7. Conclusion and Outlook 105
7.2. Outlook
The performance of the MOPA system presented here pretty much meets the require-ments for coherent optical communication applications in space. However, the charac-teristics can further be improved as follows:
The output power in combination with the beam quality can be improved by opti-mizing amplifier geometries like section lengths or opening angle. The linewidth can be reduced by realizing master oscillators with even longer cavity lengths or higher facet re-flectivities. If the cavity length of a monolithic laser can not further be increased then an extended cavity laser could be used. The size of the MOPA system can be reduced by a redesign of the optical setup. It would further be advantageous to implement a fiber cou-pling within a space-qualified package. As mentioned before, the micro-isolator needs to be space qualified and if no external isolator is used a double-stage micro-isolator could further reduce feedback sensitivity.
Furthermore, the origin of the physical effects that determine the FWHM linewidth need to be discussed in more detail. In particular it should be investigated whether the semiconductor laser itself also contributes to the technical noise and thus to the FWHM linewidth.
Coupling 1 W of optical power through a single mode fiber reliably is considered to be challenging. Large mode field fibers might accept higher power levels, however, it remains to be investigated whether coupling over several thousand hours can be realized. Fiber coupling could be avoided by designing the LCT to emit into free space. However, in this case the signals phase modulation will have to be introduced on the micro-optical bench itself. Phase modulation can be realized by integrating aLiN bO3 crystal into the micro-optical bench. The preamplifier section of the amplifier might also be used for phase modulation, however, it remains uncertain if the desired modulation depth and modulation speed can be provided.
A micro-integrated narrow linewidth laser source for integration in a coherent optical communication system might be realized as depicted in fig. 7.1 (left). An electro-optic modulator (EOM) is providing a fast phase modulation on the micro-optical bench directly. The EOM is located after the isolator to avoid any optical feedback from the EOM to the master oscillator. This location also ensures that only relatively low power levels need to be modulated. The MOPA output is collimated by a macroscopic lens as can be seen in fig. 7.1 (right). The amplifier is followed by a slow axis (SAC) lens in order to correct the astigmatism of the power amplifier. The proper way to mount the macroscopic lens is not discussed here, however, it would have to be mounted in a
mechanically stable way.
MO
PA EOM isolator
SAC
MOPA system
macroscopic lens
Figure 7.1.: Outlook of a possible implementation of a semiconductor based laser source for the use in coherent optical communication.
A shortcoming of the MOPA system presented in chapter 6 is that the emission fre-quency can only be tuned by adjusting the temperature of the entire system, including the CCP. This tuning is slow, since the entire device needs to be heated. Furthermore the output power of the system depends on the mount temperature since the performance of the amplifier is temperature dependent. To increase the tuning speed and to decouple the system temperature from the emission frequency, heater section can be placed next to the grating as depicted in fig. 7.2 (left) and already presented in the literature [125].
First experimental results look promising as can be seen in fig. 7.2 (right). The heater consists of small electric stripes with widths of 30µm which are located next to the grating. The electric current flowing through the stripe heats up the adjacent area including the grating. The wavelength can be tuned by 2.5 nm by the heater section.
However, the tuning is not continuous so far, and a "smart" simultaneous tuning of the injection current, the mount temperature, and the current through the heater sections will most likely be required to obtain fast continuous tuning over a wide wavelength range.
The frequency noise spectra recorded by the method described in 4.2.3 are only valid for large carrier offset frequencies. However, for an application like spectroscopy the close-in phase-noise might be of more importance.
Some RF spectrum analyzer can be used to record I-Q data carrying time resolved instantaneous phase information. The Fourier transform of the instantaneous phase yields the frequency noise spectrum which is also valid at small carrier offset frequencies.
Further, amplitude and phase noise can be separated by this method.
2 heater sections
heater contacts
0 100 200 300 400 500
1061.5 1062.0 1062.5 1063.0 1063.5 1064.0 1064.5
wavelength[nm]
heater current [mA]
I = 200 mA
T = 25°C
Figure 7.2.: (left) DBR laser with heater section (not to scale). (right) First results of the tuning characteristics by the heater section. The emission wavelength as obtained by a wavemeter is plotted vs. the current through the heater section.
The following scientific publications have been prepared in connection with this thesis:
Articles
K. Paschke, S. Spießberger, C. Kaspari, D. Feise, C. Fiebig, G. Blume, H. Wenzel, A.
Wicht, and G. Erbert, “High-power distributed Bragg reflector ridge-waveguide diode laser with very small spectral linewidth”. Optics Letters, vol. 35, pp. 402-404, 2010.
