Far Field Pattern

Nowadays, Semiconductor lasers play an more and more important role in the field of fiber-optic communication. To improve the coupling efficiency into the single-mode fiber of some light source, a circle shape and narrow beam divergence of the far-field pattern (FFP) is demanded.

Fig. 3.9 shows the experimental setup used to perform the FFP measurement. The same probe system and temperature controller are used.

Fig. 3.8 The setup for the measurement of lasing spectrum.

LDT-5910 Temperature

Controller

TE-Cooler KEITHLEY 2520

CW / Pulsed Laser Diode Test System

Probe Station

Laser Device

GPIB

I

OSA

Computer Lens Module

SM Fiber

The emitting ligth is focused on the Hamamatsu IR CCD camera (C5840) through the Hamamatsu FFP lens module (A3267-12). Combined with a frame grabber card built in the computer, the signal received by camera can be visulized and analyzed by computer. To prevent the gain saturation of CCD, some attenuators will be used in the high-power measurement case. After appropriate calibration for corresponding wavelength of the light source, the cross-sectional (vertical and lateral) beam divergence angle can be determined conveniently.

Fig. 3.9 The setup for the measurement of FFP.

LDT-5910 Temperature

Controller

TE-Cooler KEITHLEY 2520

CW / Pulsed Laser Diode Test System

Probe Station

Laser Device I

Computer

Lens Module

Camera Adaptor

Chapter 4

Chirped-Multilayer Quantum Dot Lasers

4.1 Introduction

Due to the inhomogeneity in shapes, sizes and composition, self-assembled QDs is subjected to lower saturated optical gain. Stacking multiple QD layers of the same growth condition (uniform-stacked) has been proposed to increase the optical gain for well-performed QD laser in the 1.3 μm region [44]. In other hand, the intrinsic characteristic of broad gain spectrum in QDs, if properly engineered, are promising for low coherence application such as OCT.

Operating at amplified spontaneous emission (ASE) and suppressing stimulated emission, SLD has been one of the candidates for low-coherence application. However, its lower output power may limit the performance on real-time imaging of OCT. Furthermore, the tilted ridge waveguide (RW) configuration for low mirror reflectivity complicates the fiber coupling and increase the package cost. Recently, it was announced that QD lasers, in normal RW configuration, can be managed to lase with a wide emission spectrum [38~40].

In this work, we propose the control of the emission wavelength in the chirped structure by changing the matrix surrounding of the QDs.

Specifically, when the InAs QDs are covered by an InGaAs strain-reducing layer (SRL), a red-shift of the emission wavelength is observed, depending on the composition and thickness of the SRL [20].

The areal density is not significantly affected by the capping, so that the available gain and the wavelength are effectively decoupled. In contrast to other approaches, this technique is reproducible and easy to implement,

since it relies on well-controlled growth parameters.

We have demonstrated 10-layer QD laser with three chirped-wavelength QD-stacks, which we called chirped-multilayer quantum dot laser diode, CMQD LD, once and for all. Some basic parameters of the CMQD LD will be measured and discussed. Empirical gain-current analysis is also performed. The measurement of interferograms will be demonstrated by our delay-tunable Mach-Zehnder interferometer to exhibit how broad a spectrum is in a more different way.

4.2 Device Growth and Fabrication

The CMQD laser structure was based on a typical p-i-n configuration grown on a silicon-doped, (100)-oriented n⁺-GaAs by the molecular beam epitaxy. Multilayer QD active region with undoped graded-index separate-confinement heterostructure (GRINSCH) of 0.5μm was sandwiched between Si-doped and C-doped Al_0.35Ga_0.65As cladding layer of 1.5 μm, and followed by heavily C-doped GaAs contact layer of 0.4 μm. The emission wavelength of InAs QDs are tuning by the subsequent In_0.15Ga_0.85As capping layer of different thickness. Multilayer QDs are spaced by GaAs and centered in the active region also by GaAs. The spacing thickness of 33 nm is chosen so that strain is not accumulated in the multilayer deposition. The wafer growth is credited to the Innolume GmbH in Germany.

To chirp the emission wavelengths of QD active region, three wavelengths designated as QD_L, QD_M and QD_S (stand for longer-, medium- and shorter-wavelength QD stacks, respectively) were engineered in the laser structure, which corresponding to the InAs QDs of 2.6 ML capped by InGaAs of 4 nm, 3 nm and 2.5 nm, respectively. The

stacking numbers for QD_L, QD_M and QD_S were 3, 4 and 3 layers, respectively. The stacking sequence of different capping thickness was interlaced. Fig. 4.1 shows the schematic diagram of chirped-multilayer QD structure.

