Introduction
Deployment of coherent optical communication systems
Currently, upgrading current optical communication networks to meet the ever-increasing data rate requirements has become the primary task in the field [7-9]. Coherent optical communication networks are likely to be widely deployed in the next decade or so, and we are now witnessing the beginning of such a transition.
Desire of the right light sources
Coherent optical communications uses more complex modulation schemes such as quadrature amplitude modulation (QAM), where information is encoded in both the intensity and phase of lasers, and therefore has higher spectral efficiency, increasing the capacity of communications links to meet the increasing demands in the field of communication. the data rate [13, 14]. Currently, major players in this field are developing coherent transceivers that support data rates of 400 Gbit/s or even higher.
Content of this thesis
The laser frequency noise PSD can be derived based on the PSD of the output of the balanced photodetectors. Equation (5.15) constitutes a general relationship between the frequency noise PSD S∆υ( )υ and the line shape function SE( )ω of the laser light.
Theory of laser coherence and high-coherence Si/III-V lasers
Coherence of semiconductor lasers
- Spontaneous emission and quantum noise
- Coupling between intensity noise and frequency noise
- Direct current modulation of semiconductor lasers
- Laser frequency noise PSD
Laser intensity noise perturbs the stimulated emission rate and causes fluctuations in the number of spin-up electrons. You might also ask 'is it possible for us to relate the line shape to the PSD frequency noise so that the phase noise is expressed as a function of the line width'.
High-coherence Si/III-V lasers
- Reduce quantum noise with mode engineering
- Hybrid Si/III-V lasers
- Laser characterization: power and spectrum
- Laser characterization: relaxation resonance frequency and alpha
- Laser characterization: frequency noise PSD
Compared to commercial III-V DFB lasers, the efficiency of the Si/III-V laser is low. Due to the response bandwidth of the MZI, we can only measure the PSD frequency noise below the relaxation resonance frequency.
Conclusions
Finally, the frequency noise PSDs of the lasers are measured under different levels of optical feedback. On the contrary, the frequency noise PSD of the Si/III-V laser is hardly changed to a feedback level of -31 dB. To determine why the frequency-noise PSD of the Si/III-V laser under coherent optical feedback differs significantly from theory, we will review the dynamical equations in section 4.2.
The exclusion of other coupling mechanisms could be the reason why the frequency noise PSD of the Si/III-V laser under coherent optical feedback deviates from theory. The intensity or frequency noise level PSD of the III-V DFB laser without incoherent optical feedback is set as a reference point. On the contrary, the BER-OSNR curves of the Si/III-V laser remain almost unchanged by coherent optical feedback.
The BER-OSNR curves of the Si/III-V laser are hardly affected by incoherent optical feedback. Furthermore, the bump reproduces the envelope of the jitter in the phase noise PSD.
Coherent optical communication
Fundamental of coherent optical communications
- Quadrature amplitude modulation
- Demodulation and detection
- Digital signal processing (DSP)
- System performance
Si/III-V lasers as the light sources
- Back-to-Back coherent communications
- Si/III-V laser vs III-V DFB laser vs external cavity laser
- ZR coherent communications
In the section we will describe the use of hybrid Si/III-V lasers as the light sources to perform coherent optical communication. In contrast, the BER of the III-V DFB laser with VV approaches a constant at high OSNR, where the system performance is limited by the phase noise. The phase noise is negligible in all but one case, the conventional III-V DFB laser with VV, where the shear deformation of the constellations indicates the existence of non-negligible phase noise.
Due to the high phase coherence of the Si/III-V laser and the ECL, the corresponding phase noise in the system is so small that as long as the phase recovery algorithm can successfully recover the constant initial phase, the performance will be good.
Conclusions
An optical isolator is placed before the PSD frequency noise measurement system to avoid any unwanted optical feedback. However, equation (5.18) predicts that the PSD frequency noise should be modified by the MZI transfer function. Second, the Central Link reveals that the PSD frequency noise at high frequencies is only related to the tail of the optical lineshape.
The frequency noise PSD of the III-V DFB laser at low frequencies is significantly reduced by very little coherent optical feedback, while at high frequencies the RF oscillations are quite weak.
Impacts of optical feedback on laser coherence
Coherent optical feedback and incoherent optical feedback
Optical feedback can be classified into two categories, namely coherent and incoherent optical feedback, as shown in Fig. Coherent optical feedback, such as reflection from an external mirror, is explicitly correlated with the laser output while incoherent optical feedback, such as enhanced spontaneous emission (ASE), originates in a different and independent light source, such as an optical amplifier, and is thus uncorrelated with the laser output. In semiconductor lasers, spontaneous emission is the single noise source, from which the intensity and phase noise arise.
By introducing optical feedback into a laser system, the source of noise or the relationship between laser intensity and phase noise changes, causing a change in the noise frequency of the PSD laser and eventually affecting the lasers.
Coherent optical feedback effects on laser coherence
Equations (4.1) and (4.2) are written in two colors, where the black part is the intrinsic dynamics of the laser noises, while the red part represents additional assemblies created by coherent optical feedback. Under the coherent optical feedback, the intensity and phase noise are related to each other, which will significantly change the laser frequency noise PSD. If the C-parameter is much larger than unity, the denominator at local minima can be much smaller than unity, leading to spikes in the laser frequency noise PSD.
