Polarization Dynamics in VCSEL-Based Gain Switching Optical Frequency Combs

A novel photonic scheme for the simultaneous generation of microwave amplitude shift keying (ASK) and frequency shift keying (FSK) signal based on gain switching in Vertical Cavity Surface Emitting Laser (VCSEL) is proposed, for the first time. In this approach, gain switched VCSEL is utilized to produce optical frequency comb (OFC) and the linearly polarised OFC signal is splitted into two orthogonal polarizations using polarisation controller and polarisation beam splitter. Two electro absorption modulators (EAM) are employed to modulate each polarization by 1Gbps data. Microwave FSK signals at 2/4 GHz and 6/8 GHz with 1 Gbps data are detected at the photodetector. Moreover, microwave ASK signals are also generated from each polarization branch at 2 GHz and 4 GHz. Simulations are performed to substantiate the proposed system. The influence of VCSEL modulating signal power and EAM reverse bias on FSK output power is also discussed. Results predict a maximum sideband suppression ratio of 33 dB, for a modulating signal current and EAM reverse bias voltage of 15 mA and 1.5 V respectively. The receiver sensitivity of the FSK signal at 2/4 GHz for a 20 km fibre transmission is found to be −16.3 dBm with a Q factor of 6, at a BER of 10−9. This scheme offers a simple, cost-effective approach and requires a low modulating current and its applications include radar, wireless and optical communications.

A novel approach to the generation of an optical frequency comb with a widely tunable center wavelength and comb spacing based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO is implemented using a polarization modulator (PolM), a phase-shifted fiber Bragg grating (PS-FBG), and a photodetector (PD). The PolM is a special phase modulator that supports phase modulation along the two principal axes with opposite modulation indexes. The joint operation of the PolM, the PS-FBG, and the PD corresponds to a frequency-tunable microwave photonic bandpass filter. When the output from the PD is fed back to the PolM, the OEO starts to oscillate and the oscillation frequency can be tuned by tuning the center frequency of the microwave photonic bandpass filter through tuning the optical wavelength. The optical comb is then generated by tapping part of the optical signal from the PolM and sending it to a second PolM. The joint operation of the two PolMs generates an optical comb with the comb spacing tunable by tuning the center frequency of the microwave photonic filter. Through introducing a second wavelength into the OEO, a duplicated optical comb at the second wavelength is generated. An experiment is performed. An optical frequency comb with tunable frequency spacing from 6.6 to 15.3 GHz and a tunable center wavelength from 1500 to 1580 nm is generated.

Recent works have shown the possibility to control through optical injection frequency combs generated by gain switching in Vertical-Cavity Surface-Emitting Lasers (VCSELs) [1] . The opposite situation, in which VCSEL is subject to injection of an optical frequency comb (OFC) deserves close attention and has not been addressed so far. In this work, we analyze the nonlinear polarization dynamics of a single-mode VCSEL with optical injection. Here, we select and reveal several bifurcations scenarios leading to generation of a two polarization OFC.

A measurement of the parameters characterizing polarization behavior of a 1550 nm single transverse mode vertical-cavity surface-emitting laser (VCSEL) is presented from the analysis of CW high-resolution optical spectra. Current-induced polarization switching (PS) is observed in our device. A minimum in the absolute value of the effective dichroism is observed at the PS point. The value of the effective birefringence has a discontinuity at the current at which PS is observed. Birefringence oscillations with the bias current are also observed. PS characteristics are used for the extraction of the parameters describing the polarization behavior of the VCSEL. Obtained values of the spontaneous emission fraction and spin-flip rate are larger than those typically used in theoretical modeling of these devices.

Optical frequency combs have been explored in different applications from optical frequency measurements to optical communications. OFCs can be formed using numerous techniques such as optical modulation, laser mode locking and optical fiber recirculating loops. An optical frequency comb generator based on an optical fiber recirculation loop operating with a Mach-Zehnder modulator was experimentally investigated using different types of amplifiers as gain media. The idea is to compare the use of semiconductor optical amplifiers (SOAs) and erbium-doped fiber amplifiers (EDFAs) and their efficiency in aiding or contributing on the generation of spectral lines. The process efficiency associated with nonlinear effects, specially four-wave mixing, is the main advantage of using SOAs over EDFAs. Under the best operational conditions, an optical comb with up to 30 lines and spacing around 20 GHz was obtained by a SOA-based loop, where an ultralong SOA was used for the first time in such an application.

