Acknowledgement
This Master thesis was accomplished within the Erasmus Mundus Joint Master Degree “Photonic Integrated
Circuits, Sensors and NETworks (PIXNET)”.
Coordinating Institution: Scuola Superiore di Studi Universitari e di Perfezionamento Sant'anna
Partners
Osaka University
Aston University
Technische Universiteit Eindhoven
Project Data
Start: 01-09-2017 - End: 31-08-2022
Project Reference: 586665-EPP-1-2017-1-IT-EPPKA1-JMD-MOB
EU Grant: 3.334.000 EUR
Website: http://pixnet.santannapisa.it
Programme: Erasmus+
Key Action: Learning Mobility of Individuals
Abstract - We investigate the influence of integrated booster semiconductor optical amplifier (SOA) on the optical power, spectra, side mode suppression ratio (SMSR) and linewidth of distributed Bragg reflector (DBR) lasers in InP platform. This thesis includes experimental characterization as well as mathematical modelling to understand the linewidth broadening effect under different current conditions in booster SOA.
Index Terms— distributed Bragg reflector laser, Indium Phosphide, photonic integrated circuit, semiconductor lasers, semiconductor optical amplifiers.
I. INTRODUCTION
IGHT Amplification by Stimulated Emission of Radiation (laser) was only invented sixty years ago, but in recent years has become an integral part of numerous fields. The first solid state laser, i.e., the ruby laser was demonstrated by H. Maiman in 1960 [1] and within the course of time, several other types of lasers such as gas lasers, dye lasers, chemical lasers and metal vapor lasers were invented and used for specialized research or industrial purposes. Currently the most widely used type of laser is the semiconductor laser. They are so popular namely because of electrical pumping, high efficiency, compact size, and broad emission range of wavelength [2]. These lasers are used for high-speed telecommunication and data-communication [3], medical imaging and bio-sensing [4] structural health monitoring of constructions [5], RF signal generation for radio over-fiber and wireless communication [5, 6], metrology and light detection and ranging (LiDAR) [7], quantum optics for secure communication links [8], optical clocks [9], laser based spectroscopy [10], sensing of strain, pressure and temperature [11] and many more.
The mentioned applications also require additional components to form a functioning device. For instance, in a typical transceiver circuit consists of a laser, modulator and a photodetector. With photonic integration, many necessary components for specific functionality, such as modulators and photodetectors on the same semiconductor chip. This allows several advantages such as mechanical robustness, high volume manufacturing, reduced footprint and energy efficiency. A crucial step in the development of InP based integration technology was the introduction of generic integration process [12], which paved the possibility for a broad range of functionalities to be realized with a small set of building block components. InP-based monolithic integration allows the most comprehensive set of photonic functionalities such as quantum well lasers and modulators, detectors as well as a range of passive components for creating interferometers, combiners and modulators.
For laser transmitters to be used in coherent communication systems, spectroscopy, sensing systems, metrology and LiDAR systems, where the frequency-noise directly impacts the system performance, there are very stringent requirements. The power of optical transmitters must be high to be detected after the losses suffered on the transmission. Low noise is also a crucial requirement in any kind of system involving optical mixing, because the noise of the laser will directly affect the fidelity of the generated signals. Thus low noise and high power lasers are in huge demand for a wide range of applications. The required linewidth and power specs were provided by Bright Photonics. The optical power and linewidth specifications of typical distributed Bragg reflector (DBR) lasers and low-linewidth lasers for different material and technologies are presented in Table 1. Table 1 shows that the low-linewidth laser with ring resonator or extended cavity satisfy the linewidth requirements, but achieving high power remains a challenge. A tradeoff has to be made between high optical power and low linewidth. Optical power can be increased by integrating booster SOA.
