We also study the reduction of nonlinear distortion in the QD VCSEL by light injection technique. Nonlinear distortion of the laser is important consideration for
SCM systems, the IMD3 has the largest impact on performance degradation because of the IMD3 signal close to the original subcarrier frequencies [4].
3.2.1 Theory
The goal of any transmission system is to transfer information as accurately as possible. However, neither device is perfectly linear, particularly when large modulation levels are involved. Several different types of distortion products are common from these components such as harmonic distortion and intermodulation distortion.
When laser is intensity modulated there will be modulated signal power at he modulating frequency; and depending on the linearity of the device, there will also be some modulating power at harmonics of the modulating frequency. Some modulated power at the second harmonic and third harmonic of the modulating frequency is very common. Harmonic distortion is defined as the ratio of modulated power in harmonic of the modulating frequency to the power at the modulating frequency. For example:
)
where HD2 is the second harmonic distortion, HD3 is the third harmonic distortion, and THD is the total harmonic distortion[5].
It is common to express the distortion in decibels (dB). In this case, the harmonic distortion is expressed as:
))
Intermodulation distortion (ID) occurs when two or more modulating signals are present. In this case, device nonlinearities cause the two modulating signals to interact,
intermodulation distortion for two signals can e measured as:
A particularly distressing form of intermodulation distortion is third-order intermodulation (IMD) for two closely spaced signals. This is because the IMD signals fall close to the original modulating frequencies.
)
3.2.2 Experimental Setup
Fig. 3-8 shows the experimental setup for measuring the IMD3 of QD VCSEL with and without light injection. A commercial DFB laser is used as the master laser in our experiments. The injection power is controlled by a variable optical attenuator at the output of the DFB laser. The polarization of the DFB laser is adjusted using a polarization controller. The polarization is chosen that the second harmonic distortion of QD VCSEL has the must reduction. An optical circulator is used to couple the DFB laser light into the QD VCSEL. The QD VCSEL is modulated as single tone or two tone measurements by RF signal generator.
3.2.3 Results and Discussion
Fig. 3-9 (a) shows the electrical spectrum of the QD VCSEL without light injection under a 1 GHz microwave modulation. The microwave power level before the bias-T is 3 dBm. The second harmonic distortion of the QD VCSEL without light injection is -8.2 dB. Fig. 3-10 (b) shows the corresponding spectrum of the QD VCSEL with light injection. The central wavelength of DFB laser is 1277.99 nm, and the injection power is 2 dBm. The second harmonic distortion has been dramatically reduced to
distortion of QD VCSEL with light injection has been suppressed by more than 14 dB from 100 MHz to 1.2 GHz.
Two-tone measurements without and with light injection varied with the input RF power is also investigated, as shown in Fig. 3-10. The two-tone frequencies are 1 and 1.01 GHz. The fundamental tone power increase of 4.5dB and the distortion suppression of 10.6 dB are observed. As a result, the dynamic range of the QD VCSEL with light injection can be enhanced 15.1 dB for the IMD3.
3.3 Conclusion
In this chapter, we report the dynamic characteristics of QD VCSEL without and with external light injection. The significant enhancement of frequency response by light injection technique has been studied. The 3 dB frequency response has been increased by as much as 4.2 times using light injection technique. Moreover, this frequency response enhancement can improve the performance of SCM system.
Experimental results show a 33 dB improvement in system performance. Furthermore, reduction of IMD3 in the QD VCSEL also has been observed. The dynamic range of the QD VCSEL with light injection can be enhanced 15.1 dB for the IMD3.These results show that external light injection is a very powerful technique to upgrade QD semiconductor lasers.
Reference
[1] N. Schunk and K. Petermann, "Noise analysis of injection-locked semiconductor injection lasers," IEEE Journal of Quantum Electronics, vol. QE-22, pp.
642-50,1986.
[2] C. H. Henry, N. A. Olsson, and N. K. Dutta, “Locking range and stability of injection locked 1.54-_m InGaAsP semiconductor lasers,” IEEE J. Quantum Electron., vol. QE-21, pp. 1152–1156, Aug. 1985.
[3] Lukas Chrostowski, Optical Injection Locking of Vertical Cavity Surface Emitting Lasers (A dissertation for the degree of Doctor of Philosophy, 2003).
[4] W. I. Way, Broadband Hybrid Fiber/Coax Access System Technologies (Academic Press, San Diego, 1999).