We show, both experimentally and theoretically, that a slave laser injected with an optical frequency comb can undergo two distinct locking mechanisms, both of which decrease the output optical comb's frequency spacing. We report that, for certain detuning and relative injection strengths, slave laser relaxation oscillations can become undamped and lock to rational frequencies of the optical comb spacing, creating extra comb tones by nonlinear dynamics of the injected laser. We also study the frequency locking of the slave laser at detunings in between the injected comb lines, which add the slave laser's frequency to the comb. Our results demonstrate the effect of the α parameter and stability of the locked states and indicate how the frequency of the relaxation oscillations affect both of these locking mechanisms. These optical locking mechanisms can be applied to regenerate or multiply optical combs.

The basic operation, implementation and control of femtosecond laser optical frequency combs will be presented. The application of frequency combs in various scientific and technological fields will be described. 150-word Biography: Scott Diddams received the B.A. in Physics from Bethel College, St. Paul, MN in 1989 and the Ph.D. degree in Optical Science from the University of New Mexico, Albuquerque in 1996. From 1996 through 2000, he did postdoctroral work at JILA, University of Colorado. In 1998 Dr. Diddams was awarded a National Research Council fellowship to work with Dr. John Hall (also at JILA) on the development of optical combs for frequency metrology. The techniques developed at that time have had a significant impact on the fields of ultrafast optics and optical frequency metrology. Since 2000, Dr. Diddams has been a staff member and project leader at the National Institute of Standards and Technology (NIST) where he and his coworkers have continued the development of optical frequency combs for optical clocks, waveform generation, low-noise microwave synthesis, spectroscopy and other applications. Dr. Diddams was a co-recipient of a Department of Commerce gold medal for revolutionizing the way frequency is measured and was additionally the recipient of the Presidential Early Career Award in Science and Engineering (PECASE) for his work on optical frequency combs. He is a Fellow of the OSA and APS and a member of IEEE. Article not available.

Optical frequency combs have been widely used in spectrum analysis, coherent optical communication, and accurate distance measurement. We propose a straightforward method to improve the flatness of optical frequency combs. First, we derived the output of the optical signal for the configuration of a cascaded MZM and two PMs. Second, we identified the parameter value when the flatness was optimal after traversing different parameter spaces. The optimal flatness conditions could be automatically determined from an existing sample dataset by using neural networks and Bayesian optimization, which significantly reduced the calculation cost. Furthermore, a broad spectrum and low power consumption were also achieved. Finally, the generated optical frequency comb signal was divided into eight carriers with 50 GHz intervals, and the optical transmission system was verified by applying a 16-QAM modulation of 40 GBaud/s to each channel. The constellation diagram proved the feasibility of this optical comb generation scheme.

Random source is important for the security of key distribution. In this paper, a novel secure key distribution scheme based on unidirectional injection of vertical cavity surface emitting laser (VCSEL) system is proposed. In the proposed scheme, a chaotic signal without time delay signature is generated by a VCSEL subject to unidirectional optical injection, which is regarded as a master laser. The chaotic signal generated by the master VCSEL is further injected into two slave VCSELs to obtain synchronized bandwidth-enhanced chaotic signals. After that, by sampling, quantizing and XOR operation on the two synchronized chaotic signals, two key streams can be obtained. Based on the well-known spin-flip model, the time delay signature of chaotic signals generated by master VCSEL and the synchronization performance between the master VCSELs and two slave VCSELs are numerically investigated in detail. It is shown that by the unidirectional injection, the chaotic outputs can be achieved in the master VCSEL in a wide range of frequency detuning and coupling strength. More importantly, no time delay signature can be observed in the auto correlation function of the chaotic intensity time series generated by the master VCSEL. Besides, we find that high quality synchronization is achieved between the bandwidth-enhanced chaotic signals generated by two slave VCSELs under the common driving of master VCSEL. The synchronization coefficient between two slave VCSELs increases up to 0.99, and the synchronization coefficient between master VCSEL and salve VCSEL is only 0.74. Note that such a high quality synchronization between two slave VCSELs while relatively low quality synchronization between the master and slave VCSEL is conducible to ensuring the security of key distribution. In addition, the effects of tunable parameters on key bit error rate are considered, and two quantization methods are employed for comparison. Numerical simulation results show that the key bit error rate between two legitimate users is as low as 1%, and the key bit error rate between legitimate user and eavesdropper is higher than 10% in the single-threshold case; the bit error rate can even be as low as 10-6 in the double-threshold case. The influence of parameter mismatch on key bit error rate is also discussed, and it is suggested that two salve VCSELs should be finely matched to ensure low bit error rate. Finally, NIST randomness test is performed for the generated key streams. Hence, the proposed scheme enhances the security of key distribution, which is valuable for further developing the chaos communication systems.

We suppress the temperature drift in the refractive-index-sensing optical frequency comb (RI-sensing OFC) by using the difference of repetition frequency between an active RI-sensing OFC and a dummy one in the mechanical-sharing dual-fiber-cavity configuration.