Tasfia Kabir
Department of Electrical Engineering, Eindhoven University of Technology (Collaboration with Bright Photonics)
*t.kabir@student.tue.nl
Study of the Performance of InP based
Semiconductor Lasers Integrated with Booster
Semiconductor Amplifier
Using SOAs as booster amplifier has certain advantages. SOAs provide very high gain with electric pumping and is desirable over EDFA or Raman amplifiers as they can be integrated on chip. But integrating SOA at the output of the laser has a detrimental effect on the linewidth, spectra and SMSR of the laser. There are few theoretical models which analyze this influence on laser linewidth, spectra and SMSR due to booster SOA [17, 18], however these effects were not experimentally characterized. Thus the motivation of this work is to experimentally determine the underlying effects of the integrated booster SOAs on the laser performance and understand the factors with the help of existing models. For this purpose we examine several lasers with different lengths of the booster amplifier. In this work, we present experimental results of characterization of a device designed
by Bright Photonics with several DBR lasers to analyze the laser parameters such as output power, spectra, SMSR and linewidth. We also present a theoretical model that estimates the linewidth of DBR laser under amplification of booster SOA derived from existing work. The report is organized as follows: Section II describes briefly the design of the device under test, Section III describes the characterization of the lasers, which includes the LI curve measurements, evolution of power, wavelength and SMSR with increasing booster SOA current. Section IV includes the theoretical linewidth model and trends of linewidth change with increasing booster current for four lasers with different lengths of boosters. The paper concludes with future the plans for measurements. Identical lasers BDBR laser s BDBR laser s BDBR laser s BDBR laser s FDBR laser s FDBR laser s FDBR laser s FDBR laser s Phase Shifter Phase Shifter Phase Shifter Phase Shifter SOA SOA SOA SOA SOA SOA SOA
Booster SOA with different lengths TABLEI
REQUIREMENTS OF HIGH PERFORMANCE DBR LASERS AND REPORTED PERFORMANCE
Required Linewidth and Optical Power State of the art Linewidth Optical Power Monolithic InP DBR lasers with intra-cavity microring
resonator [13]
63 kHz 6 mW
Linewidth ~100 kHz Optical power ~ 50 mW
Heterogeneous III-V on Si with extended ring assisted DBR lasers [14]
500 Hz 4.8 mW
Heterogeneous III-V on Si with extended DBR [14] 1 kHz 37 mW
Monolithic InGaAlAs MQW supermode DBR lasers [15]
<200 kHz 38 mW
Monolithic SGDBR lasers with booster SOA and spectral filter [16]
70 kHz 50 mW
II. DESIGN
The device that was used for the experiment and analysis was designed and provided by Bright Photonics. The layout of the device is shown in figure 1. It consists of four DBR lasers with SOAs and phase shifters in the cavity.The four laser cavities are identical (marked with dashed red lines in figure 1), with the same length, pitch and reflectivities of DBR and same length of SOAs and phase shifters. The SOA consists of a 2 μm waveguide with quantum well (QW) core of InGaAsP/InP material that is optimized for TE polarization. The lasers termed B, C and D are integrated with different lengths of booster SOA as illustrated in figure 1, and laser A does not have a booster SOA on its output. The boosters of laser B, C and D are of short, medium and long lengths respectively. This gives us the opportunity to study the effect of booster SOAs on identical lasers on the same chip. Throughout the paper we will refer to the laser cavity active section as ‘laser SOA’ and the boosters as ‘booster SOA’.
III. EXPERIMENTAL PROCEDURES AND RESULTS
A. Experimental set-up and chip operation
The schematic of the experimental setup is shown in figure 2. The chip is mounted on a water cooler heat sink and
lensed fiber is aligned using three-axis stages to couple the light out. The chip is wirebonded to a printed circuit board (PCB) and glued on top of an Aluminum block. The PCB is connected to a multichannel current source PRO 8000 with a stripe cord and breaking board. The current source supplied bias current to the laser SOA and booster SOA through the chip contacts. The current source, power meter and optical spectrum analyzer was connected to the computer via a general-purpose interface bus (GPIB) to sweep the different current conditions automatically and for recording the data in an automated way.
B. LI curve measurements
In this section we present the LI curve measurements of the lasers. For these measurements, the setup given in figure 2 is used. The resulting plots for LI curve measurements are shown in figure 3. The current was swept with multi-channel current source PRO 8000 connected to the current scan program on computer. This program was also used for data acquisition from HP Agilient power meter.
LI curve measurements of the output power evolution with current in laser SOA is shown in figure 3 (a). For these measurements, the laser SOA current was varied for 300 values from 0 to100 mA. The biasing of the booster SOA for lasers B, C and D is kept at a current density of 5 kA/cm2.
