mission required an irradiation with protons of energy of 50 MeV and fluences up to 1·1012 p/cm2. Several intermediate steps were introduced to check the behavior of the image sensor. A low-cost test camera was developed to control the image sensors, acquire the images, and monitor the currents and voltages during the tests. Each color was characterized separately according to the EMVA 1288 standard. Such treatment allowed also analysis of the bayer filter deposited on the image sensor surface. Post-radiation characterization revealed that a significant deterioration in the parameter performance was found independently of the pixel color. The most affected parameters were dark current and dark signal nonuniformities (DSNU) which have increased from about one to two orders of magnitude.
The accurate determination of the atmospheric distribution of carbon dioxide (CO2) on planetary scale is a key requirement for setting up modeling tools able to make reliable predictions of Earth climate dynamics which are essential for the understanding of such important issues as climate change and global warming. Nowadays, the concentrations of CO2 are mainly measured in-situ at a number of surface stations that are unevenly distributed over the planet. Air-borne and space-borne missions have the potential to provide a denser and better distributed set of observations to complement those provided by the surface network.
Carbon dioxide (CO2) is the major anthropogenic greenhouse gas contributing to global warming and climate change. Its concentration has recently reached the 400-ppm mark, representing a more than 40 % increase with respect to its level prior to the industrial revolution.
Semiconductor light sources like light emitting diodes (LEDs) or laser diodes (LDs) are the most important light sources for space applications. LEDs are used in the control panels or lightning systems in the spacecrafts and as growth lightning systems in a deep space.
We propose an integrated path differential absorption lidar system based on all-semiconductor laser sources and single photon counting detection for column-averaged measurements of atmospheric CO2. The Random Modulated Continuous Wave (RM-CW) approach has been selected as the best suited to semiconductor lasers. In a RM-CW lidar, a pseudo random sequence is sent to the atmosphere and the received signal reflected from the target is correlated with the original sequence in order to retrieve the path length. The transmitter design is based on two monolithic Master Oscillator Power Amplifiers (MOPAs), providing the on-line and off-line wavelengths close to the selected absorption line around 1.57 µm. Each MOPA consists of a frequency stabilized distributed feedback master oscillator, a bent modulator section, and a tapered amplifier. This design allows the emitters to deliver high power and high quality laser beams with good spectral properties. An output power above 400 mW with a SMSR higher than 45 dB and modulation capability have been demonstrated. On the side of the receiver, our theoretical and experimental results indicate that the major noise contribution comes from the ambient light and detector noise. For this reason narrow band optical filters are required in the envisioned space-borne applications. In this contribution, we present the latest progresses regarding the design, modeling and characterization of the transmitter, the receiver, the frequency stabilization unit and the complete system.
Integrated master-oscillator power amplifiers driven under steady-state injection conditions are known to show a complex dynamics resulting in a variety of emission regimes. We present experimental results on the emission characteristics of a 1.5 μm distributed feedback tapered master-oscillator power-amplifier in a wide range of steady-state injection conditions, showing different dynamic behaviors. The study combines the optical and radio-frequency spectra recorded under different levels of injected current into the master oscillator and the power amplifier sections. Under low injection current of the master oscillator the correlation between the optical and radio-frequency spectral maps allows to identify operation regimes in which the device emission arises from either the master oscillator mode or from the compound cavity modes allowed by the residual reflectance of the amplifier front facet. The quasi-periodic occurrence of these emission regimes as a function of the amplifier current is interpreted in terms of a thermally tuned competition between the modes of the master oscillator and the compound cavity modes. Under high injection current of the master oscillator, two different regimes alternate quasi-periodically as a function of the injected current in the power amplifier: a stable regime with a single mode emission at the master oscillator frequency, and an unstable and complex self-pulsating regime showing strong peaks in the radio-frequency spectra as well as multiple frequencies in the optical spectra.
We study experimentally the dynamic properties of a fully integrated high power master-oscillator power-amplifier emitting at 1.5 μm under continuous wave and gain-switching conditions. High peak power (2.7 W) optical pulses with short duration (~ 110 ps) have been generated by gain switching the master-oscillator. We show the existence of working points at very close driving conditions with stable or unstable regimes caused by the compound cavity effects. The optical and radio-frequency spectra of stable and unstable operating points are analyzed.
High power two-section tapered lasers are promising candidates to generate short optical pulses by Q-switching. The
main advantage of these devices is that high peak optical power can be generated by using a low excitation current in the
ridge-waveguide section. In this work we analyze the Q-switching dynamics of two-section tapered lasers by means of a
simplified three-rate-equation model and we compare the results with measurements in 1060 nm DBR multi/section
tapered lasers. The experiments and simulations show similar trends with repetition frequency, modulation signal
amplitude and bias conditions. The effect of the driving conditions on the peak power and pulse duration is analyzed.
