Chapter 4 Chirped-Multilayer Quantum Dot Laser
4.3 Results and Discussion
4.3.6 Coherence
To obtain the information about the coherence of CMQD LD, we measured the interferograms for all devices as increasing current by the fiber-based, delay-tunable Mach-Zehnder interferometer. In order to couple the light into the fiber-based MZI, operating the lasers at cw mode is necessary. However, only devices with lengths longer than 1.5 mm could be operated at cw mode. Besides, pulse mode operation was adopted at higher current injection to prevent severe thermal effect. Fig.
4.14(a) ~ 4.14(e) shows the results with the linear-scale spectrum operating at 10*I for each device. th
Table 4.3 The spectral dependence on current injection.
Fig. 4.14(a) The interferograms for CMQD LD with L =1.5 mm as increasing current injection and the linear-scale spectrum operating at 10*I .th
1230 1240 1250 1260 1270 1280
0.0 0.2 0.4 0.6 0.8 1.0 1.2
I = 10Ith CMQD LD_Spectrum
L = 1.5mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm T = 293K
Intensity (linear; a.u.)
Wavelength (nm)
λp = 1260.7 nm
δλ = 12.4 nm
Fig. 4.14(b) The interferograms for CMQD LD with L=2mm as increasing current injection and the linear-scale spectrum operating at 10*I . th
1240 1250 1260 1270 1280 1290
0.0 0.2 0.4 0.6 0.8 1.0 1.2
I = 10Ith CMQD LD_Spectrum
L = 2mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm T = 293K
Intensity (linear; a.u.)
Wavelength (nm)
λp = 1265.2 nm
δλ = 12.8 nm
Fig. 4.14(c) The interferograms for CMQD LD with L=3mm as increasing current injection and the linear-scale spectrum operating at 10*I . th
1230 1240 1250 1260 1270 1280 1290 0.0
0.2 0.4 0.6 0.8 1.0 1.2
I = 10Ith CMQD LD_Spectrum
L = 3mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm T = 293K
Intensity (linear; a.u.)
Wavelength (nm)
λp = 1268.5 nm
δλ = 14.2 nm
Fig. 4.14(d) The interferograms for CMQD LD with L=4mm as increasing current injection and the linear-scale spectrum operating at 10*I . th
1240 1250 1260 1270 1280 1290
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
I = 10Ith CMQD LD_Spectrum
L = 4mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm T = 293K
Intensity (linear; a.u.)
Wavelength (nm)
λp = 1268.0 nm
δλ = 14.0 nm
Fig. 4.14(e) The interferograms for CMQD LD with L =5mm as increasing current injection and the linear-scale spectrum operating at 10*I . th
1240 1250 1260 1270 1280 1290
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
I = 10Ith CMQD LD_Spectrum
L = 5mm, W = 3μm 10us / 1ms pulse operation resolution = 0.1nm T = 293K
Intensity (linear; a.u.)
Wavelength (nm)
λp = 1270.1 nm
δλ = 11.5 nm
Compared with the interferogram measured under cw operation, some noise data was observed in interferograms under pulse operation.
Power fluctuation and digitization during the current rising and falling time are the main attribution. Though there exists these noise data under pulse operation, the profile of autocorrelation can still be clearly defined.
It is obvious that the interferogram doesn’t seem to be a function of cavity length. There is no simply rule that could explain what the interferogram will look like with varying cavity length. Based on Wiener-Khinchin theorem, we know that the power spectrum density and the interferogram of a light source form a Fourier-transform pair. To understand why these interferograms do look like what they are, we start with discussing the relationship between spectra in linear scale of devices with different length. First of all, these spectra also show little dependence on cavity length, except for the longitudinal mode spacing.
We tried to acquire more ideas from simulation and demonstrated the inverse Fourier transform of two adjacent Gaussian-like spectra with the same amplitude and FWHM, as depicted in Fig. 4.15. The result showed that the peak separation in the interferogram would depend on the distance between centers of the two Gaussian functions. Specifically, the interferogram would appear like the “beat” with a Gaussian envelope. In such a simplified case, the position of side peaks in interferograms could be easily predicted. However, it is difficult to make a connection between interferogram and its power spectrum in real case. Second, these spectra we measured may not the “real” spectra owing to the specific value of resolution we adopt when measuring. Therefore, it is difficult and, in fact, unnecessary to prove the validity of interferograms for such a complex spectrum of a broadband laser by Fourier transformation. In other words, instead of judging by spectrum, measuring interferogram is a more direct and more practical way to decide if a broadband “laser” is suitable or not
2.34 μ 1014 2.35 μ 1014 2.36 μ1014 2.37 μ1014 2.38 μ 1014 2.39 μ 1014 2.4 μ 1014
Fig. 4.15 Simulation on interferogram of two adjacent Gaussian-like spectra with the same amplitude and FWHM.