The coupling loss from chip to fiber is estimated to be 3/4 dB. The resulting plots in figure 3 show that the
7
Power meter Current supply Chip Water cooler Chip glued on Aluminum block wirebonded to PCB Lensed fiber 3-axis stage GPIB to USB Connector boardFor the laser A with no booster, the threshold current was recorded at 12 mA. It is seen that for the bias current densities of 5 kA/cm2 in laser SOA, the booster SOA of laser
B provided an amplification of 2.2 dB, laser C of 5.5 dB and laser D of 7.7 dB, highest amplification achieved from the laser with the longest booster (i.e. laser D). The power increases with the bias current in the laser active region, with a sharp jump at 76 mA current. From the spectrum analysis of laser A, this jump is also observed and is termed as mode-hop. Mode hop in DBR lasers occur due to temperature anomaly [2]. It is the abrupt change of wavelength corresponding to the value of longitudinal mode spacing. This behavior is common in real laser diode LI curve and is explained in details in part B.
The plots of figure 3 (b) show the power evolution with increase of current density in booster SOA. For laser SOA at a constant bias current of 50 mA, the highest amplification was obtained for laser D with a 12 dB increase in input power with 22.4 mW on fiber. Assuming a fiber coupling loss of 3/4 dB, the on-chip output power is 50-56 mW. The power level of laser A at 50 mA SOA is shown as the constant red line to show the amplification by the boosters. The power evolution with booster bias current for three biases in laser SOA of 3 kA/cm2, 5kA/cm2 and 9 kA/cm2
current densities is shown in figure 3 (c). The trend of power evolution shows similar trends at different current densities in laser SOA. The plot in figure 3 (c) shows that for the three lasers with booster configuration, as the current density in laser exceeds 5 kA/cm2, saturation occurs. The thermal roll
off effect is also observed in the LI curves.
C. Spectra Measurements
In this section, we present the analysis of the spectra evolution with the increase of booster SOA current. For measurement of optical spectrum, the optical spectrum analyzer OSA-APEX with 20 MHz resolution is used with the same setup described before in figure 2. Analyzing the spectra data, we plot the maximum wavelength evolution of lasers A, B, C and D with respect to the current increased in booster SOAs, at constant current density 5 kA/cm2 in laser
SOA. The resulting plots are presented in figure 4.
In the plot of laser A in figure 4 (a), we observe a mode jump at 76 mA also seen previously in the LI curve in figure 3 (a). In DBR lasers, as current is applied to the laser SOA, wavelength tuning can be achieved. Wavelength is a linear function of injection current in the active section. While wavelength tuning can be achieved coarsely by changing the temperature in the device, current injection in the laser SOA also results in a finer tuning of the wavelength. Changing the injection current in the laser SOA results in increasing the temperature of the active region. This increased temperature alters the effective index and therefore shifts the cavity modes towards longer wavelengths. This explains the kinks in the LI curve of laser A and the mode hop that occurs at 76 mA. When the bias current is increased beyond 76 mA, the Fabry Perot mode hits the edge of the reflection band of the DBR and the gain shifts to a new FP mode. This is how the DBR laser stays within its operating range. As current is increased in the laser SOA, the effective index changes and shifts the cavity modes towards longer wavelengths. A trend of thermal dependency upon maximum wavelength was also observed from a previous run of measurements, which showed a maximum wavelength shift of 0.1 nm/°C. For laser
(a) (b) (c)
Figure 3: (a) LI curves of lasers A, B, C and D with booster SOA current density of lasers B, C and D set at 5 kA/cm2.
(b) Laser power dependency on the booster current at 50 mA current in laser SOA. (c) Power vs. booster current density of laser B, C and D at three different laser SOA currents.
B, the rate of wavelength shift is 1.33nm/A, for laser C is 2.7 nm/A and laser D is 2.65 nm/A.
D. Side mode suppression ratio measurements
The SMSR is an important standardization characteristics of single mode lasers. It is defined as the ratio between the power of center peak mode to the nearest higher order mode. Many applications such as communication, metrology, sensors etc. require high SMSR of atleast 40 dB. For instance, in wavelength division multiplexing (WDM) systems, high SMSR of laser source is required to ensure low crosstalk in between channels. For this reason, we also investigate the influence of booster SOA on SMSR. This section includes the analysis of SMSR evolution with booster SOA current, extracted from the spectra data of the lasers. In one work in Ghent University it was demonstrated theoretically and experimentally that integrating booster SOA can degrade the SMSR of DBR lasers. In [20] it was shown that the SMSR can be reduced by 10 dB or more with an integrated booster SOA.