Ridge waveguide of 3μm width was formed by high-density-plasma dry etching. The etching depth was in situ monitored to stop just above GRINSCH waveguide. Thin oxide was then deposited by plasma-enhanced chemical-vapor deposition. The contact window opening over the narrow ridge was done by double-channel self-aligned process. After p-metallization by Ti/Pt/Au deposition, the substrate was thin down to around 100 μm, and n-type metal of AuGe/Ni/Au was deposited afterwards. As-cleaved and uncoated laser bars with cavity lengths from 0.5 mm to 5 mm were fabricated then evaluated by standard light-current-voltage (L-I-V) and spectrum measurement. Both pulsed and continuous-wave driving conditions are performed under varied heatsink temperature.

Fig. 4.1 The schematic diagram of the chirped multilayer QD structure.

GaAs GaAs

N-AlGaAs cladding P-AlGaAs cladding QDM

QDL

QD_M QDS

QD_L QDM

QDS

QDL

QD_M QD_S

N-type : Si 1.5µm

Al0.35Ga0.65As

GaAs

0.5μm P-type : C

1.5µm

InAs QDs InGaAs QW

4.3 Results and Discussion

The areal density of InAs QDs without InGaAs capping was grown separately and characterized by AFM to be around 5*10¹⁰ cm^-2. Fig. 4.2 shows the AFM image of epitaxial surface immediately following the QD deposition. According to the room-temperature (RT) photoluminescence (PL), the dependence of QD emission wavelength on the thickness of capping InGaAs layer can be determined. Fig. 4.3 illustrates the dependence of GS and ES peaks at RT PL on the thickness of In_0.15Ga_0.85As capping layer. The peak wavelength of GS for QD_L, QD_M and QD_S in our case is about 1262 nm, 1230 nm and 1215 nm, respectively. The peak wavelength of first ES for QD_L, QD_M and QD_S is also estimated to be around 1183 nm, 1155 nm and 1145 nm, respectively.

Since the available gain and wavelength are decoupled in the chirped structure [20], stacking CMQD in which each QD layer is capped with different thickness of InGaAs SRL could effectively increase available

Fig. 4.2 The AFM image of epitaxial surface immediately following the QD deposition.

gain and spectral width.

4.3.1 Laser Characteristics

Fig. 4.4 shows the RT pulsed L-I-V characteristics of CMQD LD with a narrow RW of 3 μm and different cavity lengths. Fig. 4.5 shows the lasing spectra around threshold relevant to different cavity lengths.

Longer lasing wavelength around 1265 nm was achieved for cavity length above 0.75 mm. Only the 0.5 mm cavity length was found lasing at 1173nm. Compared it to the results of RT PL, it was expected that these two wavelength emissions was contributed by the GS and ES of QDL. For the cavity length of 5 mm, the threshold current is 34 mA, which corresponds to a threshold current density (Jth) of 226.7 A/cm². By fitting the relationship of inverse external slope efficiency 1/η_ex versus cavity length L to the equation 1/η_ex =1/η_i[1−α_iL/ln(R)] with R (reflectivity

Fig. 4.3 The dependence of GS and ES peaks at RT PL on the thickness of In0.15Ga0.85As capping layer.

0 1 2 3 4 5 6 7 8

1080 1120 1160 1200 1240 1280 1320

QD_S QD_M QD_L

ES

In0.15Ga

0.85As / InAs QDs

Peak Wavelength @ RT- PL (nm)

In0.15Ga

0.85As Thickness (nm)

GS

QD_L

Fig. 4.4 The RT L-I-V characteristics of our CMQD LD for different

Fig. 4.5 The RT lasing spectra of CMQD LD operated around threshold for different cavity lengths.

1165 1170 1175 1245 1250 1255 1260 1265 1270 1275 1280 -60

Fig. 4.6 Inverse of external differential efficiency plotted as a function of cavity length of CMQD LD.

0 1000 2000 3000 4000 5000

1.6 1.8 2.0 2.2 2.4 2.6 2.8

W=3μm @ 20o C ηi=88.3%

αi=3.0 cm-1

1/DQE

Cavity Length (μm)

at facet) = 0.32, a η_i of 88.3% and an α_i of 3.0 cm^-1 are obtained, as shown in Fig. 4.6.

4.3.2 Gain-Current Analysis

The dependence of the modal gain, G , on current density, J is one of the main characteristics of the active region in a QD laser. Due to the non-ideality such as inhomogeneous broadening of the self-assembled QD, its gain-current characteristic demonstrates higher transparency current density, lower saturated gain, and less abrupt increase of the gain on increasing the current density. After determining the current dependence of threshold modal gain, we fitted the gain curve based on the empirical equation proposed by Zhukov et al. [42], i.e. Eq. (2.41).