Power spectrum Interferogram
IFT
We try to quantitatively determine the coherence length for our CMQD LD. For a LD with specific cavity length, the difference between
interferograms of different pumping level can be well-explained. As the spectrum broader and broader (as increasing current), a narrower main peak (at zero optical delay) and a larger contrast between main peak and side mode (at elsewhere) can be observed in all interferograms we measured. This tendency consists well with our simulation of Fourier transformation for complex and multi-Gaussian profile. However, a problem occurs when it comes to determine the coherence length quantitatively. Due to the complexity of lasing spectrum, it is difficult to clearly define the whole envelope of interferogram within the limited delay range (~2 cm) in our MZI [47]. Moreover, the penetration depth of OCT at wavelength around 1.3 μm is about 2 mm. Consequently, we
“roughly” defined the FWHM of the main peak in interferograms as the
“practical coherence length”. The main peak of interferogram for 3-mm-long device was shown in Fig. 4.16. The practical coherence lengths with varying injection levels are depicted in Fig. 4.17. These show that practical coherence length could be as short as 100 μm in our CMQD laser. It is a reasonable value for spectral width of 14 nm which centered at 1260 nm. It means the extremely broad spectral width of 29 nm at high injection current we mentioned before was expected to have coherence length less than 50 μm. Indeed, it is still too long to be applied to OCT. Besides, the progressive shrink in the FWHM of main peak implies that a broader bandwidth could be obtained at higher pumping level. Fig. 4.18 shows the FWHM of spectra plotted as a function of injection current. The result consists with the characteristic of Fourier transform. In conclusion, the FWHM of the main peak in an interferogram could be viewed as an indicator for judging how broad the spectrum of a broadband laser is.
Fig. 4.16 The main peak in interferograms as increasing current injection for 3-mm-long device of CMQD LD.
Fig. 4.18 The FWHM of lasing spectra plotted as a function of injection current.
2 4 6 8 10
0 2 4 6 8 10 12 14 16
FWHM of Lasing Spectrum (nm)
Current Injection ( I / Ith )
1.5mm 2mm 3mm 4mm 5mm
Fig. 4.17 The practical coherence length plotted as a function of current injection for CMQD LD.
2 4 6 8 10
50 100 150 200 250 300 350 400
FWHM of Main Peak (μm) @ Interferogram
Current Injection ( I / Ith )
1.5mm 2mm 3mm 4mm 5mm
Chapter 5
Conclusions and Future Works
5.1 Conclusions of Present Studies
In summary, we have demonstrated a specially designed chirped-multilayer (N=10) quantum dot (CMQD) broadband laser with 3-, 4- and 3-layer of longer-, medium, and shorter-wavelength QD states, respectively. An extremely broad spectrum of 29 nm without any 3dB dip at very fine resolution of 0.1 nm was achieved. It centered at 1270 nm and was expected to have an applied coherence length less than 50 μm.
Laser parameters with ηi of 88.3% and αi of 3.0 cm-1 were extracted from devices with different cavity lengths. Instead of coming from single quantized state, the lasing peak results from the total effect of simultaneous multi-states pumping. The carrier population without well distribution and non-equal contribution to threshold modal gain are the main attribution. A higher GS saturated gain and higher transparency current density of 126.1 A/cm2 were obtained from gain-current analysis.
The structure of CMQD should be optimized to improve the vertical beam divergence for higher coupling efficiency. The characteristic temperature T of 82.4 K in the range of 283 K~353 K has been 0 measured.
Competition between thermal effect and inhomogeneous broadening was observed in spectra of devices with different gain-loss condition. A reasonable group effective index at active region of 3.73 was extracted from longitudinal mode separation. To acquire the coherence information of our CMQD, we have set up a fiber-based, delay-tunable Mach-Zehnder interferometer. The accuracy of measurement has been confirmed by mathematical simulation (Weiner-Khinchin theorem) on commercial
lasers with simple spectral profiles. Due to longitudinal modes in broad-spectrum laser, the interferogram is much more complex than that of SLD. Instead of judging by spectrum, interferogram measurement is a more direct and more practical way to decide if a “broad-spectrum” laser is suitable or not for OCT application. Finally, the FWHM of the main peak in an interferogram could be viewed as an indicator for judging the spectral width of a broadband laser.
5.2 Suggestion for Future Works
For low-coherence application such as OCT, a real broad and nearly flat-top spectrum without ripple is demanded. Owing to the high-power advantage of broadband laser over SLD, the main issue for realizing broadband laser is how to make its spectrum broader. To achieve the goal, simultaneously two-state lasing (ES and GS or two GSs) is one of the approaches. However, only FWHM of spectrum of 29nm, corresponding to coherence length less than 50μm, can be achieved in our CMQD. To improve the performance in the spectral issue, it is advised to optimize the structure of active region in CMQD, such as capping thickness, number of stacking layers and growth condition for InAs QDs.
First of all, simultaneously two-GS lasing can only be achieved in the structure where the carrier population is well-distributed. It means that a change in gain-current characteristic has to be made by readjusting the number of stacking layers of different QD states [48].
Second, the growth condition of active layer, such as InAs thickness and growth temperature, must be optimized carefully. With increasing InAs thickness, the PL peak will shift to the long wavelength side due to an increased dot size and the spectral width becomes small due to the improved dot size uniformity [49]. In other words, a broad spectral width
can be obtained when the InAs is thinner due to size dispersion. On the other hand, at lower growth temperature, the spectral width of the QDs increases and the PL efficiency decreases [49]. This can be attributed to a reduced diffusion length of atoms at lower growth temperature [50]. The size distribution of these dots is very inhomogeneous which may result in the broad emission spectrum.
Third, it is reported that the Beryllium modulation-doping could be used to further increase the PL spectral width in MQWs [51]. The physical mechanism of the broadening is considered to be partly related to the many-body effect by built-in majority holes. A similar mechanism may also play a role in the modulation-doped QD case [52].
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簡歷(Vita)
姓名：黃俊仁 (Chun-Jen Huang) 性別：男
出生年月日：民國 73 年 9 月 9 日 籍貫：台灣省台北縣
學歷：
國立交通大學電子物理系學士 (92.9~96.6) 國立交通大學電子研究所碩士班 (96.9~98.9)
碩士論文題目：
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
Characterization of chirped-multilayer quantum dot laser and the feasibility study of its application on Optical Coherence Tomography