The plot in figure 5 shows the evolution of the SMSR with increasing current in booster SOA for constant laser SOA current of 50 mA. In case of the laser A, the SMSR at 50 mA current was 57.5 dB (green line in figure 5). For laser B, as the current density in booster is increased, this SMSR reduces up to a minimum value of 39.6 dB for current density of 8 kA/cm2 in booster. For laser C, the SMSR
degrades up to 40 dB for 12.5 kA/cm2 and for laser D the
value reduces to 50 dB at 12.5 kA/cm2. The ASE from the
booster, which is proportional to the amplification of the booster, is one of the reasons why the SMSR degrades as current is increased in the booster SOA.
IV. THEORETICAL LINEWIDTH CALCULATION
In this section, we present theoretical equations to predict the linewidth broadening of a DBR laser under the influence of booster SOA. We also present a brief analysis of the factors are at play in the broadening of the linewidth. One of the reasons for linewidth broadening is the ASE coupling to the lasing mode. The coupling of spontaneous emission is a
(a) (b)
(c) (d)
Figure 4: Maximum wavelength evolution of (a) laser A with laser SOA current. (b) Laser B with booster current, at 50 mA laser SOA current (c) Laser C with booster current, at 50 mA laser SOA current. (d) Laser D with booster current, at 50 mA laser SOA current.
Figure 5: SMSR change with booster current density of four DBR laser at 50 mA bias in laser SOA.
purity. The photons emitted due to spontaneous emission effect are of different phase than ones emitted due to stimulated emission effect. This results in disturbance of phase of frequency of the electric field. ASE causes random fluctuation in the carrier density, which therefore causes random fluctuations of laser amplitude and phase. The random phase fluctuations results in Lorentzian lineshape of the emission spectrum [19], and this linewidth defined by ASE is called the intrinsic linewidth. There is also 1/f noises dominating at the low frequency range resulting from technical noises. In this work, we focus on studying the intrinsic linewidth of the DBR laser with booster SOA.
In this section, we present a theoretical model of linewidth broadening based on modification of two different expressions presented in [13] and [17]. In part A, we calculate the intrinsic linewidth of the lasers with no booster. The part B shows the linewidth broadening factor calculation that results from the booster SOA amplification. The part C includes the results obtained from the expressions and the setup for measurement of intrinsic linewidth and brief discussion on how certain parameters such as linewidth enhancement factor α and spontaneous emission factor nsp
relates to the change of linewidth.
A. Theoretical intrinsic linewidth of laser without booster
In this part we use the following expression to calculate the linewidth of the laser A at a constant bias current of 50 mA in the laser SOA. The equation is modified from [13] and [21]. The parameters for this equation are listed in table II.
Here Po is the output power. The calculation, we took the
output power of the laser A at 50 mA bias in laser SOA calculated with the coupling losses. The term C is a correcting term for the reflectivity imbalance between the two mirrors [21, 22]. It relates to the reflectivity of the DBR mirrors R1 and R2. The distributed out coupling loss
relates to the mirror losses in the cavity. The distributed internal loss is given by the passive losses per unit length of the passive section ( ) and losses per unit length in the laser SOA ( ). The values of spontaneous emission factor nsp, l
of 2 is taken for InP based SOA and value of α factor as 3, which is common for QW active material [13].
Calculating from (1) while plugging in the parameters from table II, we obtain the value of the linewidth to be ~ 249 kHz.
B. Linewidth broadening factor
This part calculates the linewidth broadening factor due to the booster SOAs. The presented model is adapted from an expression derived from rate equation analysis that provides a simplified modelling of the linewidth broadening [17]. The model is generalized for multisection DBR and DFB lasers. The linewidth broadening factor includes two factors: the back propagating ASE from booster to cavity and the ASE of the booster that causes random fluctuations in the phase of the propagating laser emission. In this model, the influence of the booster SOA upon the linewidth broadening considers only the backward propagating ASE that is injected in the laser cavity at the output side facet of the laser. The obtained expression for linewidth broadening is given by:
The parameters used in (2) are defined in Table III. The factors A is the amplification factor, ga and gl are the material
gains of the booster amplifier and the laser. The amplification factor A was calculated from the LI curve measurements presented in Section III for 50 mA biasing in laser with coupling losses considered. The values of ga and gl
were estimated from the net gain model curve at 1550 nm emission wavelength [23]. The material gain relates to the net modal gain as:
Modal gain=material gain*confinement factor-loss
The net modal gain was extracted from the smart photonics manual and material gain was calculated using the given relation, estimating 25 cm-1 loss in the booster SOA
[23]. The term αrLcavity is the ratio of the output power from the facet facing the booster amplifier and the average power inside the cavity. For a DBR laser, this depends upon the reflection of the Bragg mirrors. Calculating for average power and output power as a function all propagating powers in the cavity, we derive the expression for αrLcavity to be:
Expression from (1) and (2) combined gives us the changed linewidth with current in booster SOA. The obtained values are shown in Table IV.