The experimental data points and fitting curves are shown in Fig. 4.7. The lasing wavelengths at each threshold condition were also shown here.

Fig. 4.7 Modal gain and lasing wavelength as functions of current densities for CMQD LD.

0 500 1000 1500 2000 2500 3000 3500 4000 0

Since the lasing wavelength significantly switched to higher energy at cavity length of 0.5mm, the others could be viewed as coming from the GS. If the GS saturated gain of 20.9 cm^-1 is from 3*QDL, the GS saturated gain per QD is about 7 cm^-1. Compared to the typical value of 4

~ 6 cm^-1, this value is unreasonably high. Furthermore, the fitting transparency current density is 126.1 A/ cm² and the non-ideality parameter γ is equal to 0.38. Due to the trade-off between saturated gain and transparency current density discussed before, the relatively higher GS saturated gain and higher transparency current density manifest the fact that the total dot density is high in our CMQD structure.

On the other hand, the relatively low γ -factor and the less abrupt increase of optical gain with increasing the current density could be attributed to the simultaneous population of the higher-energy states. The contribution of the higher-energy state such as the GS of QDM can not be ignored. In other words, instead of coming from single quantized state, the lasing peak results from the total effect of simultaneous multi-states pumping. It is reasonable that this behavior occurs in our specially designed CMQD structure.

4.3.3 Far-Field Pattern

To obtain a high power coupling, not only the high output power but also narrow beam divergence is required. To reveal the superior performance on power issue than that of SLD, the divergence angle of a broadband laser is also an important parameter in the fiber-based OCT application. Fig. 4.8 shows the measured far-field patterns with increasing current density of our CMQD LD with cavity length of 2mm.

Since the divergence angle does not significantly depends on the cavity length of a device, only the 2mm one is measured. This result shows that both the lateral and vertical divergence angle almost remain the same when current density increases to 10*I_th. The vertical far-field angle (θ_⊥) is 52.0°

°

and the lateral far-field angle (θ_//) is 7.8°. The aspect ratio, defined as θ_⊥/θ_//, is 6.8. Without appropriate engineering of cladding layer structure, the FFP performance of CMQD LD is not good enough when compared to well-engineered QW laser which the optical coupling efficiency higher than 60% can be obtained. Only coupling efficiency of 10%~20% can be achieved in our case when measuring the interferograms. All parameters are listed in Table 4.1.

I

FFP 1.1 Ith 2 Ith 4 Ith 6 Ith 8 Ith 10 Ith

Lateral Angle (degree)

7.8 7.8 7.6 7.6 7.6 7.8

Vertical Angle (degree)

52.0 52.0 52.2 51.8 53 53.2

Aspect

Ratio 6.7 6.7 6.9 6.8 7.0 6.8

Table 4.1 All parameters of FFP for the 3μm-wide ridge and 2mm-long CMQD LD with increasing current density.

-50 -40 -30 -20 -10 0 10 20 30 40 50

Fig. 4.8 The measured far-field patterns for the 3μm-wide ridge and 2mm-long CMQD LD with increasing current density.

-50 -40 -30 -20 -10 0 10 20 30 40 50 20μs/0.2ms pulse mode I = 1.1 Ith

Fig. 4.9 The temperature-dependent L-I characteristics of CMQD LD with 3μm ridge width and 2mm cavity length.

0 10 20 30 40 50 60 70

0 4 8 12 16 20 24

T=10^oC

2-Side Power (mW)

Current (mA) CMQD LD

L=2mm, W=3μm

pulse operation (10μs/1ms) duty cycle = 1%

δT=10^oC

T=80^oC

4.3.4 Temperature Characteristics

Due to the non-radiative recombination rate is a function of temperature, more current is required both for threshold and the increment above threshold as the temperature increased. Fig. 4.9 shows the temperature-dependent L-I characteristics. The device under test was 3 μm in ridge width and 2 mm in cavity length. The temperature changes from 283K to 353K with an interval of 10K. The relation of the characteristic temperature and the threshold current is expressed as

] / exp[ ₀

0 T T

I

I_th = , where T is called characteristic temperature. It ₀ represents how many excited electrons and holes by lattice heating contributed to the radiative recombination. Fig. 4.10 shows the threshold current versus temperature and the fitting curve. The extracted T is as ₀ low as 82.4K (283K~353K). For broad-spectrum QD laser, many characteristics differ from conventional narrow-spectrum laser. To broaden the spectral width, a deliberate increase in the dot size

Fig. 4.10 The threshold current versus temperature and the fitting curve.