[5] Dennis Derickson, Fiber Optic Test and Measurement ( Prentice-Hall, Inc. 1998)
0.0 2.5 5.0 7.5 10.0
Fig. 3-1 (a) Schematic diagram of the quantum dot vertical cavity surface emitting laser (b) Experimental setup for the injection locking of QD VCSEL (DFB: DFB laser, VA:
variable optical attenuator, OC: optical circulator, OSA: optical spectrum analyzer, PC: polarization controller, PD: photodetector, Amp: electrical amplifier)
DC Bias
6dBm ~ 1dBm
OSA
1276 1278 1280 1282
-70
Intensity (10 dB / div)
Wavelength (nm) 3mA 3.5mA 4mA
Fig. 3-2 Small-signal frequency response of QD VCSEL at different bias currents.
0.0 2.5 5.0 7.5 10.0 -60
-50 -40 -30 -20 -10 0 10
Re sp on se (d B)
Frequency(GHz)
6dBm 4dBm 2dBm
1276 1278 1280 1282
-70 -60 -50 -40 -30 -20 -10 0
Intensity (10 dB / div)
Wavelength (nm)
4 mA free running with 4 dBm injection
0 2 4 6 8 10 12
3 dB Frequency ( G Hz)
Fig. 3-3 Small-signal frequency response of QD VCSEL at different injection powers.
PG 50 Mb/s
7 GHz ~
∆φ Phase shifter
QD VCSEL LPF
100MHz
Signal Generator
DC Bias
LPF 100MHz Oscilloscope
mixer
A
PDDFB
Bias T
PC 2 1
3 VA
mixer RFA
C
RF Spectrum Analyzer
Fig. 3-5 Experimental setup for the quantum dot VCSEL without and with light injection in a subcarrier multiplexed system. (PG:
pattern generator, LPF: low pass filter, RFA: RF amplifier, PD: photodetector)
without light injection
with light injection
In te ns it y ( 5 dB / di v)
6.90 6.95 7.00 7.05 7.10
Frequency (GHz)
(b) (a)
Fig. 3-6 7-GHz 50-Mb/s data signal at point A (a) without light injection (b) with light injection.
C RF Spectrum Analyzer PD
QD VCSEL
DC Bias PC
Bias T DFB
VA 1 2
3
DC Bias
RF Signal Generator
f2
f1
f2
f1
Fundamental IMD3
Fundamental RFA
Fig. 3-8 Experimental setup for measuring the third-order intermodulation distortion (IMD3) of quantum dot VCSEL without and with light injection.
-8.2 dB
Without light injection
-26.4 dB
With light injection
Intensity (5 dB / div)
0.5 1 1.5 2 2.5
Frequency (GHz)
(b) (a)
0.0 0.4 0.8 1
-70 -60 -50 -40 -30 -20 -10 0
Without light injectio With light injection
Second H am oni c D istri buti on (dB)
(c)
-15 -10 -5 0 5
-120 -100 -80 -60 -40 -20 0
without light injection Fundametal IMD3
with light injection Fundametal IMD3
Ou tp ut Po we r (d Bm)
RF Input Power (dBm)
Fig. 3-10 IMD3 of quantum dot VCSEL without and with light injection.
Chapter 4 Slow Light in Quantum Dot VCSEL
This investigation experimentally demonstrates tunable slow light in a 1.3 µm quantum dot vertical-cavity surface-emitting laser (QD VCSEL) at 10 GHz. The QD VCSEL fabricated on a GaAs substrate is grown by molecular beam epitaxy with fully doped n- and p-doped AlGaAs distributed Bragg reflectors. Tunable optical group delays are achieved by varying the bias currents, and the maximum delay of 42 ps at 10 GHz has been demonstrated at room temperature. Moreover, the delay-bandwidth product is 0.42.
4.1 Theory: Approaches to achieve slow light
To obtain a slow light device, in general, one must vary the medium within which the optical signal travels by either increasing the path length or reducing the signal group velocity. The former can be accomplished with the use of a fiber delay line, which will be discussed below. The latter has several possibilities. We first observe that the group velocity is defined as
where n is real part of the refractive index and k is the waveguide propagation constant. We can define a slowdown factor S as
k
From (4-2), we see that the group velocity can be reduced by introducing a large and positive waveguide dispersion n
∂
∂ or material dispersion n
∂
also possible to include both material and waveguide dispersion in one device to have an enhanced effect [2].
a. Optical Fiber Delay Lines:
Optical fiber delay lines have previously been referred to as an “optical buffer” [3].