If we continue to increase the level of coherent optical feedback, at some point the side modes will be strong enough to compete with the original lasing mode, making the lasers unstable.
Incoherent optical feedback effects on laser coherence
The first affects the properties of the laser, for example the phase coherence, so that the performance of the system changes. To study how the ASE noise affects the PSD laser frequency noise, we can still write down the dynamical equations of the laser noise and do the analysis in the same way as we treat coherent optical responses. Induced intensity noise is coupled to laser frequency noise also through the mechanism of linewidth enhancement, leading to laser coherence degradation.
Since the intensity noise produced by ASE is white noise whose PSD is proportional to the feedback power, it is laser frequency noise.
Lasers’ sensitivity to optical feedback
Note that we have made no assumptions about the physical origin of the frequency noise. It should be emphasized that the left side of equation (5.15) is essentially the phase noise PSD of the laser. To illustrate the validity of the central relation, both the PSD frequency noise and the line shape of a single laser were measured.
By properly designing the transmission spectrum of the filter, the frequency noise can be adjusted accordingly.
Sensitivity to coherent optical feedback
Sensitivity to incoherent optical feedback
System performance under coherent optical feedback
We characterize the system performance by measuring the BER-OSNR curves at different levels of coherent optical feedback. Therefore, at a response level of -41.5 dB or more, it is impossible to perform coherent optical communications successfully with the conventional III-V DFB laser. Based on the data, the Si/III-V laser is stronger against coherent optical feedback than the conventional III-V DFB laser by at least 27.2 dB.
For the record, commercial optical isolators typically provide optical isolation between 25 dB and 30 dB, suggesting that, in terms of sensitivity to coherent optical feedback, the Si/III-V laser is as stable as the commercial III-V DFB laser. with an optical isolator.
System performance under incoherent optical feedback
Even at the feedback level of -18.3 dB, the highest feedback level achievable in the experiments, there is no obvious degradation of system performance. However, those of the III-V DFB laser continue to shift upward as the feedback power increases, demonstrating the degradation of system performance. However, this type of degradation, which gradually increases as the feedback power increases, is quite different from what we observed with coherent optical feedback.
Previously, above a certain level of coherent optical feedback, -41 dB in our case, the communication system experiences a sharp transition from functional to dysfunctional.
OSNR penalty due to optical feedback
Conclusions
The general relationship precisely describes the correspondence between the frequency noise PSD and the optical line shape and therefore we experimentally demonstrate the validity of the central relationship. In this section we will discuss some corollaries of the Central Relationship, along with the experimental evidence. Equation (5.18) indicates that the frequency noise PSD at a baseband frequency υ is modified by the same transmission function of the filter at an optical frequency ω0±2πυ.
Consequently, the laser linewidth, a physical characteristic of the optical lineshape, is generally independent of the frequency noise PSD at high frequencies and thus not a good measure of laser coherence in coherent optical communication.
A general relation between laser frequency noise and lineshape
Derivation of the general relation
The line shape function (one-sided spectrum) of the laser light is represented by the PSD of the laser field, which is the Fourier transform of the correlation function of the field. Note that if we integrate the entire laser function of the line shape, we get the total power of the laser light, namely. It shows that at high frequencies there is a one-to-one correspondence between the frequency noise and the line shape function.
Empirically, frequencies greater than ten times the linewidth can be considered sufficiently high for the Central Ratio to apply; such a rule of thumb is confirmed by the experiments described in the following sections.
Validation of the Central Relation
If the lineshape is symmetric about the center frequency, which is true for the laser lineshape, then any feature in the high-frequency PSD phase noise will appear identically in the lineshape and vice versa. There is jitter in the tens of megahertz in the spectrum, which comes from the laser control circuit. The noise in the PSD phase noise is located at the same frequency as the humps in the line shape with respect to the center frequency.
Due to the measurement resolution of the spectrum analyzer, we cannot observe the individual lines in the line shape.
Insights into the Central Relation
- Optical filtering of laser frequency noise PSD
- Is laser linewidth a good measure for laser coherence
The laser frequency is set to match one of the maximum transmission frequencies of the MZI, which is shown schematically in Fig. The function is indeed the transmission spectrum of the MZI, which is sinusoidal with an FSR of 203 MHz. Apparently, little coherent optical feedback can help dramatically suppress the linewidth of the III-V DFB laser, making it as 'coherent' as the Si/III-V laser as laser linewidth as the measure of laser coherence [64- 68].
5.9, show that the narrow linewidth of the III-V DFB laser is useless under very little coherent optical feedback, as there is no obvious improvement in the performance of the laser system.
Conclusions
Steger, “A Fundamental Approach to Reducing Phase Noise in Hybrid Si/III-V Lasers,” (California Institute of Technology, 2014). Yariv, “Oxide-confined heterogeneously integrated narrow-line semiconductor lasers,” IEEE Photonics Technology Letters. Coldren, "Single Longitudinal Mode Selection Characteristics in Short Coupled Cavity (SCC) Injection Lasers," Journal of Lightwave Technology.
Petermann, "External optical feedback phenomenon in semiconductor lasers," IEEE Journal of Selected Topics in Quantum Electronics.
