Thesis overview
Introduction
There we consider the thermal properties of these lasers and how the design of the laser can be improved to mitigate the thermal effects. The orange dots are visual representations of the measurement of a symbol when corrupted with phase noise.
Applications of semiconductor lasers
The green dashed lines show the decision boundary of the detection system used to differentiate between the symbol for 001 and the adjacent symbols. Optical isolation is necessary to keep the lasing frequency stable and the output of the laser stable [21].
Semiconductor laser structures
The lasing frequency of the laser can be changed in a continuous fashion either by changing the injection current [13], or by changing the length of the lasing cavity as is done in MEMS VCSELs [25]–[27]. The phase section: in a Fabry-Perot laser, this corresponds to the entire length of the waveguide.
Platform parameters
Current flows from the top p-contact to the two n-contacts on either side of the laser. The mode gap resonator is designed by “engineering” the band edges of the grating as described in [39].
Quantum noise control review
The thickness of the quantum noise control layer (tQNCL) is used to change the confinement factor between in different materials. By decreasing the confinement factor of the optical mode in the quantum wells, the spontaneous emission into the lasing mode is decreased(Wspon.(l) ∝Q−tot1).
Output power optimization of quantum noise controlled lasers
The temporal differential gaing(equal toWspon.(l) ) is proportional to the confinement factor in the quantum well layer of the resonator. Numerical simulations (Fig- ure 2.5b) indicate that the confinement factor in the quantum wells is proportional to that of the InGaAsP.
Final platform design parameters
ForQext =Qi, often known as critical coupling, the output power is reduced by up to 25% of the optimal value. ForQext =Qi, often known as critical coupling, the output power is reduced by 25% of the optimal value.
Conclusions
Results from the 2D simulation of the temperature and heat flux in the laser are shown in Figure 3.4a. Recasting Equation 5.9 in terms of the bandwidth of the chirp(B)and the location of the reflector(τj)and the optical delay(τ= ν/ξ).
Thermal properties of high-Q silicon/III-V lasers
Thermal characteristic of semiconductor lasers
The characteristic temperature describes how the threshold current is affected by the temperature of the laser is given by the empirical value ofT0that best describes. 3.1) The above threshold characteristic temperature describes how the slope efficiency of the laser is affected when the temperature increases.
Thermal challenges for heterogeneously integrated lasers
When current flows through the laser, heat is generated in the active medium causing the temperature of the laser to rise proportionally to the thermal impedance between the heat sources and the temperature controlled stage. This increases the temperature of the quantum wells compared to the case of an all III-V laser where the thick buried oxide layer does not exist.
Thermal impedance of high-coherence silicon III-V lasers
The simulations of the temperature increase in the laser when considering the heat generated from the n-layer are shown in Figure 3.4b when the laser is operated at 200 mA. In this case, the generated heat must to be funneled through the narrow 2.5 µm waveguide, increasing the temperature of the quantum wells.
Flip-chip bonding for decreased thermal impedance
The temperature profile of the flip-chip bonder during the bonding process is shown in Figure 3.9. Unless otherwise noted, the laser is driven with 200 mA of current. a) 2D thermal simulation of the laser when it is flip-chip bonded to an AlN submount.
Flip-chip bonding results
The dashed line shows the average property across the bar before and after flip-chip bonding. As was observed in the FP lasers, many of the lasers observed a markedly improved slope efficiency, allowing them to reach much larger output powers.
Flip-chip bonding limitations
Conclusions
Reflectors at±τj appear at the same location in the Fourier transform of the acquired signal. At the edge of the depth of field (∆z = ±25 mm), the resolution decreases by approximately a factor of.
Feedback sensitivity of high-coherence Si-III/V lasers
Optical feedback sensitivity review
The reflectivity of the mirror as a function of the mirror length is found in the same simulation and summarized in Figure 4.3. If one assumes that the reflectivity of the mirrors is given by R(νL)= tanh2κL, the external quality factor of the resonator can be overestimated by as much as a factor of ten (10), as illustrated in Figure 4.3.
Feedback sensitivity of high coherence Si/III-V lasers
The effective isolation of a laser can therefore be compared to that of its all III-V counterpart through. In applications where a single isolator is necessary, a QNCL with an SiO2 thickness of approximately 150 nm can operatewithout any isolatorobviating the need for the magneto-optic material in the photonic platform.
Measurement of the feedback sensitivity of QNCL lasers
The fringe visibility of the MZI is characterized as the magnitude of the reflection is increased. The black line shows the measured intensity modulation response of the laser as a visual guide.
Conclusions
Swept source 3D imaging has become an important tool for many scientific and industrial applications due to its high dynamic range, and high axial resolution [80]. 3D imaging systems based on SSOCT typical employ beam steering to acquire volumetric 3D imaging characterizing each lateral location in isolation.
Swept frequency reflectometry
The finite duration of the chirp can be modeled mathematically by a rect function in the time domain. The axial resolution is given by the optical bandwidth of the chirp and cannot be improved by increasing the signal to noise ratio.