S. Spießberger, M. Schiemangk, A. Wicht, H. Wenzel, O. Brox, and G. Erbert, “Narrow linewidth DFB lasers emitting near a wavelength of 1064 nm”. Journal of Lightwave Technology, vol. 28, pp. 2611-2616, 2010.
S. Spießberger, M. Schiemangk, A. Wicht, H. Wenzel, G. Erbert, and G. Tränkle, “DBR laser diodes emitting near 1064 nm with a narrow intrinsic linewidth of 2 kHz”. Applied Physics B, vol. 104, pp. 813-818, 2011.
S. Spießberger, M. Schiemangk, A. Sahm, A. Wicht, H. Wenzel, A. Peters, G. Erbert, G. Tränkle, "Micro-integrated 1 Watt semiconductor laser system with a linewidth of 3.6 kHz". Optics Express, vol. 19, pp. 7077-7083, 2011.
A. Bawamia, G. Blume, B. Eppich, A. Ginolas, S. Spießberger, M. Thomas, B. Sumpf, and G. Erbert, “Miniaturized tunable external cavity diode laser with single-mode oper-ation and a narrow linewidth at 633 nm”. IEEE Photonics Technology Letters, vol. 23, pp. 1041-1135, 2011.
Conference Contributions
S. Spießberger, M. Schiemangk, A. Wicht, and G. Erbert, "Compact narrow linewidth laser sources for coherent optical communication". Laser Optics Berlin, Berlin, 2010.
S. Spießberger, M. Schiemangk, A. Wicht, and G. Erbert, "Narrow linewidth DBR-RW lasers emitting near 1064 nm". Conference on Lasers and Electro-Optics (CLEO), OSA, CtuKK6, San Jose, 2010.
S. Spießberger, M. Schiemangk, A. Sahm, A. Wicht, H. Wenzel, J. Fricke, and G. Er-bert, "1 W semiconductor based laser module with a narrow linewidth emitting near 1064 nm". Photonics West, SPIE, 7953-36, San Francisco 2011.
S. Spießberger, M. Schiemangk, A. Sahm, A. Wicht, H. Wenzel, J. Fricke, G. Erbert, and G. Tränkle, "1 W narrow linewidth semiconductor-based laser module emitting near 1064 nm for coherent optical communication in space". IEEE International Conference on Space Optical Systems and Applications (ICSOS), Santa Monica, pp. 322-324, 2011.
E. Luvsandamdin, G. Mura, S. Spießberger, A. Wicht, A. Sahm, H. Wenzel, G. Erbert, and G. Tränkle, "Micro-integrated ECDLs for precision spectroscopy in space". IEEE International Conference on Space Optical Systems and Applications (ICSOS), Santa Monica, pp. 381-383, 2011.
S. Spießberger, M. Schiemangk, A. Sahm, A. Wicht, H. Wenzel, G. Erbert, and G. Trän-kle, "Narrow-linewidth high-power semiconductor-based laser module". Conference on Lasers and Electro-Optics (CLEO) Europe, Munich, CB-P10, 2011.
C. Kürbis, A. Kohfeldt„ E. Luvsandamdin, M. Schiemangk, S. Spießberger, A. Wicht, A. Peters, G. Erbert, and G. Tränkle, "Mikrointegrierte Lasersysteme für die höchstau-flösende Atomspektroskopie und die kohärente Nachrichtenübertragung im Weltraum".
60. Deutscher Luft- und Raumfahrtkongress, Bremen, 2011.
S. Spießberger, M. Schiemangk, A. Sahm, F. Bugge, J. Fricke, H. Wenzel, A. Wicht, G. Erbert, and G. Tränkle, "All-semiconcutor based, narrow linewidth, high power laser system for laser communication applications in space at 1060 nm". Photonics West, SPIE, 8246-18, San Francisco 2012.
A. Wicht, E. Luvsandamdin, A. Kohfeldt, M. Schiemangk, S. Spießberger, A. Sahm, F.
Bugge, J. Fricke, H. Wenzel, A. Peters, G. Erbert, and G. Tränkle, "Micro-integrated, high power, narrow linewidth diode lasers for precision quantum optics experiments in space". Photonics West, SPIE, 8265-21, San Francisco 2012.
B. Sumpf, A. Bawamia, G. Blume, B. Eppich, A. Ginolas, S. Spießberger, M. Thomas, and G. Erbert, "Continuously current-tunable, narrow line-width miniaturized external cavity diode laser at 633 nm". Photonics West, SPIE, 8277-40, San Francisco 2012.