One basic design typically consists of a 2×2 optical switch connected with a fiber loop. Other components such as optical isolators, amplifiers, and dispersion compensation devices have also been included to reduce impairments due to reflection, loss, and dispersion.
The optical switch is first set to direct the data train into the fiber loop and subsequently is closed to allow the data to recirculate in the loop. The storage capacity, i.e., amount of data stored, is limited by the time required to travel one loop τloop subtracted by that required to set the switch. This is because when the optical data stream is longer than τloop, the data of the leading part of the packets will overlap with that in the back to cause interference. The storage time, i.e., how long the data is kept in the loop, is an integer multiple of. The turn-off (release) time is also determined by τloop. This is because once a packet enters the delay line, it can only emerge at a fixed duration of time later. It is impossible to remove the packet from the delay line before that fixed time interval.
b. Slow Light Using Waveguide Dispersion:
Studies on the light propagation in highly dispersive structures with a very slow group velocity have drawn much attention. Grating structures have been used extensively in DFB lasers and grating waveguide couplers. Recent progress on the fabrication of grating structures in fiber has opened new research areas using fiber Bragg gratings [4]. Recently, another method for achieving slowed light based on a Moiré fiber Bragg grating has been suggested [5]. A Moiré grating is formed if the
2 ) cos(
2 ) cos(
)
( 0
Λ + Λ
= z
s n z
n z
n δ π π
where Λ is the Bragg period and Λ s is the Moiré period. The theoretical analysis shows that the group velocity of light in the transmission band can be slowed down substantially although with a very small signal bandwidth.
c. Progress in EIT-Based Slow Light:
EIT refers to an artificially created spectral region of transparency in the middle of an absorption line due to the destructive quantum interference arising from two transitions in a three-level system [6], [7]. There are three basic energy level schemes for implementing a three-level EIT system interacting with two near-resonance electromagnetic fields. In a ladder or cascade system, levels are arranged as E1<E2<E3; in a V scheme, levels are arranged as E2<E1 and E3; whereas in a scheme, levels are arranged as E1, E3 <E2. In all three cases, we label the transitions the same way: |1> to |2>, and |2> to |3> are strong dipole-allowed transitions, while
|1> to |3> is a dipole-forbidden transition.
The signal field connecting |1> to |2> is the light field that one desires to slow down in a controllable fashion. The pump field is the control field connecting |2> to
|3>, whose intensity controls the amount of slowing down. In the literature, the pump laser is sometimes called the control laser.
As a result of the coherent coupling between the atomic system and the laser beams, atomic levels |1> and |2> are no longer eigenstates of the system. Instead, they are dressed by the pump laser and become two new states |2d> and |3d>. This (destructive) quantum interference between two absorption paths produces a transparency spectral window in the middle of the strong |1> to |2> absorption line. The width of this (4-3)
imaginary part χ 〞 of the optical susceptibility χ, must be accompanied by a dispersive-shaped variation in the real part χ’ of the susceptibility. Such a variation leads to a very large positive derivative (or gradient) of the index of refraction ( n≡Re 1+χ ) with respect to frequency inside the center of the EIT transparency region. This slope results in a very large group index of refraction and, thus, a reduced group velocity [8]–[10].
4.2 Tunable Slow Light Device of Quantum Dot VCSEL
4.2.1 Experimental Setup
The spectrum and LIV curve of the QD VCSEL in this experiment is shown in Fig.
4-1. The threshold current of the QD VCSEL is about 0.7mA. Fig. 4-2 shows the experimental setup for measuring the optical group delays in the QD VCSEL. A probe signal is generated by a tunable laser modulated with an electro-optical modulator.
The OMI value of the modulated signal is about 50% and the signal is modulated at the linear region of the electro-optical modulator. The signal power is controlled by a variable optical attenuator at the output of the electro-optical modulator. The polarization of the tunable laser is adjusted by using a polarization controller before injecting into the QD VCSEL. The adjusted probe signal injects into the port 1 of optical circulator then passing into the QD VCSEL at the port 2.The light output at port 3 of optical circulator is divided into 2 parts: 90% of the light signal is transmitted into photo detector then convert into electrical signal amplified by RF amplifier and sent into oscilloscope to measure the delay condition. 10% of the light signal is connected to optical spectrum analyzer to observe the detuning wavelength of the probe signal and the QD VCSEL. The polarization of the probe signal is adjusted to reach the maximum time delay in the QD VCSEL.
