In this work we present experimental results on the radiation hardness of InP based Photonic Integrated Circuits (PICs) fabricated in a multi-project wafer through an open access platform. The PIC includes different types of building blocks: semiconductor optical amplifiers (SOA), waveguides, electro-optical phase modulators (EOPM), multi-mode interference couplers and photodiodes (PD). Three chips were submitted to gamma radiation up to 106 krad in four steps and another three to proton radiation up to 1.5 x 1011 p+/cm2. The effects of the radiation were evaluated by measuring on-wafer the power-current-voltage (P-I-V) characteristics and emission spectra of several tunable four-section Distributed Bragg Reflector (DBR) lasers. The P-I-V characteristics measured before radiation showed kinks corresponding to modal changes and stable regions with single longitudinal mode emission. After the radiation some changes were observed: i) the value of the threshold current in some of the lasers, either increasing or decreasing its value; ii) slightly different emission wavelength and evolution of the modal jumps with current. However, these changes were not systematic in all devices, and they were not dependent of the radiation dose. In consequence, they were attributed to the lack of reproducibility of the on-wafer measurements and the high sensitivity of the modal selection to temperature changes in these DBR lasers. In conclusion, the building blocks of the PICs involved in the DBR laser (SOA, EOM, waveguide, PD) can be considered radiation hard up to the levels in the tests, which were typical for space applications.
An experimental and theoretical analysis of the effect of optical injection on optical frequency combs generated by gain-switching a single mode laser is performed. Combs with a frequency separation fR in the GHz range are generated by gain-switching a discrete mode laser (DML). The effect of optical injection on the comb has been analyzed for a wide range of the injected power and detuning of the injected master laser from the DML frequency. A rich variety of nonlinear behaviours is found when the injected power is increased for a fixed value of the detuning. At low injected powers two combs due to the gain-switched laser and the injected field are obtained with the same frequency spacing fR. As the injected power is increased, the comb from the gain-switched laser locks to the master laser. In the parameter space spanned by detuning and optical injection strength, we obtain several regions of locked combs with a tongue shape around detuning values given by multiples of the frequency spacing of the comb fR. When injected power is further increased combs with frequency spacing equal to rational fractions of fR are obtained. We also obtain irregular behaviour with low values of the ratio between the intensity of comb lines and noise level. Numerical simulations are in very good agreement with the experimental results.
We have evaluated the performance for space applications of commercial off-the-shelf fiber coupled optical switches with no-moving parts, based on different technologies. The technical requirements of several space applications of optical switches were defined. After the technology selection, a tradeoff was performed to select the final optical switches to be tested, which are based on three technologies (Magneto-Optic MO, Bulk Electro-Optic B-EO, and Waveguide Electro-Optic W-EO) and fabricated by four different manufacturers. Other potential technologies (acoustooptic, liquid crystal, thermo-optic, micro/nano photonic waveguides) were not selected due to the lack of commercial products. A test campaign was carried out, consisting of thermal vacuum cycles, mechanical tests (vibration and shocks) and radiation tests (gamma radiation). The main performance parameters were the insertion loss, crosstalk, and switching speed. After the final electro-optical characterization, a destructive physical analysis was made to some optical switches. The results of the tests indicated that B-EO and MO technologies are excellent candidates for the analyzed space applications. They respond very well under typical space conditions as radiation, vibration, shocks and thermal vacuum; B-EO technology presents lower switching time but its crosstalk is worse. WG-EO technology is very fast, but a mechanical failure in one device was observed, the insertion losses are very high and the crosstalk is very low.
This paper presents the results obtained in the frame of an ESA-funded project called “Screening and Preevaluation of Shortwave Infrared Laser Diode for Space Application” with the objective of verifying the maturity of state of the art SWIR DFB lasers at 2.1μm to be used for space applications (mainly based on the occultation measurement principle and spectroscopy). The paper focus on the functional and environmental evaluation test plan. It includes high precision characterization, mechanical test (vibration and SRS shocks), thermal cycling, gamma and proton radiation tests, life test and some details of the Destructive Physical Analysis performed. The electro-optical characterization includes measurements of the tuning capabilities of the laser both by current and by temperature, the wavelength stability and the optical power versus laser current.