A. Bawamia, B. Sumpf, G. Blume, B. Eppich, A. Ginolas, S. Spießberger, M. Thomas, and G. Erbert, "Miniaturized, current-tunable, external cavity diode laser with single-mode emission and a narrow line-width at 633 nm". Laser Optics Berlin, submitted, Berlin 2012.
In the following the dependence of the threshold gain and the mode discrimination of a DFB laser in dependence of the facet-grating phase of the rear facet will be presented.
The threshold gain and the detuning from the Bragg condition of three modes on either sides of the stop band are shown in fig. B.1 with κL (κ: coupling coefficient of the grating,L: length of the cavity) as a parameter. The simulations are carried out as described in [68]. Fig. B.1 (left) and (right) present simulation results for a DFB laser with a rear facet-grating phase ΦL= 0 andπ/2, respectively. For a given κL, the mode with the lowest threshold gain will start to oscillate (modes possible are marked for κL
= 0.2 and 0.4). ForΦL= 0 the mode m = -1 exhibits the lowest threshold gain and thus a single mode operation can be expected. In contrast, forΦL =π/2 the modes m = -1 and m = 1 have the same threshold gain and thus oscillation of both modes would be supported. Accordingly, depending on the facet-grating phase, DFB lasers with a high reflective coating on the rear side can either emit in a stable single mode or theoretically in two modes. Note, that well above threshold typically only one mode would lase for ΦL =π/2 as well, however, mode hops between the two modes are likely to happen.
-15 -10 -5 0 5 10 15
0.0 0.5 1.0 1.5 2.0 2.5 3.0
kL = 0.4 kL = 0.2 m=-3
m=-2
m=-1
m=+3
m=+2
L
detuning from Bragg condition L m=+1
-15 -10 -5 0 5 10 15
0.0 0.5 1.0 1.5 2.0 2.5 3.0
L
detuning from Bragg condition L kL = 0.4 kL = 0.2 m=-3
m=-2
m=-1
m=+3
m=+2
m=+1
Figure B.1.: Threshold gain and detuning from Bragg condition for three modes on each side of the stop band of DFB lasers with κL as a parameter. (left) DFB laser with RL = 0.95, R0 = 10−4,ΦL = 0,Φ0 = 0. (right) DFB laser with RL= 0.95, R0 = 10−4,ΦL=π/2,Φ0 = 0.
Pumped Solid State Laser
Current coherent LCT demonstrators in space make use of Nd:YAG NPRO laser sources.
In the following, the linewidth results of the MOPA systems presented in 6.3 are there-fore compared to commercially available free-running NPRO lasers (Lightwave 122 and Lightwave 124) that have been provided by the "AG Optische Metrologie, Humboldt Universität zu Berlin". Heterodyne linewidth measurement results with a "weak" lock are depicted in fig. C.1. For comparison, the results of the MOPA systems are also displayed. The negative feedback signal, required for the "weak" lock, was fed into the fast modulation port of the NPRO drivers.
The FWHM linewidth corresponds to 130 kHz and 6 kHz for the micro-integrated MOPA systems and the NPRO lasers, respectively as obtained by the linear plot (fig.
C.1 (left)) of the RF beat note spectrum (note that this is the linewidth of the beat note signal and not that of a single laser).
The logarithmic plot (fig. C.1 (right)) reveals pronounced peaks that most likely are caused by injection current noise.
-0.4 -0.2 0.0 0.2 0.4
0.0 0.2 0.4 0.6 0.8 1.0
beat note, MISER = 6 kHz
normalizedRFpower
detuning from carrier [MHz]
MOPA
MISER
beat note, DBR
= 130 kHz
-5 -4 -3 -2 -1 0 1 2 3 4 5
-80 -60 -40 -20 0
RFpower[dB]
detuning from carrier [MHz]
MOPA
MISER
Figure C.1.: Comparison of RF beat note spectra of optically pumped solid state lasers (NPRO) with the micro-integrated MOPA laser systems. (left) Linear scale.
(right) Logarithmic scale.
Even if the linewidth results presented within this work are, to our knowledge, the best that have been achieved for semiconductor lasers so far, the results presented in fig. C.1 show that optically pumped solid state lasers can still provide a narrower linewidth. However, as presented in table 2.1, these extremely narrow linewidths are not required for coherent optical communication. Furthermore, if required, the linewidth of
semiconductor lasers can be reduced by electrical feedback or by locking to an external cavity. In addition, semiconductor lasers provide a variety of other important advantages compared to NPRO lasers already mentioned in the introduction like compactness, high efficiency, mechanical stability, and direct modulation capability.