Optical sources for swept frequency reflectometry
The precision of the measurement along the illumination axis becomes dependent on the signal to noise ratio and limited not by the bandwidth of the chirp. More recently, MEMS VCSELs have been demonstrated with optical chirp repetition rates between T ≈ 1 and 10 µs limited by the mass and stiffness of the MEMS mirror.
The linearly swept frequency laser
A phase-lock loop is used to ensure that the frequency and phase of the signal from the MZI track that of the electrical reference oscillator. While the described system utilizes a feedbackloop to ensure the linearity of the sweep, this requirement can be relaxed by using afeed-forward system.
Linearly swept frequency lasers at 850 nm and 1064 nm
The two sidebands on either side of the main peak are artifacts from the finite electrical bandwidth of feedback loop. The phase noise limited regime is apparent for delays±100 ps away from the delay of the MZI.
Conclusions
A system diagram of a typical configuration of the tomographic imaging camera (TomICam) is shown in Figure 6.2. Contrary to the case of the intensity modulator, only the reference field of the interferometer is modulated.
3D imaging with the tomographic imaging camera
High resolution 3D imaging
In Chapter 7, we show how interferometric consistency across the entire field of view can be used to digitally refocus the image far beyond the conventional depth of the field of the lens. The voxels indicate the lateral (determined by the focusing optics) and depth resolution (determined by the chirp bandwidth of the optical source) of the imaging system. b) The proposed imaging system whereby individual depth slices, shown in in blue, are acquired one at a time with a standard digital sensor.
TomICam principle of operation
The electric field out of the beam expander in the presence of an intensity modulator is given by:. Since the modulation frequency is electronically controlled, its value is only limited by the bandwidth of the mod- ulator, typically in the GHz range.
Comparison between different 3D imaging modalities
For static scenes, where N can be assumed in a separate measurement, one only needs two measurements with θmod = 0, π/2, each estimating the in-phase and in-quadrature components of the beat signal. Just as for the case of time domain OCT, this poses a stringent path- length matching requirement on the reference optics, which can burden the design of the imaging system and cannot be realized for cameras with ranges approaching a meter.
Imaging optics
As will be shown in Chapter 7, we will use the relaxed path length matching requirement to greatly extend the depth of field beyond that of the lens using holographic reconstruction. Within the depth of field of the imaging lens, the wavefront from scatterers can be considered to be a Gaussian beam with constant phase and a radius equal to the lateral resolution of the image.
Signal to noise ratio for shot noise limited measurements
An important trade-off between lateral resolution and depth of field exists in any imaging configuration. As one increases the lateral resolution, the minimum separation between two resolvable reflectors δx decreases, but the depth of field decreases as(δx)2.
Compatible modulation formats
We choose Im(AM) = 2Im to ensure that the average power received from the mirror is the same as in the case of the intensity modulator. 2 times larger than the case of acquiring TomICam with an intensity modulator, resulting an increase of 3 dB of the SNR.
Experimental results at 850 nm
This makes the SNR of the TomICam system identical to that of a high-speed photodetector. We show tomographic measurements from three different depth slices in Figure 6.7 (a-c) corresponding to the depth of the coin, the bevel of the coin, and the paper.
Experimental results at 1064 nm
A conventional imaged, acquired with coherent illumination, and a sketch of the setup are shown in Figure 6.9. Each camera frame is acquired in 1.8 ms matching the sweep duration of the DFB laser.
Conclusions
This insight has also been applied to OCT imaging systems to image beyond the depth of field of the focusing optics [114]. This demonstration shows that a six fold increase in the depth of field of the is possible by applying a filter to the measured field profile.
Imaging beyond the depth of field
The effects of the finite resolution of an optical system are readily observable by simulating a Siemens star target [111]. It can be shown that propagation in free space of the electric field located at a distance∆z = z− fo.
Computationally refocusing images acquired with TomICam
The advantage of the TomICam over other forms of depth resolved microscopy is that the system acquires a hologram at a particular depth location in a small number (three to four frames) of successive frames limiting the stability requirement of the system.
Experimental results with a resolution target
To explore the limit of digital we place the resolution, we placed the resolution target 383 mm away from the plane of focus of the imaging system. As shown in Figure 7.8, even more than 15×away from original depth of field, digital reconstruction is able to adequately refocus the image and resolve many of the features of the resolution target.
Experimental results for retro-reflective polystyrene beads
Figures 7.7(a-c) show the results of applying the digital refocusing technique when the target is placed between±50 mm and±168 mm away from the plane of focus. In contrast, the letterT is nearly unrecognizable without refocusing because it is located at a position≈ 6× away from the depth of field.
Conclusions
Favre, “Theoretical analysis of external optical feedback on DFB semicon- ductor lasers,”IEEE Journal of Quantum Electronics, vol. Schatz, “Longitudinal spatial instability in symmetric semiconductor lasers due to spatial hole burning,” IEEE Journal of Quantum Electronics, vol.