100 ps pulses with 4.2 W peak power are obtained at 900 MHz repetition frequency.
Highly efficient 670 nm-tapered lasers with a vertical divergence of 31° (FWHM) will be presented. The devices are
based on a GaInP single quantum well embedded in AlGaInP waveguide layers. Compared to previously reported
material, the structure has an improved material quality with a transparency current density jtr = 165 A/cm2, an
internal efficiency ηi = 0.75, small internal losses αi = 1.2 cm-1, and a good temperature stability with T0 = 120 K.
2 mm long tapered lasers were fabricated in a standard process, using reactive ion etching for the index-guided
structures and ion implantation for the definition of the contact window in the tapered section. The properties of
devices with 500 μm or 750 μm long ridge waveguide (RW) section and a flared section with 3° or 4° taper angle
will be compared. In CW-operation an output power up to P = 1 W with a conversion efficiency of 30% and a beam
propagation ratio M2 (2nd moments) smaller than 2.3 were obtained. In pulsed mode up to 3.3 W output power was
measured.
650 nm tapered laser diodes with nearly diffraction limited beam quality are requested for laser display applications.
In this paper, results for 2 mm long 650 nm tapered lasers diodes with different lateral geometries will be presented.
The vertical structure is based on a 5 nm thick InGaP single quantum well embedded in AlGaInP waveguide and n-
AlInP and p-AlGaAs cladding layers. The ridge waveguides of lengths LRW = 200 μm, 300 μm, 500 μm, and 750 μm
were fabricated using selective etching. The contact window for the tapered section was defined applying ion
implantation. Devices with a taper angle of 4° were manufactured. The facets were passivated. The rear side was
high reflection coated and the taper side anti reflection coated. The devices were mounted p-side down on CVDdiamond
heat spreaders and standard C-mounts.
All devices reached a maximal output power larger than 1 W. They had a threshold current density of about
700 A/cm2 and a slope efficiency of 0.8 W/A. The best conversion efficiency was 20%. The devices with the shortest
RW-length LRW = 200 μm had the best beam quality (beam waist width 7 μm, far field angle 8.8°, 85% of the
emitted power in central lobe, M2 of 1.3 (all values measured at 1/e2-level)) at P = 1 W. The beam quality for devices
with longer RW-section deteriorates up to M2 = 4.4 for a LRW = 750 μm laser.
High-brightness narrow line-width 1060 nm tapered lasers with an internal distributed Bragg reflector were realized.
The devices reach a maximal output power of 12 W with a narrow spectral line-width below 40 pm (95% power). A
nearly diffraction limited beam quality was measured up to a power of 10 W. The vertical structure is based on an
InGaAs triple quantum well (TQW) active region embedded in a 4.8 μm broad AlGaAs super large optical cavity.
This leads to a narrow vertical divergence of 15° (FWHM). Tapered devices were processed a total length of 6 mm
consisting of 2 mm long ridge waveguide (including 1 mm DBR mirror) and 4 mm tapered sections. A full taper
angle of 6° was manufactured. The input currents to both sections can be independently controlled. The devices had a
conversion efficiency of about 50%. A first reliability test showed failure-free operation at 5 W without a
deterioration of the beam quality and the spectral properties.
High-brightness 650 nm tapered lasers with output powers up to 1 W and nearly diffraction limited beam quality at
500 mW were realized. The vertical structure is based on an InGaP single quantum well (SQW) embedded in
AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers. The tapered structure consists of a 750 μm
long ridge waveguide section and a 1.25 mm long flared section. Taper angles of 2°, 3° and 4° were manufactured.
At 15°C, the devices achieve 1 W at an operating current below 2 A in CW operation. The conversion efficiency is
about 20%. At 500 mW output power a nearly diffraction limited beam quality with a beam propagation ratio of
about 1.5 was measured.
The reliability was studied in a long-term test for five tapered diodes at 250 mW over 1,000 h and than at 500 mW
over 2,000 h. All diodes survived this test. The beam quality remains nearly stable over the complete reliability test.
High hydrostatic pressure can be used for wavelength tuning of semiconductor laser diodes in a wide spectral range. Coupling the laser with external grating leads to wavelength tuning within the gain spectrum (i.e. in a narrower range than with pressure) but allows for a narrow emission line and nearly continuous tuning (mode-hop free if anti-reflecting coating is applied). Here we demonstrate a combination of pressure and external-resonator tuning for the GaInNAs laser emitting at 1343 nm at ambient conditions. Using the specially designed liquid pressure cell working up to 20 kbar we shift the emission down to 1170 nm while the external grating (used in Littrow configuration) allows for fine tuning in the ~10 nm range (at each pressure).