The traditional approach for analyzing high-resolution optical spectra involves locating and measuring line frequencies, then assigning and fitting these to an appropriate model for the molecular energy levels. The alternative approach of fitting the spectrum directly is particularly appealing in the case of congested spectra with many overlapped features. The capabilities and limitations of direct fitting are explored in application to the 0–0 band of the OH A→X transition, as recorded on a charge-coupled-device array using a high-pressure Tesla discharge source. Monte Carlo calculations confirm that for Poisson (counting) data, the parameter errors from the variance–covariance matrix are trustworthy, unless weights are neglected, in which case the error estimates can be wildly optimistic. In the fitting of actual spectra, a major barrier is the derivation of a suitable instrumental line-shape function. In particular, the often adopted triangular slit function is grossly inadequate. A systematic procedure has been devised for obtaining the line shape as a sum of Gaussian and Lorentzian components. This method has facilitated the analysis of experimental spectra spanning the region 3071–3103 Å. A model having 46 adjustable parameters determines the temperature (320 K) of the low-N′ levels of the A state with σ < 1 K and corroborates theoretical intensity branching ratios within 2%. It also determines within 10% the contribution to the emission from OD, which is present in natural abundance (H:D≈7000) in the source.

The monolithic integration of soliton microcomb devices with active photonic components and high-frequency electronics is highly desirable for practical applications. Among many materials, silicon nitride ( SiN x ) waveguide layers prepared by low-pressure chemical vapor deposition (LPCVD) have been the main platform for on-chip optical frequency comb generation. However, the high temperatures involved in LPCVD render it incompatible as a back-end process with complementary metal oxide semiconductor (CMOS) or active III-V compound semiconductor fabrication flows. We report the generation of coherent soliton frequency combs in micro-ring resonators fabricated in deuterated silicon nitride ( SiN x : D ) waveguides with a loss of 0.09 dB/cm. Deposited at 270°C by an inductance-coupled plasma chemical vapor deposition (ICP-CVD) process, the material preparation and fabrication flow are fully CMOS-compatible. These results enable the integration of silicon-nitride-based optical combs and photonic integrated circuits (PICs) on prefabricated CMOS and/or III-V substrates, therefore marking a major step forward in SiN x photonic technologies.

The present experimental work studies the dynamics of dual-polarization optical frequency combs (OFCs) based on gain switching (GS) vertical-cavity surface-emitting laser (VCSEL) diodes under optical injection locking (OIL). This study presents two main results. First, we have obtained an overall comb formed by two orthogonally polarized sub-combs with comparable span and power. The overall comb shows enhanced optical span and flatness and high coherence between its modes. The second result is that we have been able to control the polarization state of the overall comb by tuning the polarization state of the injected light by locking the same single teeth of the comb. This produces an overall comb with single polarization that is parallel or orthogonal. These are novel findings that provide for the development of efficient and compact OFCs based on GS VCSEL sources with versatile polarization dynamics.

A wideband optical frequency comb (OFC) generator using a fiber re-circulating loop (FRL) including a phase modulator and optical amplifier, cascaded with a spectrum expander containing a high-power erbium-doped fiber amplifier and highly nonlinear fiber (HNLF) is proposed and experimentally demonstrated, in which the FRL is used as a source to generate seed comb lines, and then generated seed comb lines are extended to a wideband optical frequency comb using cascaded four-wave mixing in the HNLF. Only using one stage spectrum expander, a stable 20-GHz optical comb with 201 comb lines within 30-dB power deviation, spanning 40-nm bandwidth, is achieved. The results show that the HNLF has unique ability to broaden output spectrum from the FRL. In addition, an optical frequency comb is dramatically broadened, which may be predicted by optimizing gain and dispersion of the FRL or configuration of the spectrum expander, or both.

: This research project was aimed at the stabilization of the frequency and phase of a set of optical combs from a semiconductor laser operating in an active mode-locked regime. To achieve this, an intracavity Pound-Drever-Hall technique was used on a 10 GHz harmonically mode-locked semiconductor ring laser and obtained a simultaneous optical frequency comb stabilization within 3 MHz range and supermode phase noise suppression. Together with an additional phase-lock-loop, the timing jitter integrated from 10 Hz to 10 MHz (5 GHz) was 63.5 fs (161 fs). Approximately 25 fsec of the integrated noise is attributed to line noise, resulting in a net jitter of 38 fsec. This work represents, to our knowledge, the first stabilized modelocked diode laser using PDH that achieves both supermode elimination and optical frequency comb stabilization. The resulting optical comb source may be useful for advanced RF imaging radar for optical sampling in ADC or in novel waveform generation (DAC's).