280 290 300 310 320 330 340 350 360 20

25 30 35 40 45 50

Threshold Current (mA)

Temperature (K)

Temperature Characteristic Exponential Fitting

T₀ = 82.4 K Ith = 0.575*exp(T/82.4)

distribution is necessary. However, due to the Gaussian-like DOS (not delta-function any more), there are trade-offs in threshold current density and characteristic temperature. In other words, the relatively low T ₀ obtained in our CMQD structure is not a surprising result. H. S. Djie reported a low T of 40.3K in the range of 278K ~ 323K for the ₀ broadband InGaAs/GaAs QD LD with cavity length of 800 μm [40].

4.3.5 Spectral Characteristics

The main issue in this thesis focuses on whether our CMQD LD could exhibit broad spectral width and centered near 1.3 μm to meet the demand of low-coherence applications. The RT lasing spectrum with varying injection levels and cavity lengths for CMQD LD with 3 μm ridge width are shown in Fig. 4.11.

It is obvious that the spectral ripple is severe for short-cavity device in low injection current. This phenomenon can be attributed to the longitudinal mode existed in a Fabry-Perot cavity. The longitudinal mode

Fig. 4.11(a)~(h) The RT lasing spectrum operating from lasing 53

threshold to well-above threshold current for CMQD LD with 3μm

1150 1160 1170 1180 1190 1200 1210 1220

2 Ith DO 1560_Spectrum

L = 0.5mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1220 1230 1240 1250 1260 1270 1280 1290

DO 1560_Spectrum L = 0.75mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1230 1240 1250 1260 1270 1280 1290

DO 1560_Spectrum L = 1mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1220 1230 1240 1250 1260 1270 1280 1290

DO 1560_Spectrum L = 1.5mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1230 1240 1250 1260 1270 1280 1290

DO 1560_Spectrum L = 2mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1230 1240 1250 1260 1270 1280 1290

DO 1560_Spectrum L = 3mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1230 1240 1250 1260 1270 1280 1290 1300

DO 1560_Spectrum L = 4mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

1230 1240 1250 1260 1270 1280 1290 1300

DO 1560_Spectrum L = 5mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm

separation measured in the output spectrum can be expressed by 2nL

2/ λ λ ≈

Δ , where n and L are the group effective index of active region and the cavity length, respectively. The longer the cavity length is, the closer the longitudinal mode separation will be. In other words, the longitudinal mode spacing cannot be specifically defined for devices with longer cavity length. In our case, the longitudinal mode spacing can only be defined as 0.37 nm, 0.28 nm and 0.21 nm for devices with L of 0.5 μm, 0.75 μm and 1 μm, respectively. The corresponding group effective index of 3.73 is obtained. It is believed to be a rational value in our InAs/InGaAs active region. In fact, whether the longitudinal mode is clear or not strongly depends on the resolution, an adjustable parameter when measuring power spectrum with OSA. However, due to the Fourier-transformation relation between power spectrum and interferogram, it is the narrow peak and random-like behavior of longitudinal mode in spectrum that severely affects the profile of interferogram. That is why the resolution must be mentioned when the spectral width of a broadband laser is quantitatively determined.

Table 4.2 lists the corresponding peak wavelength and FWHM as increasing the injection level for these devices. Fig. 4.11 shows the plotof peak wavelength versus current injection level for each device. The broadest spectrum equals to 14.2 nm occurs in the 3-mm LD at 10*I . _th As stated before, the lasing wavelength is expected to be coming from the GS of QD_L for all devices except for the one with cavity length of 0.5 mm.

Around the threshold current, a slight blue-shift occurs when cavity length decreases. It could be attributed to the moving of maximum gain spectrum curve towards higher photon energies as the injection current increases below the lasing threshold current, which is proposed by Maximov et al [45]. Due to the mirror loss is inversely proportional to the cavity length, the threshold current density of short-cavity device is

higher than that of long-cavity one. As a result, the degree of blue-shift for short-cavity diode laser will be much more apparent. This is consistent with our data.