C. Results
The theoretical linewidth plots with the presented theoretical model is shown for the laser B, C and D in figure 6. Table IV shows the increase of linewidth with booster current. Obtained values indicate that the linewidth is broadened due to the increasing amplification of booster SOA. When bias current in booster is increased, the linewidth value increases and the longer boosters SOAs have broader linewidth. There are current dependent parameters that will lead to changing the linewidth broadening factor that was not taken into account for the presented model. For our calculations, the α-factor and the inversion factor nsp,a
were taken to be constant. In reality they are current density dependent terms that will also change with the output power. The α-factor is a function of the real and complex TABLEIV
THEORETICAL LINEWIDTH BROADENING OF DBR LASERS AT CONSTANT BIASING IN LASER ACTIVE REGION
Laser Bias current Maximum Linewidth in kHz
Laser A 50 mA in laser SOA 249 kHz
Laser B 0-50 mA in booster 255 kHz Laser C 0-120 mA in booster 300 kHz Laser D 0-250 mA in booster 390 kHz TABLEIII PARAMETERSINEQUATION2 Parameter Description A Amplification factor (Po/Pin)
Lboos Length of booster SOA
Lcavity Length of laser cavity
nsp,a Spontaneous emission factor of booster
SOA
Γa Confinement factor of booster SOA
Γl Confinement factor of laser
ga Material gain of booster SOA
gl Material gain of laser
αrLcavity Extraction efficiency of output facet of
laser TABLEII PARAMETERS IN EQUATION 1 Parameter Description vg Group velocity hv Photon energy
αm Distributed out coupling loss
αi Distributed internal loss
Po Output power
C Reflectivity imbalance correcting term
nsp,l Spontaneous emission factor of laser
refractive index terms. When the output power of the laser is close to 10 mW, the α-factor deviates from its low power values [24]. The laser linewidth broadens because of fluctuations of the phase of the optical field. These fluctuations, as mentioned before, arise from spontaneous emission events. Due to spontaneous emission effect, there will be a net gain changes. The α-factor is the ratio of the change of the refractive index (n) with carrier density (N) to the change in optical gain (g) with carrier density, this is expressed as:
……...
(4)where is the wavelength.
An approximate value of α can be derived from measurements of refractive index with gain change. There are several different ways to measure the value of α [25-29]. However the values of α factor for booster SOA has not been characterized.
The spontaneous emission factor nsp also plays an important role in determining the spectral linewidth of semiconductor lasers. The nsp factor is the transition rate of the spontaneous emission coupled into the lasing mode to the stimulated emission, and in principle can be calculated from the spontaneous emission power spectra [30]. The value of (1+ α2) n
sp,a can be extracted from the fitting of experimental
value of linewidth with the theoretical value in the model.
V. CONCLUSION
To summarize, we have presented the experimental and theoretical analysis of booster SOAs upon the performance of DBR lasers. The measurement data presented in this work indicates that the output power of the laser increases up to 12
dB for booster, but the ASE of the booster SOA deteriorates the spectra, SMSR and linewidth of the laser. The theoretical calculation of linewidth that was presented indicates that the back propagating ASE broadens the linewidth of the lasers by a factor of 1.5. In the next phase of the work, we plan to conduct measurements of linewidth that will be fitted to the theoretical model for a more accurate valuation of the model. We also plan to characterize the device under test for temperature change in the heat sink to evaluate how the effect of temperature will affect the chip performance.
ACKNOWLEDGMENT
I want to acknowledge my supervisor Nicola Calabretta, co-supervisor Katarzyna Ławniczuk and previous co-supervisor Valentina Moskalenko for their patience and constant support. I am also grateful to Bright Photonics for designing and providing the chips that was the basis of this work. Lastly, I want to thank my husband, who taught me how to code with Python from scratch and constantly supported me throughout the process.
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