4.2.2 Results and Discussion
Fig.4-3 shows the measurements of time delay for a 10 GHz probe signal at the various bias currents of QD VCSEL. The probe signal is tuned to the resonance of the QD VCSEL cavity, and the signal power is -14 dBm. Fig.4-4 is the cavity resonance dip at 0mA in 200C. The injection spectrum of QD VCSEL biased 1mA is shown as Fig.4-5. Increasing the bias current of QD VCSEL can increase the time delay of probe signal. The maximum group delay of 42 ps is observed, and the driving current is at 1 mA. The delay-bandwidth product is 0.42. Given the active region of 1.13µm this delay corresponds to the slow down factor of about 3280. Fig.4-6 shows the waveform at different modulation frequencies of probe signals when the bias current of QD VCSEL is at 1 mA. For 8 GHz and 6 GHz, the time delays are 44 ps and 60 ps, respectively, and the time-bandwidth product are 0.35 and 0.36, respectively.
Moreover, the relationship between the time delays and modulation frequencies of probe signal are shown in Fig.4-7. The time delay in the QD VCSEL increases when the modulation frequency decreases. Fig.4-8 shows the waveform at different powers of probe signals when the bias current of QD VCSEL are at 1 mA and 0.6 mA. The time delays are 36 ps and 27 ps for -12 dBm and -10 dBm, respectively, when the bias current are at 1 mA. In addition, the time delays as a function of bias currents of QD VCSEL and optical power of probe signal are shown in Fig.4-9. We observe that the time delay increases as the signal power decreases.
4.3 Conclusion
We experimentally demonstrate tunable slow light using a 1.3 µm QD VCSEL at room temperature. Tunable optical group delays 42 ps for 10 GHz are achieved by varying the bias current, and the delay-bandwidth product is 0.42. This delay
Reference
[1] G. Lenz, B. J. Eggleton, C. K. Madsen, and R. E. Slusher, “Optical delay lines based on optical filters,” IEEE J. Quantum Electron., vol. 37, pp. 525–532, Apr.
2001.
[2] P. C. Ku, C. J. Chang-Hasnain, J. Kim, and S. L. Chuang, “Semiconductor all-optical buffers using quantum dots in resonator structures,” in Proc. OFC’03, Atlanta, GA, Mar. 2003.
[3] R. Langenhorst, M. Eiselt, W. Pieper, G. Grosskopf, R. Ludwig, L. Kuller, E.
Dietrich, and H. G. Weber, “Fiber loop optical buffer,” J. Lightwave Technol., vol. 14, pp. 324–335, Mar. 1996.
[4] K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightwave Technol., vol. 15, pp. 1263–1276, Aug. 1997.
[5] J. B. Khurgin, “Light slowing down in Moire fiber gratings and its implications for nonlinear optics,” Phys. Rev. A, Gen. Phys., vol. 62, p. 013 821, July 2000.
[6] S. E. Harris, “Electromagnetically induced transparency,” Phys. Today, vol. 50, pp.
36–42, July 1997.
[7] J. P. Marangos, “Electromagnetically induced transparency,” J. Modern Opt., vol.
45, pp. 471–503, Mar. 1998.
[8] L. V. Hau, S. E. Harris, Z. Dutton, and C. H. Behroozi, “Light speed reduction to 17 meters per second in an ultracold atomic gas,” Nature,vol. 397, pp. 594–598, Feb.
1999.
[9] D. F. Phillips, A. Fleischhauer, A. Mair, R. L.Walsworth, and M. D. Lukin,
“Storage of light in atomic vapor,” Phys. Rev. Lett., vol. 86, pp. 783–786, Jan. 2001.
[10] A. V. Turukhin, V. S. Sudarshanam, M. S. Shahriar, J. A. Musser, B. S. Ham, and P. R. Hemmer, “Observation of ultraslow and stored light pulses in a solid,” Phys.
Rev. Lett., vol. 88, p. 023 602, Jan. 2002.
0.0 0.3 0.6 0.9 1.2 0
2 4 6 8 10
Power (µw)
Bias Current (mA)
1276.4 1276.6 1276.8 1277.0 1277.2 1277.4 -90
-80 -70 -60 -50 -40 -30
Intensity (10 dB/div.)