As shown in Fig. 4.12, ES of QD_L emitted by device with 0.5-mm-length was red-shifted severely after the threshold current due to thermal effect which is caused by high injection current density. However, this red-shift is compensated by the blue-shift caused by smaller dots in

Ith

I /

L 1.1 2 4 6 8 10

λp (nm) 1173.4 1173.9 1179.2 1183.9 1187.7 1193.8 0.5mm

λ

Δ (nm) 1.4 6.4 6.2 9.1 5.9 8.7

λ_p (nm) 1254.3 1255.7 1256.1 1257.9 1259.7 1261

0.75

mm λ

Δ (nm) 2.2 3.7 4.3 5.1 5.3 5.3

λp (nm) 1259.3 1259.5 1261.3 1262.3 1265.3 1268.3

1mm

Δ (nm)λ 2.2 2.9 4.9 4.3 4.0 4.3

λp (nm) 1259.1 1257.8 1257.1 1260.6 1260.6 1260.7

1.5mm

Δ (nm)λ 1.2 4.7 8.1 9.8 11.6 12.4

λp (nm) 1264.3 1264 1264.8 1265.1 1264.9 1265.2

2mm

Δ (nm)λ 1.5 3.9 8.4 10.2 12.5 12.8

λ_p (nm) 1268.3 1267.9 1268.2 1268.4 1268.3 1268.5

3mm

Δ (nm)λ 1.1 4.7 8.8 12.6 13.7 14.2

λp (nm) 1270.4 1270.4 1269.3 1270.2 1270.1 1267.9

4mm

Δ (nm)λ 1.2 3.9 6.5 9.3 12.3 14.0

λp (nm) 1273 1272.1 1271.7 1270.2 1270.6 1270.1

5mm

Δ (nm)λ 1.4 3.8 6.8 9.0 10.5 11.5

Table 4.2 The list of corresponding peak wavelength and FWHM as increasing injection levels for all devices.

Fig. 4.12 The plot of peak wavelength versus current injection level for each device.

0 2 4 6 8 10

1170 1180 1190 1260 1280

0.75 / 1.5 / 1 / 2 / 3 / 4 / 5 mm

Peak Wavelength (nm)

Current Injection ( _{I / Ith}) 0.5mm

the devices with cavity length of 0.75 mm, 1 mm and 1.5 mm, and furthermore the blue-shift becomes dominant for longer devices. For our unpackaged and uncoated devices, the temperature-controllable heat sink plays an important role to prevent severe thermal effect. With larger contact area with the heat sink, thermal effect of long-cavity devices can be effectively suppressed.

A progressive broad bandwidth in the lasing spectrum is noticed with increased injection current because of the inhomogeneous broadening. Larger dots with lower energy would be filled at first as increasing current injection. A blue-shift in gain spectrum caused by filled smaller dots with higher energy occurred at high pumping. However, the spectral width is not as broad as expected due to non-uniform enhancement of gain spectrum as increasing pumping. Furthermore, the effect of 4*QD_M and 3*QD_S in the structure of CMQD are not apparent from our measurement at injection level up to ten times of threshold

Fig. 4.13 The evolution of spectrum at high injection level for CMQD.

1200 1220 1240 1260 1280 1300

-10 -8 -6 -4 -2 0

2.0 A 2.5 A 3.0 A

Normalized Intensity (dBm)

Wavelength (nm) (W,L)=(³μm,3mm)

(τ,T)⁼(⁵μs,1ms) Pulsed @ 293K resolution = 0.1nm

current. It means that the carrier population cannot be well-distributed and results in a non-equal contribution of threshold modal gain. On the other hand, Fig. 4.13 shows the measured emission spectrum at much higher injection level under pulse operation from a 3-mm-long device of CMQD LD. Table 4.3 lists the spectral dependence on current injection.

The spectral FWHM decreased at current level from 1.5 A to 2.3 A is due to the dip at 1270 nm. Simultaneous lasing from GS of QD_L and a higher state occurred wherein incomplete gain clamping and retarded carrier relaxation process are the main attribution [35]. It is believed that this higher state was contributed by both the ES of QD_L and GS of QD_M. Besides, an extremely broad spectrum of 29 nm centered at 1270 nm was observed. It is worth noting that these measurements are all under a fine resolution of 0.1 nm. However, the energy spacing between GS and ES is too large ( > 60 meV for S-K type QDs generally [46]) to joint these two spectra to be a really broad one. To overcome these problems, it is advised to optimize the structure of active region in CMQD, such as capping thickness, number of stacking layers and growth condition for

I (A) 0.3 0.5 0.8 1 1.3 1.5 1.8 2 2.3 2.5 2.8 3 FWHM

(nm) 12.0 16.6 18.7 19.6 20.8 29.0 6.0 24.7 17.8 27.7 28.0 28.3

InAs QDs. By the way, simultaneous two GS lasing for CMQD with different structure at active region can be observed [47], which implies

InAs QDs. By the way, simultaneous two GS lasing for CMQD with different structure at active region can be observed [47], which implies

在文檔中 啁啾式多層堆疊量子點雷射之特性暨其適用於光學同調斷層掃描系統之可行性分析研究 (頁 52-0)