Nowadays the interest in high power semiconductor devices is growing for applications such as telemetry, lidar system or free space communications. Indeed semiconductor devices can be an alternative to solid state lasers because they are more compact and less power consuming. These characteristics are very important for constrained and/or low power supply environment such as airplanes or satellites.
Lots of work has been done in the 800-1200 nm range for integrated and free space Master Oscillator Power Amplifier (MOPA) [1]-[3]. At 1.5 μm, the only commercially available MOPA is from QPC [4]: the fibred output power is about 700 mW and the optical linewidth is 500 kHz.
In this paper, we first report on the simulations we have done to determine the appropriate vertical structure and architecture for a good MOPA at 1.58 μm (section II). Then we describe the fabrication of the devices (section III). Finally we report on the optical and electrical measurements we have done for various devices (section IV).
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.
We present experimental results on a three-section master oscillator power amplifier at 1.57 μm to be applied in an integrated path differential absorption LIDAR system for column-averaged atmospheric CO2 measurements. The application requires high power and good beam quality together with spectral purity and modulation capacity to be used in a random modulation CW LIDAR system. The device consists of a distributed feedback laser acting as master oscillator, a bent modulator section and a tapered optical amplifier section with a tilted front facet to avoid coupled cavity effects. The modulator section acts as an absorber or amplifier when driven at zero or positive bias. Devices with different geometries and epitaxial structures were fabricated and characterized, presenting CW output powers higher than 350 mW and stable single mode emission. At the frequency required by the application (12.5 MHz) a high optical modulation amplitude and extinction ratio were achieved.
Fully integrated semiconductor master-oscillator power-amplifiers (MOPA) with a tapered power amplifier are attractive sources for applications requiring high brightness. The geometrical design of the tapered amplifier is crucial to achieve the required power and beam quality. In this work we investigate by numerical simulation the role of the geometrical design in the beam quality and in the maximum achievable power. The simulations were performed with a Quasi-3D model which solves the complete steady-state semiconductor and thermal equations combined with a beam propagation method. The results indicate that large devices with wide taper angles produce higher power with better beam quality than smaller area designs, but at expenses of a higher injection current and lower conversion efficiency.
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 experimentally investigate high-frequency microwave signal generation using a 1550 nm single-mode VCSEL subject to two-frequency optical injection. We first consider a situation in which the injected signals come from two similar VCSELs. The polarization of the injected light is parallel to that of the injected VCSEL. We obtain that the VCSEL can be locked to one of the injected signals, but the observed microwave signal is originated by beating at the photodetector. In a second situation we consider injected signals that come from two external cavity tunable lasers with a significant increase of the injected power with respect to the VCSEL-by-VCSEL injection case. The polarization of the injected light is orthogonal to that of the free-running slave VCSEL. We show that in this case it is possible to generate a microwave signal inside the VCSEL cavity.
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.
High-power (more than 500 mW) and high-speed (more than 1 Gbps) tapered lasers at 1060 nm are required in freespace
optical communications and (at lower frequencies of around 100 MHz) display applications for frequency
doubling to the green. On a 3 mm long tapered laser, we have obtained an open eye diagram at 1 Gbps, together with a
high extinction ratio of 11 dB, an optical modulation amplitude of 530 mW, and a high modulation efficiency of 13
W/A. On a 4 mm-long tapered laser, we have obtained an open eye diagram at 700 Mbps, together with a high
extinction ratio of 19 dB, a high optical modulation amplitude of 1.6 W, and a very high modulation efficiency of
19 W/A. On a 6 mm-long tapered laser, we have obtained a very high power of 5W CW and a very high static
modulation efficiency of 59.8 W/A.
We present results on the electrical characterization of commercial fiber pigtailed 1. 55 μm 2.5 Gb/s VCSEL based on
InAlGaAs active region, tunnel junction (TJ), air-gap aperture and InAlGaAs/InAlAs mirrors. The current-voltage (I-V)
characteristics were measured and the results were fitted to the analytical expressions of an equivalent circuit considering
the TJ in series with the active junction and a series resistance. Carrier capture/escape effects were considered in order to
account for the reduced value of the drop in the measured differential resistance at threshold. The electrical parameters of
both junctions were determined, showing that the TJ was responsible for most of the voltage drop at threshold. High
frequency electrical impedance measurements were used to determine internal parameters as well as the role of external
parasitics. The results were analyzed using a small signal equivalent circuit which includes the TJ, carrier capture/escape
effects, the cavity parasitics, and the electrical access. A good agreement between the experimental and the equivalent
circuit impedances at different bias was obtained by considering the differential resistances of the active and tunnel
junctions extracted from the I-V characteristics, yielding reasonable values of the dynamic time constants and of the
recombination coefficients.