Wavelength(nm)
Oscilloscope Trigger Power splitter
RF Signal Generator
C
PD
QD VCSEL
DC Bias PC
Tunable Laser
VA 1 2
3 RFA
Mod
DC Bias OSA
OC Tunable Delay Line
Fig.4-1. Optical spectrum and light-current characteristics of the QD VCSEL.
Fig.4-2. Experimental setup for measuring the optical group delays in QD
0.9 mA 0.7 mA 0.6 mA Ref.
1 mA
Time (20 ps / div.)
Intensity (a. u.)
△T = 42 ps
Fig.4-3. The measurements of time delay for a 10 GHz probe signal at the various bias currents of QD VCSEL.
1276.6 1276.8 1277.0 1277.2
-90 -80 -70 -60 -50 -40 -30
Intensity(10dB/div)
QD VCSEL injection spectrum
-1.0 -0.5 0.0 0.5 1.0
-10 -9 -8 -7 -6 -5
Inten sity(1d B/d iv)
Wavelength Detuning(nm)
Fig.4-4. Absorption dip of QD VCSEL
8 GHz
Intensity (a. u.)
Time (20 ps / div.)
Ref.
1 mA
△T = 44 ps
Time (50 ps / div.)
Intensity (a. u.)
6 GHz
Ref.
1 mA
△T = 60 ps
Fig.4-6. The waveform at different modulation frequencies of probe signals.
4 5 6 7 8 9 10 11 20
30 40 50 60 70 80
Dealy (ps)
Modulation Frequency (GHz)
Fig.4-7. The relationship between the time delays and modulation frequencies of probe signals.
-10 dBm -12 dBm
Time (20 ps / div.)
Intensity (a. u.)
△T = 36 ps
1 mA
△T = 27 ps
Intensity (a. u.)
Time (20 ps / div.)
△T = 9 ps
△T = 7 ps
0.6 mA
-12 dBm
-10 dBm
Fig.4-8. The waveform at different powers of probe signals.
-15 -14 -13 -12 -11 -10 -9 -8 -7 -10
0 10 20 30 40 50 60 70
Del ay (p s)
1.0 mA 0.9 mA 0.7 mA 0.6 mA
Signal Power (dBm)
Fig.4-9. The time delays as a function of bias currents of QD VCSEL and optical power of probe signal.
Chap 5 Summary
5.1 SummaryIn summary, we have studied high speed characteristics of quantum dot vertical-cavity surface-emitting laser. In Chapter 2, we demonstrate monolithically single-mode QD VCSELs with high side-mode suppression ratio. The QD VCSELs have adapted fully doped structure on GaAs substrate. The output power is ~ 330 µW with slope efficiency of 0.18 W/A at room temperature. Single mode operation was obtained with side-mode suppression ratio of > 30 dB. The high speed characteristics were also investigated. The free running bandwidth of QD VCSEL is ~2GHz. The modulation current efficiency factor (MCEF) is ~ 2.5 GHz/(mA)1/2. Finally, we illustrate the eye diagram at 1.25 Gb/s and 2.5 Gb/s.
In Chapter 3, we report the experimental characterization of 1.3µm QD VCSEL with and without external light injection. Significant frequency response enhancement has been observed. The 3 dB frequency response has been increased by as much as 4.2 times using light injection technique. Furthermore, we demonstrate that this frequency response enhancement allows us to improve the performance of subcarrier multiplexed (SCM) system. A 33 dB improvement in systems performance is obtained with a SCM system for a 7-GHz 50-Mb/s data signal. We also report the third-order intermodulation distortion (IMD3) of QD VCSEL with and without external light injection. We observed that the dynamic range of the QD VCSEL with light injection can be enhanced 15.1 dB for the IMD3. These results show that external light injection is a very powerful technique to upgrade QD semiconductor lasers.
In Chapter 4, we report the slow light in the monolithically single-mode QD VCSEL. Tunable optical group delay can be achieved by adjusting the bias current. A
demonstrated. The delay-bandwidth product is 0.42. We also study the relationship between the signal input power and the tunable optical group delay.
5.2 Future Work
Future work will be focus on the slow light in QD VCSEL. There will be some simulations about slow light mechanism of QD VCSEL. Further discussions on quantum dot medium and cavity resonance effect will be studied. Modulated laser signal with pseudo-random binary sequence (PRBS) data will pass the QD VCSEL to observe the delay condition.