• 8 gRMS, all three axes, for 120 s:
20 Hz-400 Hz 0.0045 g2/Hz 400 Hz-600 Hz 0.0675 g2/Hz 600 Hz-1300 Hz 0.0045 g2/Hz 1300 Hz-2000 Hz 0.0675 g2/Hz
• 21 gRMS, all three axes, for 180 s:
20 Hz 0.11 g2/Hz 20 Hz-50 Hz +6 dB/oct 50 Hz-300 Hz 0.7 g2/Hz 300 Hz-500 Hz -7.4 dB/oct 500 Hz-1000 Hz 0.2 g2/Hz 1000 Hz-2000 Hz -6 dB/oct 2000 Hz 0.05 g2/Hz
• Pyro shocks, all three axes:
100 Hz 20 g 1500 Hz 1500 g 10000 Hz 1500 g
• 29 gRMS, all three axes, 180 s:
20 Hz-70 Hz +6 dB/oct 70 Hz-300 Hz 1.35 g2/Hz 300 Hz-406 Hz -12 dB/oct 500 Hz-1100 Hz 0.4 g2/Hz 1100 Hz-2000 Hz -12 dB/oct
The following abbreviations and symbols are widely used within this work:
Abbreviations
Al aluminum
As arsenide
BPSK binary phase shift keying COD catastrophical optical damage DBR distributed Bragg reflector DFB distributed feedback
DLR Deutsches Zentrum für Luft- und Raumfahrt e. V.
ECDL external (sometime also called extended) cavity diode laser ESA European space agency
FWHM full width at half maximum
Ga gallium
GEO geostationary orbit
In indium
LCT laser communication terminal LEO low earth orbit
MOPA master oscillator power amplifier NPRO nonplanar ring oscillator
PSK phase shift keying RF radio frequency
Symbols
α linewidth enhancement factor
αef f effective linewidth enhancement factor γ FWHM of Lorentzian signal power spectrum
∆f frequency detuning from carrier
∆ν spectral linewidth
∆νsp intrinsic spectral linewidth
∆νtechnical technical linewidth
κ coupling coefficient of the grating
λ wavelength
ν photon frequency
Φ phase of an optical field Φ0 front facet-grating phase ΦL rear facet-grating phase
ω angular frequency of an optical field c vacuum speed of light
f frequency
Fwhitenoise constant floor in frequency noise spectrum
h Plancks constant
I injection current
IM O injection current into master oscillator Iphot total number of photons in the laser mode
IP re injection current into the ridge waveguide preamplifier section IT A injection current into tapered amplifier section
k amount of 1/f noise
K longitudinal spontaneous emission enhancement factor L length of the cavity
ng group index
nsp spontaneous emission factor
P optical power
q elementary charge Rf reflectivity of front facet RL reflectivity of rear facet
Rsp rate of spontaneous emission into the lasing mode S0 amount of white noise
In the following, the FBH internal nomenclature of the devices used in this work are given.
Type length Rf Rr presented in figures charge testing bar diode field
DFB 1 mm AR 95% 5.3, 5.8 (left) C1421-6-1 06 18 08
DFB 1 mm AR 95% 5.3, 5.4 (left), 5.5 (left), C1421-6-1 06 18 09 5.6, 5.8 (left)
DFB 2 mm AR 95% 5.3, 5.7, 5.8 (right) C1421-6-1 06 16 08
DFB 2 mm AR 95% 5.3, 5.4 (right), 5.5 (right), C1421-6-1 06 16 09 5.7, 5.8 (right)
DBR 4 mm 1% AR 5.10, 5.11 (left), 5.12 (left), C1353-3 00 02 03 5.14, 5.15
DBR 4 mm 30% AR 5.10, 5.11 (right), 5.12 (right), C1353-3 00 01 01 5.13, 5.16, 5.17
DBR 4 mm 30% AR 5.10, 5.16, 5.17 C1353-3 00 01 02
MOPA 1 6.7, 6.8, 6.9, 6.10 (left), 6.10 (right)
MO: DBR 4 mm 30% AR 6.11, 6.12, 6.14, 6.15, C1353-3 00 01 03
PA: TA 4 mm AR AR 6.16, 6.17 (right), 6.18 C0385-3 04 05 18
MOPA 2
MO: DBR 4 mm 30% AR 6.11, 6.12, 6.16, 6.18 (right) C1353-3 00 03 02
PA: TA 4 mm AR AR B1072-3 04 01 18
MOPA 3
MO:DBR 4 mm 30% AR 6.5, 6.6, 6.13 C1353-3 00 03 09
PA: TA 4 mm AR AR B1072-3 04 08 19
Table F.1.: List of devices according to FBH nomenclature and their appearance in graphs.
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