A two-dimensional self-consistent laser model has been used for the simulation of the facet heating of red emitting
AlGaInP lasers. It solves in the steady-state the complete semiconductor optoelectronic and thermal equations in the
epitaxial and longitudinal directions and takes into account the population of different conduction band valleys. The
model considers the possibility of two independent mechanisms contributing to the facet heating: recombination at
surface traps and optical absorption at the facet. The simulation parameters have been calibrated by comparison with
measurements of the temperature dependence of the threshold current and slope efficiency of broad-area lasers. Facet
temperature has been measured by micro-Raman spectrometry in devices with standard and non absorbing mirrors
evidencing an effective decrease of the facet heating due to the non absorbing mirrors. A good agreement between
experimental values and calculations is obtained for both devices when a certain amount of surface traps and optical
absorption is assumed. A simulation analysis of the effect of non absorbing mirrors in the reduction of facet heating in
terms of temperature, carrier density, material gain and Shockly-Read-Hall recombination rate profiles is provided.
Tapered semiconductor lasers have demonstrated both high power and good beam quality, and are of primary interest for
those applications demanding high brightness optical sources. The complex non-linear interaction between the optical
field and the active material requires accurate numerical simulations to improve the device design and to understand the
underlying physics. In this work we present results on the design and simulation of tapered lasers by means of a Quasi-
3D steady-state single-frequency model. The results are compared with experiments on Al-free active region devices
emitting at 1060 nm. The performance of devices based on symmetric and asymmetric epitaxial designs is compared and
the influence of the design on the beam properties is analyzed. The role of thermal effects on the beam properties is
experimentally characterized and analyzed by means of the numerical simulations. Tapered lasers with separate electrical
contacts in the straight and tapered sections, based on symmetrical and asymmetrical epitaxial designs are also presented
and analyzed.
High-power (more than 500 mW) and high-speed (more than 1 Gbps) tapered lasers at 1060 nm are required in freespace
optical communications and (at lower frequencies of around 100 MHz) display applications for frequency
doubling to the green. On a 4 mm-long tapered laser, we have obtained an open eye diagram at 700 Mbps, together with
a high extinction ratio of 19 dB, a high optical modulation amplitude of 1.6 W, and a very high modulation efficiency of
19 W/A. On a 3 mm long tapered laser, we have obtained an open eye diagram at 1 Gbps, together with a high
extinction ratio of 11 dB, an optical modulation amplitude of 530 mW, and a high modulation efficiency of 13 W/A.
High-brightness diode lasers at 1060 nm are useful in display applications (to provide green light by frequency doubling)
and in free-space optical communications. On Al-free active region laser structures, we have obtained low optical losses
of 0.9 cm-1, a high internal quantum efficiency of 98% and a low transparency current density of 64 A/cm2. On uncoated
broad-area lasers (2 mm x 100 μm) at 20°C CW, we have obtained a high maximum wall-plug efficiency of 66%, and an
optical power higher than 3W per facet.
Based on these good results, we have realized 3.7 mm long gain-guided tapered lasers, delivering a high power of 3W at
10°C CW, together with a low M2 of 3 at 1/e2 and a high maximum wall-plug efficiency of 57%.
We have also realized separate electrode lasers, in which the ridge and tapered sections are biased separately. In this
configuration, the current through the ridge section is only a few tens mA while the current on the tapered section is
several Amps. This allows to control a large output power with only a small change of the ridge current. By moving the
ridge current from 0 to 50 mA, keeping a constant 4A current through the tapered section, we have obtained a large
change of the output power from 0.09 W to 2.6 W, which corresponds to a high modulation efficiency of 50 W/A under
static operation. In dynamic regime, the separate electrode laser can be operated at 700 Mbps, showing a high
modulation efficiency of 19 W/A, optical modulation amplitude of 1.6 W and extinction ratio of 19dB [1]. These
modulation efficiencies are, to our knowledge, record values.