We demonstrate wide-range wavelength tunability of high-power laser diodes emitting at 660 nm, 808 nm, and 980 nm. Pressure shifts of the emission wavelength are due to the increase of bandgaps of III-V semiconductors under pressure with the rate of about 10 meV per kbar. For the 980 nm InGaAs/GaAs laser the threshold currents and the quantum efficiencies remain constant with pressure which allows for the constant operating current and the emitted power in the full tuning range. For 808 nm GaAs/AlGaAs and 660 nm InGaP/AlGaInP lasers there is an increase of threshold currents with pressure related to the direct-indirect crossover in the conduction band of AlGaAs and AlGaInP. This limits the tuning range unless we operate the laser at lower temperature. We designed the pressure cell with Peltier cooling allowing for independent control of temperature down to 0 Celsius and pressure up to 20 kbar. This device allows for the tuning of 980 nm laser down to 840 nm, 808 nm laser down to 720 nm, 660 nm laser down to 620 nm.
Two InGaP/AlGaInP lasers (emitting at 660 nm and at 690 nm) and one GaAs/AlGaAs laser (emitting at 780 nm) have been studied under hydrostatic pressure up to 20 kbar and at temperatures from 240 K to 300 K. The power-current characteristics and the spectra have been measured in the specially designed pressure cell. The emission spectra shifted in agreement with the pressure/temperature variation of the bandgaps in active layers of the lasers. Since at high pressure the Γ-X separation in the conduction band is strongly reduced (both in AlGaInP and in AlGaAs) the dominant loss mechanism of the lasers is the carrier leakage to X minima in the claddings. This, in turn, leads to high sensitivity of threshold currents to temperature. The dependence of threshold currents on pressure and on temperature is in good agreement with the simple phenomenological analysis taking into account the carrier leakage and the radiative and nonradiative recombination. Good description of the pressure and temperature variation of the threshold currents is obtained using three adjustable parameters. Our fits indicate that the dominant contribution to electronic leakage is drift rather than diffusion. These results are important for the application of pressure/temperature tuning of laser diodes in the 600-800 nm range. In particular, we were able to turn red laser diodes into yellow (emitting below 600 nm) and infrared 780 nm lasers into bright red. By simultanous control of pressure and temperature it is possible to obtain constant emission power of the lasers in the full tuning range (at a fixed operating current).
Wide-range wavelength tunability is demonstrated for commercial high-power laser diodes emitting at 980 nm, 830 nm, and at 808 nm. High pressure shifts the emission wavelength of the lasers due to the increase of bandgaps in the active layers with the rate of about 10 meV per kbar. For the 980 nm InGaAs/GaAs laser the threshold currents and the differential efficiencies remain constant with pressure which allows for the constant operating current and the emitted power in the full tuning range. For 830 nm and 808 nm GaAs/AlGaAs lasers there is an increase of threshold currents with pressure related to the leakage through X minima in the conduction band of AlGaAs. This limits the tuning range unless we operate the laser at lower temperature. We designed the pressure cell with Peltier cooling allowing for independent control of temperature down to 0 Celsius and pressure up to 20 kbar. The laser beam passes through the sapphire window or through the multi-mode fiber. Our device allows for the tuning of 980 nm laser down to 840 nm, 830 nm laser down to 745 nm, and 808 nm laser down to 720 nm. We were able to keep the output power fixed in the full tuning range: 300 mW for the 980 nm laser and 400 mW for the 830 nm and 808 nm lasers.
Direct bandgap of most III-V semiconductors (AlGaAs, InGaAs, InGaP, InAs) increases with hydrostatic pressure at the rate of about 10 meV per kbar. Thus the emission wavelength of semiconductor lasers shifts to the blue under the application of high pressure. We demonstrate that this effect can be used for wavelength tuning of laser diodes in a very wide spectral range. Using the specially designed liquid pressure cell working up to 20 kbar the 1550 nm laser was tuned down to 1270 nm, the 1300 nm laser was tuned down to 1100 nm, and the 980 nm laser was tuned down to 840 nm. The emitted light passes through the sapphire window or through the fiber directly coupled to the laser. The threshold current and the quantum efficiency for the 980 nm laser remained constant with pressure, for the two other lasers the thresholds decreased with pressure. Thus we obtained the constant emission power in the full tuning range. We hope that this compact device will find applications as a tool for characterization of some optical network devices or parts of optical transmission lines.
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