We have realized an asymmetric laser structure at 975 nm, based on an Al-free active region. The quantum wells are
located near the p-cladding, so that most of the waveguide is n-type, which allows for reduced optical losses and series
resistance. On uncoated broad-area lasers, we have obtained very low optical losses of 0.4 cm-1, together with a high
internal quantum efficiency of close to 100%. Based on this structure, we have realized index-guided tapered lasers
delivering 1.3W CW, with a narrow far-field angle of 2.5° (FWHM) and 5.8° (at 1/e2) in the slow axis, and a good beam
propagation ratio M2 = 1.6. The lasers reach a high maximum wall-plug efficiency of 56%. These tapered lasers deliver
a maximum power of 1.5W CW with M2 < 3. The results on the asymmetric structure are compared with those of a
symmetric laser structure.
Several types of high-brightness near-infrared tapered quantum well and quantum dots tapered lasers are reviewed and
compared. Recent developments include record-high brightness quantum well tapered lasers at 810 nm and 975 nm (up
to 8.3 W CW, diffraction limited), high wall-plug efficiency gain-guided and index-guided tapered lasers (up to 57%),
narrow slow axis far-field (2.5° FWHM) index-guided tapered lasers at 975 nm, wavelength-stable (down to
0.09 nm/K), high-brightness quantum dots tapered lasers at 920 and 975 nm, and quantum dots tapered laser bars (up to
14 W CW) at 920 nm, with narrow far-field in the slow axis (3° FWHM).
KEYWORDS: Continuous wave operation, Near field, High power lasers, Laser vision correction, Near field optics, Transparency, Internal quantum efficiency, Indium gallium phosphide, Semiconductor lasers
We have developed high-power lasers, which are based on an Al-free active region at 915 nm. The laser structure has very low internal losses of 0.5 cm-1, a very low transparency current density of 86 A/cm2, and a high internal quantum efficiency of 86%. Based on these good results, we have realised narrow-aperture, index-guided tapered lasers which deliver 1 W CW with and M2 beam quality factor of 3.0 using both the 1/e2 and standard-deviation methods. We have also fabricated index-guided tapered lasers with a Clarinet shape, which deliver 0.65 W CW with an M2 beam quality factor of less than 1.5 at 1/e2, and less than 2.5 using the standard deviation method.
A comparative simulation study of the optical output characteristics of tapered lasers with different epitaxial structure was performed. The simulation model self-consistently solves the steady state electrical and optical equations for the flared unstable resonator and was previously backed by experiments on one of the simulated structures. Three different epitaxial designs emitting at 975 nm were analyzed: a standard single quantum well symmetrically located in the confinement region (s-SQW), a double quantum well also symmetrically located (s-DQW) and an asymmetrically located double quantum well (a-DQW). The symmetric structures have different confinement factor but a similar ratio between the active layer thickness and the confinement factor, dQW/Γ, while the a-DQW has similar confinement factor than the s-SQW, but double dQW/Γ. A better performance is predicted for the a-DQW design, reaching considerably higher output power with good beam quality. The results are interpreted in terms of a lower density of power in the QW in the case of the a-DQW design, thus delaying to higher output power the onset of the non-linear effects that degrade the beam quality. The role of dQW/Γ as a figure of merit for high brightness tapered lasers is emphasized.
High indium content InGaAs/GaAs laser structures have been grown on (111)B GaAs substrates by Molecular Beam Epitaxy (MBE). The laser devices showed room temperature CW emission, low threshold current densities and emission wavelength up to 1100 nm. The influence of the internal piezoelectric field on the emission properties is studied theoretical and experimentally. A self consistent model was developed in order to simulate the gain and spontaneous emission spectra. Modeling results were compared with measurements of the spontaneous emission spectra, and a good qualitative agreement was obtained. By analyzing these results, we conclude that the piezoelectric field is not completely screened out even for high injection currents, and that the screening level is strongly dependent on the In content. Spontaneous emission measurements in two different configurations (top and edge emission) were compared, yielding similar results for a range of experimental conditions.
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